Geologic Systems

8/11/2019 Geologic Systems

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2 Geologic SystemsE arth is a dyna mic planet beca use the materials of its various layers are in motion. The ef-

fects of bot h the hydrologic and the tectonic systems are drama tically expressed in this

space photogra ph of eastern No rth America. The most obvious motion is that of the surface

fluids: air and wa ter.The complex cycle by which wa ter moves from the oceans into the at-

mosphere, to the land, and ba ck to the oceans again is the fundamental movement within

the hydrolo gic system. The energy source that drives this system is the Sun. Its energy evap-

orates water from t he oceans and causes the atmosphere to circulate, as shown above bythe swirling clouds of hurricane D ennis. Water va por is carried by t he circulating at mos-

phere and eventually condenses to fall a s rain or snow, which gravity pulls back to E arths

surface. Still acted on by the force of gravity, the water then flows ba ck to the oceans in sev-

eral subsystems (rivers, groundwater, and glaciers). In every case, gravity causes the water

to flow from higher to low er levels.

E arths lithosphere may a ppear to be permanent and stationary, but like the hydro-

sphere, it is in constant mot ion, albeit much, much slower. There is now overw helming evi-

dