Chapter 2 Exploring Geology

8/20/2019 Chapter 2 Exploring Geology

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Exploring

Geology

Third Draft

November 2006


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C H A P T E R

2 Investigating GeologicQuestions

OUR WORLD IS FULL OF GEOLOGIC MYSTERIES. Investigating these mysteries requires knowing what

to observe, how to interpret what we see, and how to analyze the problem from different viewpoints. The

investigation of geologic questions leads to new ideas and theories about Earth. This chapter explores

ways to investigate geologic questions, beginning with a mystery about the Mediterranean Sea.

This image of the Mediterranean region shows the seafloor coloredin shades of blue according to depth, with darker blue representingdeeper water. On land, satellite data show rock and sand in shades ofbrown or tan and areas with forests and grasslands in shades of green.

Trace with your finger the entire coast of the Mediterranean Sea. Wheredoes the Mediterranean Sea connect with the rest of the oceans?

Drilling from a research ship encountered thicklayers of salt within sediments at the bottom of theMediterranean Sea. Such salt layers usually form whenlarge volumes of water evaporate, such as in hot, dryclimates.

How do you get salt deposits at the bottom of a sea?

Geologists exploring for oil inNorth Africa discovered a series ofburied canyons beneath the NileRiver Valley and the sands of the

Saharan Desert.

When and how did these buriedcanyons form?

Mediterranean SeaStrait of

Gibraltar

EUROPE

Strait of Gibraltar

The Mediterranean Sea loses more water to evaporation than it receives fromthe rivers of Europe and Africa. To keep the sea full, water from the shallowlevels of the Atlantic Ocean flows through the Strait of Gibraltar. Astronautsphotographed this flow () as huge subsurface currents funneled through

the Strait and spread out into theMediterranean (toward the right). At evengreater depths, some water also flows from

the Mediterranean out to the Atlantic.

What would happen if this flow to andfrom the Atlantic Ocean was blocked?

Atlantic Ocean

Atlantic

Ocean

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Topics in this Chapter

• What Can We Observe in Landscapes? 2.1

• How Do We Interpret Geologic Clues? 2.2

• How Do We Investigate Geologic Questions? 2.3

• How Do Scientific Ideas Get Established? 2.4

• How Was the Hypothesis of Continental DriftConceived and Received? 2.5

• How Are Earth’s Surface and SubsurfaceDepicted? 2.6

• How Are Geologic Problems Quantified? 2.7

• How Do We Measure Geologic Features? 2.8

• How Do Geologists Refer to Rates and Time? 2.9

• What Are Some Strategies for ApproachingGeologic Problems? 2.10

• What Does a Geologist Do? 2.11

• Application: How Did This Crater Form? 2.12

• Investigation: What Is the GeologicHistory of Upheaval Dome? 2.13

I N V E S T I G A T I N G G E O L O G I C Q U E S T I O N S 3

When the Mediterranean Sea was a Desert

The picturesque Mediterranean Sea is 4,000 km (8,700 mi) long

and except for its connection to the Atlantic Ocean through the

Strait of Gibraltar is surrounded by land. The Mediterranean is,

on average, 1,500 m (4,900 ft) deep and holds 1.6 million cubic kilo-

meters (1.0 million cubic miles) of water.

In the 1960s and early 1970s, geologists made a series of puzzling

discoveries in the Mediterranean Sea. Surveys of the seafloor made

using sound waves revealed some unusual features, similar to those

observed for large subsurface layers of salt. To investigate these fea-tures, scientists brought in a research ship, the Glomar Challenger ,

which was capable of drilling holes in the seafloor and retrieving

samples, even from depths of several kilometers. This and later drill-

ing revealed that layers of salt hundreds to thousands of meters thick

were within the sediments on the seafloor. Surprisingly, the drill-

ing also encountered sands that show evidence of being deposited

by wind. From these and other studies arose an amazing hypothe-

sis — that in the past the Mediterranean Sea had totally dried up.

According to this hypothesis, the flow of Atlantic water into the

Mediterranean Sea was blocked six million years ago by bedrocknear the Strait of Gibraltar. The blockage occurred because of vol-

canism, uplift of bedrock by mountain building, or a world-wide

drop in sea level. As the water in the Mediterranean evaporated, it

deposited layer upon layer of salt. The large thickness of salt requires

that seawater spilled into the Mediterranean basin from the Atlantic

Ocean many times and then evaporated. After several hundred thou-

sand years, the Mediterranean Sea evaporated totally and became a

dry, hot, salt flat, similar to parts of Death Valley, but 1,500 to 3,000

m (5,000 to 10,000 ft) below sea level. Rivers draining into this new

deep basin eroded down through the land and cut canyons hundred

of meters deep. The drying of the Mediterranean Sea caused pro-

found climate changes in the region, and perhaps across much of the

planet, leading to extinction of local organisms on a massive scale.

By 5.3 million years ago, a global rise of sea level caused Atlantic

Ocean water to spill over the bedrock and cascade into the Mediter-

ranean Sea. A gigantic waterfall formed at the Strait of Gibraltar was

800 m (2,600 ft) high and is estimated to have carried 1,000 times

more water than Niagara Falls. Even with this torrent of water,

geologists calculate that it took more than 100 years to refill the

Mediterranean basin. As the Mediterranean Sea rose, adjacent riv-

ers deposited sediment, which filled and buried their recently cut

canyons. This hypothesis for the Mediterranean Sea illustrates

how an observation can lead to questions and to a possible hy-

pothesis, which can become an accepted scientific explanation

after being tested thoroughly.

About six million years ago, many species of animalsaround the Mediterranean Sea and Black Sea suddenlybecame extinct. Marine organisms that lived in the deeperparts of the Mediterranean Sea were most affected.

What dramatic changes in the environment caused thesespecies to perish?

Black Sea

Mediterranean Sea

AFRICA

ASIA

MIDDLE

EAST

Nile

River


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2 .1

I N V E S T I G AT I N G G E O L O G I C Q U E S T I O N S 2

Before You Leave This PageBe Able To

✓ Summarize or sketch the maincomponents of a relatively simplelandscape, like those shown here.

✓ Summarize the different features youcan observe in a landscape.

✓ Summarize the strategies forobserving a landscape, roadcut, orphotograph of a geologic scene.

Strategies for Observing Landscapes

Certain strategies are useful for ob-

serving natural landscapes or rock

that is exposed in a construction

cut along a road (called a roadcut).

• Begin by carefully observing the entire

landscape or roadcut, and then focus

on smaller parts, one part at a time.

• Identify the different types of features,such as layers, fractures, or loose piec-

es of rock.

• Focus on one type of feature at a time

and try to determine where the feature

is present, where it is not, and how it is

expressed in the scene.

• Examine relationships between dif-

ferent features, such as whether loose

pieces of rock are mostly below a cer-

tain cliff.

• Finally, in this book you will learn how

to use the characteristics of each rock

type to infer the environment in whichthat rock formed, and how the envi-

ronment might have changed through

time. That is, you will observe a scene

and begin to interpret its geologic his-

tory.

If we are focusingon the sequence ofrock layers in thisscene, distractionsinclude the loosebrown soil in theforeground and theshape of the hill,especially the smallridges jutting outfrom the hill. [Petri-fied Forest, Arizona]

To recognize key geologic information in a landscape, it helps to determine which parts of the scene are dis-tractions. Both photographs below have aspects that distract our view from the exposed rocks.

These rocks formcliffs separated bynarrow, snow-cov-ered slopes andledges, the ex-pression of nearlyhorizontal layers.A scenic distrac-tion is providedby trees and fallcolors. [Durango,Colorado]

Observe the photograph below and try to recognize some of the same features, such as layers and fractures,that were present in the earlier photographs. After you have made your observations, read the accompany-ing list that identifies some aspects to observe in any landscape.

Should We Pay Equal Attention to Everything in a Landscape?

What Are Some Aspects to Observe in a Landscape?

• Shapes into which the rocks have eroded

• Colors of the different rocks, including whethersome colors are a natural stain on only the out-side surfaces of the rock

• Cliffs and ledges, which generally representharder rocks, and slopes or soil-covered areas,which commonly mark materials that are moreeasily weathered

• Rock that is still a part of the bedrock (solidparts of the land) and loose pieces of weatheredor eroded sediment

• The presence of rock layers and the orientationand geometry of the layers, including whetherthe layers are horizontal, tilted, or folded

• Fractures in the rock, including their orientationand variation in spacing from place to place

• The different kinds of rocks, such as sandstoneversus granite, that appear to be present


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How Can We Infer the Environment in Which a Rock Formed?

LANDSCAPES AND ROCKS CONTAIN MANY CLUES about their geologic history. From the character-

istics of a rock, we can infer the environment in which it formed. We can also apply some simple princi-

ples to determine the age of one rock unit or geologic feature relative to another. Changes in landscapes

through time provide additional clues about how a place has changed.

How Do We Interpret Geologic Clues?

To infer how a rock formed, compare the characteristics of the rock, such as the size and roundness of its

clasts, to those of deposits in modern environments and decide which environment is the best match. Ob-

serve the characteristics of the rock in the large photograph and compare the rock with the small photos oftwo modern environments. Which of these environments is most similar to the one in which the rock formed?

Most landscapes evolve so slowly that we rarely notice any large changes in our lifetime. To get around this

limitation, geologists use a strategy called trading location for time. This strategy means observing differentparts of a landscape and mentally arranging the parts into a logical progression of how the landscape is inter-preted to have changed, or will change, through time. The approach is illustrated below using three models,

each of which might represent a real place or an interpreted stage in the evolution of a landscape.

Steep mountain fronts typi-cally contain large, angular rocksof many sizes in a matrix of mud,sand, or small rock fragments.Which environment looks mostlike the rock in the photograph?

Many river channels containlarge rounded, stones, likethese cobbles, surroundedby a matrix of sand. Note themarking pen for scale.

Erosion attacks a sequence of rock layers,carving a mountain with steep sides and abroad top. Such a flat-topped mountain iscommonly called a mesa.

With time, erosion wears away the edgesof the mesa, forming a smaller, steep-sided mountain, which geologists com-monly call a butte.

Erosion continues to strip away the terrain,producing a series of low, rounded hillsand isolated knobs. This is how a mesa orbutte might look like in the future.

or

How Can We Envision the Slow Change of Landscapes Through Time?


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✓ Describe the overall philosophy usedto infer the environment in which arock formed.

✓ Summarize or sketch what is meant by

trading location for time.

✓ Sketch or summarize four principlesused to determine the relative ages ofrocks and features.

When exploring the geology of an area, we would like to know the sequence of events that formed different

rocks and geologic features. We determine the relative ages of rocks and geologic features by using com-

mon-sense principles, including the four shown below.The youngest rocklayer is on top. Anysedimentary or volcaniclayer must be youngerthan any rock unit onwhich it is deposited.Here, the reddish layeron top is the youngest.

A younger rock caninclude pieces of anolder rock. For suchpieces to have beenincorporated into theyounger rock, thepieces had to alreadyexist.

A geologic feature is younger than any rock orfeature it crosscuts. Forexample, the fault shownmust be younger thanrock layers it crosses andoffsets. This fault is alsoyounger than formation

of the land surface.

A younger magma canbake or metamorphoseolder rocks. Hot mag-mas bake or otherwisechange pre-existingrocks with which themagma comes indirect contact, eitherunderground or on the

surface.

How Do We Determine the Sequence of Past Geologic Events?

Figure It Out!

Areally fun part of geology

is visiting a new place and

trying to figure out what

events happened and in what or-

der. This outcrop shows several

meters of an upper, tan and gray

unit that overlies a dark volcanicrock unit. Use the principles de-

scribed above to interpret which

rock layer is older: the upper

gray one or the lower black one.

Not all principles may apply.

A small canyon exposesa tan sandstone thatoverlies darker coloredmetamorphic rocks. Themetamorphic rocks hadto already be there tohave the sand depos-ited on top. The canyonformed later.

A small fault cuts acrossvolcanic layers in aroadcut, so the cross-cutting fault must beyounger than the layers.Scratch marks on therocks were made dur-ing construction of thehighway.

Tan sedimentary rocks,in the bottom half ofthe photograph, show areddish zone caused bybaking next to a magmathat solidified into thedark igneous rock in thetop of the photograph.

Weathering and erosionbroke off pieces of thegray bedrock on the low-er right, and the piecesbecame incorporated intoa younger reddish sedi-ment that was depositedon top.


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1 2 . 3

What Are Observations?

How Do We Investigate Geologic Questions?IN ANY REGION, there is a wealth of interesting geologic questions. Geologists observe Earth and its

processes through their senses and by using scientific instruments. Geologists use these observations to

ask questions and then employ a series of logical steps that build from observations to explanations.

We learn about our world by gathering observations with our senses. Scientific instruments can provideadditional information about aspects of the world that we cannot sense, or discriminate finely enough. For

example, we might sense that the temperature outside is below freezing, but a thermometer can be used tomeasure the precise value. Every day we make judgments about our observations to determine whether they

are worth remembering or are unambiguous and reliable enough to base some action on.

2. Compasses andother scientific instru-ments are checked and calibrated to make sure that they function properly and ensurethat the measurements become valid and trustworthy data. All measurements are re-

corded in a field notebook or in a portable computer and then analyzed and archived.

Data, by themselves, are not very useful, until we begin analyzing them in the context of existing ideas, per-haps identifying the need for a new interpretation of some problem. The recent history of volcanic eruptions

near Yellowstone National Park is used below to contrast data and interpretations.

1. Geologists, like otherscientists, are trainedto constantly judge thevalidity of their obser-vations, such as whenexamining these layersof volcanic ash. An ob-servation that is judgedto be valid becomes apiece of data that canbe used to developpossible explanations.

3. Evaluating the validityof observations is critical,so geologists commonlyrepeat measurements tocompare values and maybring other geologists outto the field to check anddiscuss their observationsand ideas.

DATA: The belt of smooth topographyends near Yellowstone, an active volca-

nic area in the corner of Wyoming.

DATA: This map

shows a belt of rela-tively smooth, lowerelevation terrain,outlined in red. Thebelt trends northeastacross the moun-tains of southernIdaho and northernNevada.

DATA: The ages (shown in white) ofvolcanic rocks, as measured in the lab,get younger toward the northeast,from 15 millions years in Nevada to

610,000 years near Yellowstone.

How Are Interpretations Different from Data?

INTERPRETATION: Some processformed a belt of lowtopography aftermost of the moun-tains had formed.

INTERPRETATION: Recent volcanismat Yellowstone may be related tothe process that smoothed the belt’stopography.

INTERPRETATION: The smoothed beltdid not form all at once, but is the re-sult of a continental mass moving overa deep, stationery source of magma.


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2 . 3



Explain how observations becomedata.

Describe how data differs from aninterpretation, and provide at leastone example of each.

Summarize how data andinterpretations lead to new

explanations.

Describe how a single visualobservation led to an explanation forchanging lake levels at Yellowstone.

Data Interpretation

A belt of smoothed topography

extends in a northeast direction.

The belt of smoothed topography is

younger than the mountains it cuts.

The belt ends at Yellowstone Na-

tional Park.

Volcanism at Yellowstone is probably

related to the smoothed topography.

Volcanic rocks along the belt get

younger to the northeast.

The smoothed belt didn’t form all at once

but rather sequentially.

The North American continent is

moving slowly to the southwest

based on satellite observations.

The linear belt is parallel with the move-

ment of the continent relative to a source

of magma in the underlying mantle.

A Possible Explanation

For the last 15 million years, the North Ameri-

can continent and its lithosphere have been

moving southwest over a deep thermal distur-

bance, or hot spot , that is causing melting at

depth and volcanism on the surface. As North

America moves over the hot spot, new volca-noes erupt and then become inactive once that

area moves past the hot spot. If North America

continues to move southwest, the hot spot may

cause new volcanism northeast of Yellowstone.

When geologists examine various data associated with a system, several interpretations may fit together intoa coherent story or explanation. The table below summarizes the data-interpretation pairs from Part B. The

last item in the table is a new piece of data obtained from other studies. These data and interpretations com-bine to form a possible explanation, or hypothesis, for how the belt of smooth topography formed.

Geologists have known that very large volcanic eruptions have occurred at Yellowstone. Geologist BobSmith had studied Yellowstone for decades when, in 1973, he noticed that lake levels along the south side of

Yellowstone Lake had risen and trees were being drowned. What did he do to investigate these observations?

2. This view shows Yellow-stone Lake, with north to theleft. When Smith comparedthis new survey with thelast survey done in 1920,he discovered thatthe elevation of thearea shown in red

had increased in aremarkably shortperiod of time.

3. The rising areanorth of the lake wascausing the lake tospill over its south-

ern shoreline,drowning trees

in the areasshown in light

blue.

1. Smith’s visual observation led toa new, detailed survey of the area’stopography, done with high-preci-sion surveying equipment.

The discovery of drowned trees

along Yellowstone Lake and the

follow-up investigations nicely il-

lustrate how geologists develop and in-

vestigate questions. An observation (the

drowned trees) led to the question, What

is going on here? The question led to a

possible interpretation that parts of the

land around the lake may be actively ris-

ing or sinking. To test this hypothesis,

geologists collected new observations

with surveying equipment that preciselyrecorded (within a millimeter or two) the

elevation of the land surface. Once these

new measurements were scrutinized

and validated as reliable data, a new hy-

pothesis developed — that the land be-

neath the north part of the lake had risen

and displaced water that drowned trees

along the south part of the lake.

This Yellowstone example also illus-

trates why it is useful to incorporate dif-

ferent types of data and different scales

of observation into our thinking. The

local uplift around Yellowstone Lake is

interpreted as a result of heating from a

large magma chamber beneath the sur-

face. The origin of this magma is related

to deep melting and upward movement

caused by an underlying more regionalhot spot. As North America moved over

the hot spot, a belt of smoothed topogra-

phy, the Snake River Plain, was etched

across the mountains. Many geologists

accept this explanation, but other expla-

nations are still being considered.

Observations, Interpretations, and Hypotheses

What Is an Explanation?

What Is Happening at Yellowstone National Park?


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1 2 . 4

Steps in theInvestigation

How Do Scientific Ideas Get Established?WHY DO SCIENTISTS HESITATE TO SAY that a theory can be proven? In the science of geology, expla-

nations are examined, criticized, and tested through a logical process. Those explanations that survivethis rigorous process may be promoted to the level of a theory, which is still subject to scrutiny and revi-

sion. Science is a way to evaluate which theories are likely to be correct and which ones are not.

Another explanation isthat the buried tank isnot the source of the

contamination.

Gasoline has beenfound in groundwaterfrom a local well. An oldgasoline tank is buriednearby.

Observations

QuestionsDerived fromObservations

Science proceeds as scientists explore the unknown — making observations and then systematically investi-

gating questions that arise from observations that are puzzling or unexpected. In many cases, we try to de-velop several possible explanations and then devise ways to test the various explanations. The normal stepsin this process are illustrated below using an investigation of groundwater contaminated by gasoline.

Scientists often propose several explanations, orhypotheses, to answer the question. A hypothesis isan explanation that can be tested by additional in-

vestigation or by examining data that already exist.Predictions are developed for each explanation. Ifthe tank is the source of the contamination, we pre-dict that it should have a leak or display evidence ofspillage. Also, the type of gasoline used in the tankshould match that in the groundwater. We then de-vise ways to test the predictions, such as by inspect-ing the tank or obtaining data about the chemistryof gasoline in the tank and groundwater.

The first step in an investigation is to make observa-tions, recognize a problem, and state the problemclearly and succinctly in terms of the data. Statingthe problem in this way simplifies it into a moremanageable form and helps scientists focus on themost important aspects of the problem.

Did the gasoline inthe groundwatercome from a leak inthe buried tank?

Questions arise naturally from the observations.The questions may be about what is happeningcurrently, what happened in the past or, in this case,who or what caused the problem.

When the results of the investigation are available,they are compared with the predictions to deter-mine which of the possible explanations is moreconsistent with the new data.

Any explanation that is not consistent with the

data is unlikely to be correct, so we pursue otherexplanations, such as the possibility that gasolineis coming from a nearby, underground pipeline.We can revisit the rejected explanation later if wediscover a new way by which it might be consistentwith the data.

• No holes were observedin the tank.• No gasoline spillage wasfound around the tank.• Records show that thetank held unleaded gaso-line.• Gasoline in the groundwa-ter is leaded.

The data collected duringthe investigation support theconclusion that the buriedtank is not the source of the

contamination.

How Do We Test Alternative Explanations?

ProposedExplanations

and Predic-tions fromEachExplanation

One explanation isthat the buried tankis the source of

contamination.

Results ofInvestigations

Conclusions


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2 .4

T H E N A TU R E O F G E O L O G Y 2


Explain the logical steps taken toevaluate an explanation.

Summarize how a hypothesisbecomes an established theory.

Describe what causes changes inscientific understandings.

Discuss why scientific explanations arenever proven to be “true.”

A hypothesis that survives examination and investigation can be elevated to a higher standard of acceptabil-

ity called a theory . A theory explains the existing data and can also be used to predict data that have yet to

be collected. Most theories are rather encompassing and include more than just minor ideas. Many hypoth-eses are rejected before becoming a theory. It can take years of investigation and experimentation to accu-

mulate the data needed to support a theory that will then be accepted by a majority of scientists.

How and Why Scientific Understandings Change Over Time

Geology, like other sciences, is a

way of investigating the world

around us. It is an evolving frame-work of knowledge and methods, not a

static collection of facts. Explanations

and theories that are accepted by the

geologic community can change through

time as new data, new scientific instru-

ments, and new ideas become available.

Whereas many scientific explanations

are considered to be acceptable and are

supported by many lines of evidence, the

history of science warns us not to trust

any explanation as “final truth.” Pro-

posed hypotheses or theories have too

often become unacceptable or modified

into better theories upon the finding of

new data. In many cases, the accepted

scientific explanation needs only to be

revised slightly to account for new data

or for other advances in the science.

Other examples can be cited in which

the science of the time was not sophis-

ticated enough to produce explanations

that could hold up under scrutiny. Scien-

tists operate under the principle that no

explanation in science is ever proven.

1. A hypothesis arose from theobservation that fossils of thesame land animals are found inSouth America and Africa, eventhough these continents areseparated by an ocean.

2. The hypothesis proposedthat long ridges of land calledland bridges crossed theocean, and once linked thetwo continents (as hypotheti-cally shown here). The landbridges allowed land ani-mals to walk from one con-tinent to the other. Accord-ing to the hypothesis, thebridges later collapsed orwere submerged beneaththe oceans.

3. The hypothesis predicted that the AtlanticOcean should contain submerged ridgesconnecting the two continents. When surveysof the ocean floor failed to find ridges thatcould have functioned as land bridges, thehypothesis had to be abandoned. So, we

need another way to explain the similarity offossils in South America and Africa.

4. A land-bridge hypothesis was also pro-posed as a way to explain the migration ofanimals and humans from Asia to NorthAmerica during the ice ages (not shownhere). This hypothesis, however, is sup-ported by a wide array of data and by acredible explanation of why a land bridge

could have existed. A submerged ridgeof continent does link Alaska and Asiaand would have been dry land when sealevel dropped as the glaciers grew. Thishypothesis has evolved into a theory.

There are no final answers, just logical,

well-tested explanations that are based

on the best data available.In the 1700s, for example, the domi-

nant scientists of the time could not ac-

cept that stones ( meteorites), such as the

ones shown here, fell out of the sky. For

a time, all meteor sightings and meteor-

ites were considered to be the result of

lightning that had fused dust with other

particles in the air. This explanation was

rejected when chemists noted that some

meteorites had iron-nickel alloys that

were not found in any Earth rocks. Also,

some meteorites fell in plain view when

there were no lightning storms. Stones

really were falling from the sky!

Although we have learned an amazing

amount using the methods of science,

we are very likely in the early stages of

understanding the physical universe.

There are important theories left to

imagine and countless interesting ques-

tions left to investigate. We not only lack

reasonable explanations for many scien-

tific topics, we do not yet even know the

right questions!

How Does a Hypothesis Become an Established Theory?


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SouthAmerica

Africa

Antarctica

India

A u s

t r a l i a

SouthAmerica

Africa

Antarctica

A u s

t r a l i a

India

Madagascar

O t h e r P i e c e s o f C r u s t

1 2 . 5

Is the Distribution of Fossils Consistent with Continental Drift?

How Was the Hypothesis of Continental

Drift Conceived and Received?SOME CONTINENTS HAVE MATCHING SHAPES that appear to fit together like the pieces of a giant jigsaw puzzle. Alfred Wegener (1880-1930) observed the fit of these continents and tried to explain this

and other data with a new hypothesis called continental drift. This hypothesis put forth the idea that

the continents were at one time joined together but later drifted apart. To explore how geologic explana-

tions are developed and tested, we investigate the controversy of continental drift.

In this arrangement, thebulge on the easternside of South Americafits nicely into theembayment on the west-ern coast of Africa.

Another piece of evidence supporting continental drift is the correspondence of fossils of plants and landanimals on continents that are now several thousand kilometers apart and separated by wide oceans.

This “cut-and-paste” fit of the continents is in-triguing and leads to predictions for testing thehypothesis of continental drift. Parts of con-tinents that were supposedly joined should

have similar rocks and geologic structures.Geologists find such similarities whenthey compare the rocks and structuresin southern Australia with the rocks andstructures exposed around the edges ofice sheets on Antarctica.

This figure illustrates

that fossils of someland animals exist onseveral continents thatare now separated bywide oceans. The ani-mals lived more than200 million years agoand are now extinct.They are the kinds ofland animals that could

not have swum acrossthe wide oceans thatcurrently separate thecontinents.

Fairly accurate world maps became available during the 1800s and scientists, including Alfred Wegener,

noted the apparent fit of the continents. Wegener used this apparent fit and some other observations todevelop his hypothesis of continental drift.

Other fossil data suggest that at one timeAntarctica was farther north, away from theSouth Pole. Such data include coal beds

interpreted to have formed from plantsthat grew in warm-weather swamps. Oneexplanation is that Antarctica moved to itspresent polar location after the coal formedmore than 200 million years ago.

Were the Continents Once Joined Together?

This figure shows how thesouthern continents are inter-preted to have fitted together200 million years ago. Notehow the coastlines of adjacent

continents match.

The fit of the continents and other supporting evidence pre-served in rocks and fossils inspired Wegener and others to sug-

gest that the continents, including most of India, were once joined together but then drifted apart.

The distribution of fossils is consistent with the idea

that the continents once were joined and was a keypiece of evidence in favor of continental drift. Thehypothesis of continental drift provided an alterna-tive to the hypothesis of land bridges because itsuggested that land animals moved across a singleland mass that later split into separate continents.

In this match, we included the continental

shelves, which are just a part of the conti-nent that is currently underwater.


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2 . 5

Africa

SouthAmerica

Mid-AtlanticRidge

Ice Cap


✓ Summarize the observations thatWegener used to support thehypothesis of continental drift.

✓ Summarize why the hypothesis wasnot widely accepted.

✓ List some discoveries that broughta renewed interest in the idea ofcontinental drift.

Geologists working on continents in the Southern Hemisphere were puzzled by evidence that ancient gla-

ciers had once covered places that today are close to the equator. It was a real geologic mystery.

1. This rounded outcropin South Africa has apolished and scratchedsurface that is identicalto those at the bases ofmodern glaciers.

Old and New Ideas about Continental Drift

T

he hypothesis of continental drift

received mixed reviews from

geologists and other scientists.

Geologists working in the SouthernHemisphere were intrigued by the idea

because it explained the observed simi-

larities in rocks, fossils, and geologic

structures on opposite sides of the At-

lantic Ocean. Geologists working in the

Northern Hemisphere were more skepti-

cal, in part because many had not seen

the Southern Hemisphere data for them-

selves.

We now know that Wegener (with

the evidence he gathered) was on the

right track. A crucial weakness of his

hypothesis, however, was the need for

a mechanism that allowed continents

to move. Wegener imagined that conti-

nents plowed through or over oceanic

crust in the same way that a ship plows

through the ocean. Scientists of his day,

however, could demonstrate that this

mechanism was not feasible. Continen-

tal crust is not strong enough to survive

the forces needed to move such a large

mass across such a great distance while

pushing aside oceanic crust. Because

scientists of Wegener’s time could show

with experiments and calculations that

this mechanism was unlikely, they prac-

tically abandoned the hypothesis, inspite of its other appeals. The hypothesis

probably would have been more widely

accepted if Wegener or another scientist

of that time had been able to conceive

a mechanism by which continents could

move.

In the late 1950s, the idea of drifting

continents again surfaced as new infor-

mation about the seafloor’s topography,

age, and magnetism became available.

These data showed, for the first time,

that the ocean floor had deep trenches

and long, submarine mountain belts,

such as the one () in the middle of the

Atlantic Ocean. Harry Hess and Robert

Dietz, two geologists familiar with We-

gener’s work, examined the new data on

ocean depths, as well as new data on mag-

netism of the seafloor. The magnetic data

had largely been acquired in the search

for enemy submarines during World War

II. Hess and Dietz both proposed that oce-

anic crust was spreading apart along these

mountain belts, carrying the continents

apart. This process of seafloor spreading

rekindled interest in Alfred Wegener’s idea

of continental drift.


How Did Continental Drift Explain Geologic Deposits in Unusual Places?

4. The overall directions of glacial movement inferredfrom the scratch marks made it seem as if the glaciers hadcome from the oceans, something that is not seen today.Wegener discovered that these data made more sensewhen the continents were pieced back together into alarger, ancient continent, as shown in this illustration. Ac-cording to this model, a polar ice cap was centered overSouth Africa and Antarctica, and the directions of glacial

ice movement were those

shown by the bluearrows.

2. Sedimentary rocks abovethe polished surface containan unsorted collection of clastsof various sizes. Some of theclasts have scratch marks, likethose seen near glaciers.

3. The scratch marks on the polishedsurface are used to interpret the direc-tion that glaciers moved across theland as they gouged the bedrock. Thevarious observations are most easily

interpreted as evidence that glaciersmoved across the area about 280 millionyears ago. [Kimberly, South Africa]


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1 2 . 6

How Do Maps and Satellite Images Help People Study Earth’s Surface?

MAPS AND DIAGRAMS OF THE LAND and underlying geology are essential tools for visualizing and

understanding Earth. We represent the land surface with several types of maps and with satellite images

that are informative and sometimes beautiful. We use two-dimensional and three-dimensional diagrams

to depict the subsurface geometry of rock units and how these units interact with the surface.

How Are Earth’s Surfaceand Subsurface Depicted?

Satellite images and various types of maps are the primary ways we portray the land surface and the geol-

ogy exposed at the surface. Maps of SP Crater in northern Arizona provide a particularly clear example of therelationship between geologic features and the land surface.

A shaded relief map emphasizes the shape of the land by simulating light and shadows onthe hills and valleys. The unusual hills on this map are small volcanoes called cinder cones.Much of the area is dissected by straight and curving stream valleys. The simulated lightcomes from the left of the image.

A geologic map represents the distribu-tion of rock units and geologic features onthe surface. This one shows the SP Craterlava flow and older rock units. Comparethe four maps to match specific features.

A satellite image is produced by measuring different wave-lengths of light to portray the distribution of different types

of plants, rocks, and other features.

The dark feature in the center of theimage is a black, solidified lava flowthat erupted from the base of SPCrater. SP Crater is the dark cindercone connected with the flow.

Shaded Relief Map

Topographic Map

Sa telli te Image

Geologic Map

A topographic map shows the elevation above sea level of the land surface with a seriesof lines called contours. Each contour line follows the same elevation on the surface.

Adjacent contour lines are widely spaced where the land sur-face is fairly flat (has a gentle slope).

Contour lines are more closely spaced where the land surfaceis steep, such as on the slopes of the cinder cones.

This photograph,taken from the air,shows the smallvolcano at SP Cra-ter and a dark lavaflow that eruptedfrom the base ofthe volcano.


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2 . 6


Summarize how different types ofmaps depict Earth’s surface.

Sketch or describe the types ofdiagrams geologists use to represent

the subsurface and sequences of rockunits.

Sketch or describe what is shown by aseries of evolutionary diagrams.


Sketching Geology

One of the more interesting chal-

lenges of geologic field studies

is trying to visualize how geol-

ogy exposed at the surface continues at

depth. Sketches drawn in the field while

studying the geology are an excellent

way to capture one’s thoughts while they

are still fresh and while the ideas can betested by making additional field obser-

vations. The field sketch to the right is a

simplified geologic cross section drawn

to summarize the field relationships for a

faulted sequence of rocks. Such sketch-

es are an excellent way to conceptualize

and think about geology, either in the

field or from a textbook.

Most of the planet’s geology is hidden from our view beneath Earth’s surface. We are most aware that rock

units exist where they are exposed in a mountainside or deep canyon, but such units are also present be-

neath areas of relatively flat topography. Geologic diagrams help us envision and understand the thickness-es, orientations, and subsurface distributions of rock units. Such diagrams are also one of the main ways thatgeologists document and communicate their understanding of an area.

1. A block diagram portrays in three dimensions the shape of the land surfaceand the subsurface distributions of rock units and geologic features such asfaults and folds (if present).

2. A crosssection showsthe geologyas a two-di-mensionalslice throughthe land. This

example isequivalent tothe front-leftside of theblock diagram.

3. A stratigraphic section with appropriate relativethicknesses shows therock units stacked on topof one another.

Present: Erosion of LayersIntermediate Stage: Deposition of Layer

How Do We Represent Geologic Features in the Subsurface?

Block Diagram

Stratigraphic SectionCross Section

Evolutionary Diagrams

Earliest Stage: Arrival of Sea

Evolutionary diagrams areblock diagrams or crosssections that show the his-tory of an area as a series ofsteps, proceeding from theearliest stages to the mostrecent one. Here, a tan rocklayer is deposited in the seaand later eroded.

5. One edge of thediagram (here the leftedge) typically conveysthe relative resistance of the different rock units to weatheringand erosion. A more easily weathered and eroded unit is shownrecessed, like the orange unit, whereas more resistant units areshown protruding farther out, like the two gray units.

4. The patterns withineach rock unit visually

represent the character ofthe unit, such as roundedpebbles in this orange-colored sedimentary unit.

2 7


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1 2 . 7

How Are Geologic Problems Quantified?GEOLOGISTS APPROACH PROBLEMS IN MANY WAYS. Geologists ask questions about Earth processes

and then try to collect the data that can help answer these questions. Some questions require quantita-tive data, which can be analyzed and visualized using graphs and numerical calculations.

Geologists often describe features qualitatively, but many of the data they collect are quantitative, consist-ing of numbers measured with scientific tools or instruments. The cube of rock below illustrates some of the

quantitative data with which geologists may work.

AGE : Certain rocks, including a volcaniclayer shown here, can be dated usingprecise analytical instruments that mea-sure the ratios between different types ofradioactive elements.

COMPOSITION: The abundances of

different types of grains in a rock provideinformation about what materials arepresent and the conditions under whichthe materials formed or were modified.

WATER FLOW AND QUALITY : Measure-ments can record the velocity and volumesof water flowing in rivers and in groundwater,while chemical analyses measure what thewater contains. Below, the USGS measuresflow in the Rappahannock River of Virginia.

MAGNETISM : When a rock forms, it mayrecord Earth’s magnetic field in its internalstructure. If the rock is later rotated and/ortilted, we can use the original magnetism toestimate how much its orientation and posi-

tion have changed.

VELOCITY : All rocks are in motion because

they are part of a moving plate. Somerocks are also being uplifted or dropped bydeformation. The velocity of this motion canbe measured using satellite data and othermethods.

Qualitative data are conveyed with wordsor labels. We can describe this recentlyformed volcanic unit on Augustine Vol-cano with phrases such as “contains large,

angular fragments”, “releases steam”, or“represents a dangerous situation.”

Quantitative data are numbers that rep-resent measurements. They are collectedwith scientific instruments, such as thethermal camera above, or with simple

measuring devices like a compass. Geolo-gists collect such data in the field and lab.

SPECTRA: The way that light is reflect-

ed off the surface of the rock tells usabout the composition and the surfacetexture or roughness.

What Is the Difference Between Qualitative and Quantitative Data?

What Kinds of Quantitative Data Do Geologists Use?

PHYSICAL PROPERTIES : Density,strength, and other physical propertiesof a rock, as measured in the laboratory,form the basis for modeling how rocksbehave when subjected to forces.

ORIENTATION: Geologists observeand measure the orientation of geologicfeatures, such as layers, fractures, and

folds. This geologist is using a level ona hand-held compass to determine howmuch these sedimentary layers havebeen tilted during faulting.

When Augustine Volcano in Alaskaerupts, geologists collect various types ofobservations and measurements. Someobservations are qualitative, like simple

descriptions, and others are measure-ments that are more quantitative.



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2 .7


✓ Explain how qualitative data differfrom quantitative data.

✓ Describe several types of quantitativedata that geologists use.

✓ Explain why graphs are useful.

✓ Provide an example of a geologicquestion that can be answered usingnumeric data and calculations.

How Are Graphs Useful for Answering Geologic Questions?


Numeric data are often easier to understand if they are presented graphically. Graphs allow scientists to visu-

ally interpret data sets and see relationships that might not be apparent by examining a long list of numbers.

Graphs are powerful tools for evaluating correlations, extrapolating trends, and making predictions.

How much time didpeople have before di-saster struck? A simplecalculation providesan estimate for howfast the Indian Oceantsunami wave wastraveling:

Many geologic questions involve quantitative data and calculations. How fast will contamination reach a waterwell? How much damage could an earthquake along the New Madrid Fault cause in St. Louis? How long will

Yucca Mountain protect us from the nuclear waste we store under it? The answers to such questions require usto collect data and then do calculations, as in this example from the 2004 Indian Ocean tsunami.

The December 26, 2004, Indian Ocean tsunami was caused by alarge earthquake and resulting upward movement of the seafloor.A huge area of the seafloor was displaced forming both a moundand a trough on the surface of the overlying ocean. This movementstarted the tsunami wave, which spread across the Indian Ocean.

The blue lines indicate the amount of time in hours that the tsunamitook to get to each region that it impacted. The velocity of thetsunami is related to the depth of the ocean — the wave goes fasterin deeper water.

The X value for eachdata point is plottedagainst its corre-sponding Y value.

In most graphs, the horizontal axis is called the X-axis and the vertical axis is the Y-axis. In most graphs, bothaxes start with low values near the origin (lower leftcorner of the graph) and have increasing values awayfrom the origin.

A line drawn through the data shows the average rela-

tionship between X and Y. In some cases, the line canbe used to predict the value of X for any value of Y.

Each of the graphs below provides an answer to a question a geolo-gist might ask.

Is therecalcitein thisspeci-men?

Why Types of Calculations Can Be Applied to Earth?

Aretheserocksamplesthesameage?

At whatdepthshouldwe stopmin-ing forgold?

Howmuchwatercan wepumpfromthiswell?

In the case of the Indian Ocean tsunami, which originated in about 5,000 meters ofwater, scientists estimate that the top wave velocity was 748 kilometers per hour (468miles per hour)! If you lived along the Indian Ocean within 100 kilometers of the originof the tsunami, you may have had less than 10 minutes to reach safety.

Velocity (m/s) = Square root (9.8 m/s2

* ocean depth (m))

1 2 8


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1 2 . 8

How Do We Measure Geologic Features?MEASUREMENTS ARE AN IMPORTANT PART OF SCIENCE, including geology. The physical proper-

ties of an object include length, area, volume, weight, and density. For topographic features, we measureelevation, relief, and slope. On these pages, we summarize some important units of measurements used

in geology.

The standard unit for length, width, thickness, or diameter of geologic objects is the meter . Objects smallerthan a meter are measured in centimeters, millimeters, and even smaller units. Features much larger than a

meter are measured in kilometers.

How Do We Refer to the Linear Dimensions of Geologic Features?

How Do We Refer to Differences in Topography?

For longer distances, we use the kilome-ter , which is 1,000 meters or a little lessthan 11 football fields (one km =1,093 yds). One kilometeris 0.6 miles, andone mile is1.6 kilome-ters.

An inch, a nonmetric unit, is just a littlelarger than the diameter of a U.S. quarter.

A centimeter (1/100 ofa meter) is equivalentto half the diameter ofa U.S. nickle. One inchequals 2.54 centimeters.

() Although this figure is not shown in its actual size, it illustratesthat a meter is equivalent to 39 inches, a little more than threefeet. Three feet is also called a yard .

A millimeter (1/1000 of ameter) is ap-

proximatelyequivalent tothe thicknessof a U.S. dime.

Ten centimeters is 10/100 of a meter (or 1/10 of a meter) and isequivalent to 4 inches.

The height of a feature above sea level is calledelevation. Elevation can be described in meters orkilometers above sea level, but it is also commonlygiven in feet on many older maps and signs.

We can also refer to the height of a feature above an adjacent valley.The difference in elevation of one feature relative to another is calledtopographic relief. Like elevation, relief is measured in meters or feet.

Beneath water, werefer to depth asreferenced to some

datum, like sea lev-el. We use meters for shallow depthsand kilometers fordeep ones.

The steepness of a slopecan be described indegrees from horizon-tal, such as a 30-degreeslope. We also refer togradient , such as a 70-meter drop in elevationover one kilometer (alsoexpressed as 0.070, or70 m / 1000 m).

On a larger scale, 100 meters is a little longerthan a 100-yard-long, American-style footballfield, minus the end zones

Meters Centimeters and Millimeters

Hundreds of Meters and Kilometers



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2 . 8


✓ Explain what units we use to measurelength, area, and volume.

✓ Calculate the area and volume ofsimple objects.

✓ Explain elevation, depth, andtopographic relief.

✓ Describe what weight and density areand how to calculate density.

How Do We Refer to Areas and Volumes of Geologic Features?


What Is the Difference Between Weight and Density?

In addition to linear measurements, geologists refer to areas of the land surface and to volumes of geologicmaterials, such as rock, water, and magma. The meter is the standard unit, and such measurements are metric .

The area of moder-ate-sized geologicobjects is measuredin the square meter.One square meter isequivalent to a sur-face that is one meter

long and one meter wide. It is written as 1 m2. Larger areas are

measured in square kilometers (1 km by 1 km, written as km2).

The volume of geologic objects is mea-sured in units of a cubic meter , which isequivalent to the volume of a cube 1 mlong, 1 m wide, and 1 m high. One cubicmeter is written as 1 m3.

Larger volumes are measured in the

cubic kilometer . A volume of one cubickilometer is equal to a cube 1 km by 1km by 1 km, and is written as 1 km3.

This object is 3 m long,1 m wide, and 2 m high.To calculate the volume of the object, multiplyits length times its widthtimes its height. In this

case, the volume is3 m x 1 m x 2 m = 6 m3 .Count the number ofboxes here to verify thatthis is true.

The weight of an object is how much downward

force it exerts under the pull of gravity. It dependson how much mass the object contains and wherewe measure it. A person weighing 180 pounds onEarth will weigh only 30 pounds on the Moon.

Density refers to how much mass (sub-

stance) is present in a given volume. Here,the wooden block, cube of water, and stoneblock all have the same volume but dif-ferent amounts of mass. The wood is lessdense than water and floats, but the stoneis more dense and sinks. The cube of waterhas the same density as the surroundingwater and so does not sink to the bottom orfloat on the surface.

Area

Volume

To calculate the area of a rectangular surface, you multiply thelength of the two sides. For the example above, the area is

2 m X 3 m = 6 m2 .

Calculation of Area

Calculation of Volume

Density Weight

Density is calculated using this formula:

Density = mass/volume

Mass is typically measured in grams or kilo-grams, and volumes are measured in cubiccentimeters, cubic meters, or liters. Densi-ties are written in units such as gm/cm3.Water has a density of 1 gm/cm3.

1 2 . 9


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1 2 . 9

How Do Geologists Refer to Rates and Time?TIME IS ONE OF THE MOST IMPORTANT ASPECTS of geology and, in part, makes geology different than

most other sciences. Geologists commonly investigate events that happened thousands, millions, or billions of years

in the past. Many geologic events and processes occur through very long durations of time and may occur at rates

that are so slow as to be nearly imperceptible. Geologic time needs a special language and calendar.

The geologic history of Earth is long, so geologists commonly refer to time spans in millions of years (m.y.) or

billions of years (b.y.). If we are referring to times before the present, we use the abbreviation Ma (mega-an-num) for millions of years before present and Ga (giga-annum) for billions of years before present. We also

use a special calendar to refer to the four main chapters of Earth History. Each chapter is called an era.

Geologic processes that are muchfaster than this runner include themotion of the ground during earth-quakes or the speed of an explo-sive volcanic eruption.

A runner provides a good reminder ofhow to calculate rates. A rate is howmuch something changed divided bythe amount of time required for thechange to occur.

The most recent chapter in Earth his-tory is the Cenozoic Era, which beganat 65 Ma (65 m.y. ago) and continues tothe present.

How Do We Refer to Rates of Geologic Events and Processes?

We calculate rates of geologic processes in the same way that we calculate rates such as the speed of a caror the speed of a runner. The major difference is that some geologic rates are measured in millimeters per

year or centimeters per year instead of miles per hour or kilometers per hour.

How Do We Subdivide Geologic Time?

If this runner sprinted 40 meters in 5seconds, the runner’s average speedis calculated as follows:

40 m/5 s = 8 m/s

Geologic processes that are muchslower than the runner include the

movement of groundwater, themotion of continents, and the upliftand erosion of the land surface.

The next oldest chapter is the Me-sozoic Era, which refers to the timeinterval from 251 Ma to 65 Ma. It largelycoincides with the time when dinosaurs

roamed the planet.

The Paleozoic Era started at 542 Ma,a date which marks the first record ofcreatures with shells and other hardbody parts. It ended at 251 Ma whenmany Paleozoic organisms becameextinct.

The oldest chapter in Earth’s history is

the Precambrian. It represents most ofEarth’s history and extends from 4,500Ma (that is 4.5 billion years before pres-ent) to 542 Ma, the start of the Paleo-zoic Era.

Each of the foureras represents adifferent amountof geologic time.This figure ()shows the entirePrecambrian Era,which began withthe origin of Earth

at 4.5 Ga (b.y.ago). The Precam-brian Era, whichhas early and lateparts, representsapproximately90 percent ofgeologic time,whereas theCenozoic Era

represents only1.4 percent ofgeologic time.



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2 .9


✓ Give an example of a rate and how arate is calculated.

✓ Summarize or sketch the four mainchapters (eras) of Earth history,showing which era is longest and

which era is shortest.

✓ Discuss what the geologic time scaleis and the kinds of data that wereused to construct it

What Are Some Important Times in Earth’s History?

If the entire 4.5-billion-year-long history of Earth is scaled to a singlecalendar year, the Precambrian Era takes up the first ten months and

part of November. On this calendar, Earth formed on January 1st.

On Earth’s calendar, the oldest dated rocks (about 3.9to 4.0 b.y. old) would fall in early March. The oldest

known fossils are only a little younger (in late March).

Animals having hard shells arrived at about 542 Ma. On our calendar, this isthe middle of November. This event is used to define the beginning of thePaleozoic Era, the second of Earth’s main chapters. The long time beforethe Paleozoic Era is called the Precambrian Era (shown in brown).

Earth’s final two chapters began in December. The Paleo-zoic Era ended and the Mesozoic Era started at 251 Ma inmid-December. The Mesozoic Era ended and the Ceno-zoic Era began at 65 Ma equivalent to December 26.

Fossils, Absolute Ages, and the Geologic Time Scale

M

any rocks include fossils, which

are shells, bones, leaf impres-

sions, and other evidence of

prehistoric animals and plants. Geolo-gists have discovered that the types of

fossils change from one layer to another

up or down in a sequence of sedimentary

rocks. By comparing one location to an-

other, geologist defined certain age pe-

riods on the basis of their fossils. These

age periods are named from the place

where they were first recognized, such

as the Pennsylvanian Period for Pennsyl-

vania. When arranged in their proper or-der, these periods and their subdivisions

constitute the geologic time scale.

The generalized time scale presented

here has four main divisions. The Pre-

cambrian is the time before life devel-

oped shells and other hard body parts.

The Precambrian is followed by the Pa-

leozoic, Mesozoic, and Cenozoic, from

oldest to youngest. These four divisions

of geologic time appear throughout this

book and are an essential part of the vo-

cabulary of geology.

Geologists calculate the actual, or

numeric age of rocks in thousands, mil-

lions, or billions of years before present.

Many rocks contain atoms that change by

natural radioactive decay into a different

type of atom, such as potassium (K) toargon (Ar). Calculating the numeric age

of a rock involves precisely measuring

in the laboratory the rock’s abundance

of both types of atoms. These calcula-

tions, when combined with a knowledge

of fossils, provide ages for the boundar-

ies between the divisions of the geologic

time scale. When referring to a geologic

event, we may use the name of the time period, such as the Mississippian, or the

age in millions of years before present.

The time scale is not set in stone, as

evidenced by a re-

cent change () in

the subdivisions

of the Cenozoic.

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How Does the Scale of a Problem Affect Our Approach?

What Are Some Strategies for Approaching Geologic Problems?

GEOLOGIC PROBLEMS ARE OFTEN COMPLEX. Geology is a science with unique challenges that come with

trying to study old rocks, big areas, and long intervals of time. Where do you begin an investigation? A com-

mon strategy in geology is to break a problem down into simpler parts or to focus on one aspect of it at a time.

Geologic problems come in all sizes. Some can be answered on a single hill or with one rock sample. Others

require examining the geology of an entire state or even the entire Earth. The types of data we collect andhow we think about a problem depend on the scale. Most geologic problems require using different scales

of observation. Data and interpretations gained at one scale often guide our approach to a problem at a dif-

ferent scale.

Built at the foot of steep, unstableslopes, La Conchita is a small oceansidetown in southern California. In 1988, 1995,and 2005, parts of the slopes collapsedand landslides plowed through a subdivi-sion. The medium-size scale of this prob-lem allows a few geologists to map thelandslide and suggest ways to save lives.

Geologists have recommended abandon-ing the town.

Geologists study modern processes or investigate past events by studying rocks that may record thousands,millions, or billions of years of Earth’s history. Studying the geologic past is like walking into a vast laboratory

where experiments have already occurred and then trying to reconstruct when and how these experimentstook place. The figures below show five steps in the history of a rock.

1. Deposition on asandy beach forms

layers of sandstone.

2. The sandstone iseroded, which pro-

duces clasts that aretransported away.

3. The sandstoneclasts are deposited

as part of anothersediment.

4. The sediment withthe clasts is buried

and lithified intosedimentary rock.

5. Thesedimen-tary rockis buried,heated,

and converted into a metamor-phic rock. With the rock in this

condition, it might be difficult toreconstruct the entire history.

Landslides can also be viewed at a broader scale, as in this map of landslide hazardsfor the United States. Areas shown in red, yellow, and green have significant risk for land-slides, with red areas showing the highest risk. This map required the efforts of hundredsof geologists, each mapping and studying individual landslides. Viewing landslides atthis scale is useful for allocating funds in anticipation of future landslide disasters.

Why Are Some Geologic Problems Unique?



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2 .1 0


✓ Explain why some geologic problemsare unique.

✓ Summarize an example of viewing a

problem from different scales.

✓ Explain how you could simplify acomplex problem.

✓ Summarize an example of theproblem of incomplete data.

Many geologic problems have complexities at various scales that can be somewhat overwhelming. One

strategy for addressing a complex problem is to simplify it by appropriately combining or excluding some

aspects of the problem based on the goals and purposes of the study.

If anticipating the severity of groundshaking during an earthquake caused bymovement on the fault, the most hazard-

ous areas would be near and above thefault or on loose, easily shaken sedimentalong the river.

If exploring for petroleum, we may onlyneed to consider the sedimentary rocks,the most common rocks in which petro-

leum is found.

This terrain has a simple landscapebut moderately complex geology inthe subsurface.

The Problem of Incomplete Data

How does groundwater flow?

Where are potentialearthquake hazards?

Where would we ex-plore for petroleum?

A n important problem that plagues

many geologic investigations is

working with incomplete data.

When reconstructing the geology of a

mountain range, it is impossible to walkover and examine every exposure of

rock and every square meter of ground.

A field geologist must make many gen-

eralizations, such as this rock type is

the same as the one observed 20 meters

away. We rarely have as much data as

we would like or have time to collect.

The problem of incomplete data is il-

lustrated in the figure to the right, which

shows two water wells drilled near eachother. One well is in a sandstone that

contains abundant water between the

sand grains, but the other well inter-

cepted an unseen granite having water

Successful well Poor well

If evaluating the flow of groundwater, theterrain can be simplified into three unitseach having certain properties that con-

trol the movement of groundwater. Thefault could also affect groundwater flow.

A fault has offset all therock units except the toplayer of river gravels.. Different questions can be addressed by considering

only the aspects that pertain to that question.

The sedimentary rocks overlie theolder metamorphic and igneousrocks and are different thicknesseson the two sides of the fault.

There are four types of materials: a thinupper layer of reddish-brown sedi-ment, a tan-colored sedimentary unitwith cobbles, a gray metamorphic unit,and some pinkish igneous rocks.

Are There Ways to Simplify and Conceptualize Complex Geologic Problems?

only in its fractures. Both wells encoun-

tered water at the same depth, but the

well drilled into granite went dry when

pumped because there was so little wa-

ter in the fractures. Although from thesurface and the initial water levels the

two wells seemed comparable, the well

in granite was later abandoned — anoth-

er victim of incomplete data. For this

situation, we would need to know the

geometry of the rocks in the subsurface

and those characteristics of the rocks

that influence the flow of groundwater.

The number of fractures in the granite

would be relevant information, but the

age of the granite is probably not.

A thin layer of sediments, includ-ing some loose sediment recentlydeposited next to the river, is at thesurface.

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GEOLOGISTS ADDRESS A DIVERSE RANGE OF PROBLEMS relevant to our society. There are many

types of geologists, each with interesting questions to explore. Some questions can be explored directlyin the field, whereas others require sophisticated computers or other technology to study places that are

inaccessible, such as other planets or the interior of Earth. Some questions address active processes,

and others address the interpretation of ancient processes from rocks, structures, and fossils.

What Does a Geologist Do?

During their fieldstudies, geologistsobserve various as-pect of the naturalenvironment, re-cord these observa-tions, and proposeexplanations forwhat they discover.A common goal isto understand thearea’s geologic pro-cesses and history.Sometimes, justgetting to the fieldsite is an outdooradventure. [Antarc-tica]

The traditional view of a geologist is a person outdoors with backpack, hiking boots, and rock hammer travers-

ing a scenic mountain ridge. From field studies, geologists produce descriptions, maps, and other data need-

ed to reconstruct a piece of Earth’s history and determine how this history affects how and where we live.

Many field geologists construct a geologic map by identifyingrock types in the field, finding the boundaries between adjacentrock units, and drawing these on a map. The geologic map be-comes the main basis for making interpretations about subsurfacegeology and the geologic history of the area. This geologic mapshows rock layers and river deposits near St. Paul, Minnesota.

Geologists help find most of the energy and mineral resourceson which we depend. They conduct field studies to find areas

favorable for a certain resource and to determine through drillingthe location and size of the resource. Energy and mineral com-panies have traditionally employed many geologists and havetaken their employees to many parts of the world, such as 3 km(10,000 ft) below the ground in a South African gold mine.

Some field geologists study volcanoes, earthquakes, land-

slides, floods, or other natural hazards. To help us avoid thesehazards, geologists study the processes that are operating,determine how often hazardous events occur, and map areasthat are most likely to be affected. This geologist is extractingsamples of lava on the big island of Hawaii.

How Do Geologists Investigate Questions in the Field?


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What Would You Observe at the Crater?

A CRATER ON THE PLATEAU OF NORTHERN ARIZONA is more like those on the Moon than any nearby fea-

ture. The crater is a huge pit, more than 1,250 m (4,100 ft) across and 170 m (560 ft) deep, with a raisedrim of broken rocks. Some dramatic geologic event must have occurred here. What could it be? We ex-

plore this mystery to see how competing explanations for the origin of a geologic feature are evaluated.

Observe the photographs below and think about what your observations might imply about how the crater

formed. What other types of information would you like to have to understand the crater’s origin?

The raised rim of the craterconsists of huge, angular blocks oflimestone and sandstone, many ofwhich are fractured and shattered.Because they are pieces of the rocklayers exposed in the carter walls,these blocks are interpreted to havebeen thrown out of the carter.

This geologic block diagram shows thegeometry of rock layers through the cra-ter. A geologist constructed this crosssection on the front sideby observing therocks exposed inthe crater and in thesurrounding plains, byexamining results fromdrilling in the floor of thecrater, and by extrapolatingthese observations into thesubsurface.

Application: How Did This Crater Form?

Rocks in the walls ofthe crater contain unusualminerals that can beseen using a microscope.Laboratory experimentsshow that these miner-als can form only underextremely high pressures.

The layers of sandstone and limestone are nearlyhorizontal away from the crater, but the layers alongthe rim of the crater have been peeled back and arenow locally completely upside down.

What Are Some Ways to Show the Geologic Setting of this Crater?Numerous meteorites are scattered

on the surface around the crater.Geologists have collected many

small and large pieces tounderstand what informationthe meteorites provide aboutthe origin of the crater andabout the solar system.



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✓ Summarize or sketch the threeexplanations for the origin of thecrater and which observations supportor do not support each explanation.

✓ Summarize how geologists interpretMeteor Crater to have formed.

Predictions fromEach Explanation

Of the three explanations, only the

meteorite-impact explanation re-

mains viable because the crater

does not contain the rock types predict-ed by the other two explanations. Other

types of data also support an origin by

meteorite impact. Laboratory experi-

ments indicate that the unusual minerals

in the walls of the crater can form only

at very high pressures, such as when

a shock wave from an impact passes

through rocks. These experiments also

show that rock layers near the surface

can be peeled back and shattered by thecollision. Computer models of meteorite

impacts predict that large meteorites,

such as the one required to form this cra-

ter, are mostly vaporized by the impact.

Therefore, little of the meteorite would

remain in the crater, but scattered pieces

might survive around the crater. It is es-

timated that 30 tons of meteorite speci-mens have been collected around the

site. The accepted origin for this crater

is reflected in its famous name, Meteor

Crater .

From the presently available informa-

tion, geologists conclude that the crater

formed about 50,000 years ago when

a meteor 30 to 50 meters in diameter

smashed onto the surface at a speed of

about 11 km/s (25,000 miles/hr). A recentstudy suggests that the meteor broke

apart and slowed before it hit. Humans

were not yet living in this area to observe

the fireball blazing across the sky, but

animals, such as the woolly mammoth,

were around to witness this unexpected

catastrophe.

Since the crater was discovered, geologists have proposed and tested several explanations for its origin.

Among these explanations are a volcanic explosion, warping by a rising mass of salt, and excavation by a

meteorite impact.

An Incident 50,000 Years Ago

Volcanic materials, suchas ash, should be presentaround the crater and solidi-fied magma might underliethe crater floor.

A mass of salt should existdirectly beneath the floor of thecrater.

Fragments of a meteorite mightremain beneath the crater or onthe surrounding plains. Much ofthe meteorite could have beenvaporized during the blast.

No volcanic material wasfound around the crater orby drilling beneath the crater,so this explanation seemsimprobable.

No salt was found beneath thecrater, despite extensive drillinginto the crater floor.

Drilling did not encounter alarge mass of meteorite be-neath the crater, but numeroussmall meteorite fragments arescattered across the surround-ing plains.

A volcanic explosion blastsopen the crater during aviolent eruption of gas andmagma.

The rock layers are upwarpedby a rising mass of salt. Rain-water later dissolves the salt toform the crater.

A large meteor streaks throughEarth’s atmosphere and blaststhe crater when it collides withthe surface.

What Are Some Possible Explanations for the Origin of the Crater?

Volcanic ExplosionWarping by a Rising

Mass of SaltMeteorite Impact

PossibleExplanation

Results of Testing

Each Explanation

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What Are Some Observations and Questions About the Dome?

A PERPLEXING GEOLOGIC FEATURE CALLED UPHEAVAL DOME is located in Canyonlands National

Park of Utah. In most of Canyonlands, the colorful sedimentary rocks are nearly horizontal, but around

Upheaval Dome they are abruptly warped upward and eroded into a very unusual circular feature.

Investigation: What Is the GeologicHistory of Upheaval Dome?

• Make some observations and develop some questions about Upheaval Dome.

• Determine the sequence of geologic events that formed the rock layers in the dome.

• Suggest some ways to test possible explanations for the origin of the dome.

Goals of This Exercise

The rocks shown in this image are all sedi-mentary and of Mesozoic age. The rock layersaway from the dome are nearly horizontal andform various benches and flat-topped mesas.

The three-dimensional perspective below shows an unusual circular feature called Upheaval Dome. Makesome observations about this landscape, and record your observations on a sheet of paper or on the work-

sheet provided to you by your instructor. Also record any questions that you have as you study this land-scape.

The rock layers in the dome aretilted outward in all directions.



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E

DC

B A

ED

F and G

SN

Procedures

1. For each of the three explanations, draw a simple sketch in the worksheet illustrating which types of rocks you predict to find atdepth.

2. List a prediction that follows from each explanation. Then, explain how that prediction could be tested.

3. List which types of information you would like to know about this location to further constrain the origin of the dome.

The origin of Upheaval Dome is controversial, and three competing explanations currently are being debat-

ed.

Shown below are (1) a stratigraphic section with the sequence of rock layers and (2) a geologic cross section

across the dome. Using these two figures and the strategies in Section 2.2, determine the order in which the

layers were formed, and write your answers on the worksheet.

Units A, B, and C have a Jurassic age, whereasunits D and E are Triassic. Units F and G arePermian (see time scale in Chapter 2.2).

Many faults and folds (notshown here) have thick-ened the rock layers insome places and thinnedthem in others.

This is the cen-

ter of the dome.

Unit A: upper sandstone

Unit B: mudstone withthin sandstone layers

Unit C: middle sandstone

Unit D: mudstone withpebble-rich layers

Unit E: lower mudstone

Unit F: lower sandstone

Unit G: sandstone with

abundant pebbles

The letters A–G mark the units shown in the stratigraphic section to the left. Theletters are assigned in order from top to bottom, not in the order in which theunits formed. Some units, such as B and D, contain a series of related sedimen-tary rock layers rather than just a single type of rock.

What Sequence of Geologic Events Formed the Rocks and the Dome?

How Would You Test Possible Explanations for the Origin of the Dome?

The age of the dome is poorly constrained. The dome is younger than all of the rock layers in the vicinity. When the dome formed,the presently exposed rocks were buried several kilometers deep beneath overlying rock layers that have since been eroded away.This erosion, therefore, removed some key evidence for the origin of the dome.

1. The rock layers were warped upward by a rising mass of salt. A thick salt layer is known to be present beneath much of theCanyonlands region, and the salt would have risen upward because it is less dense than the surrounding rocks.

2. The dome formed as a result of rising magma. Igneous rocks formed from such magmas are common elsewhere in the region,where they have bowed up and baked the surrounding rock layers.

3. The dome is part of a larger, circular crater formed by a meteorite impact. Many of the larger meteorite-impact craters on theMoon and elsewhere have a central peak, or dome, which is interpreted to form by converging shock waves.

Cross SectionStratigraphic Section

All of the rock units havebeen folded into thedome. Any layers de-posited after the domewas formed have beeneroded away.

It is uncertainwhat types ofrocks lie at depthbelow the dome(i.e., below units Fand G).

Chapter 2 Exploring Geology

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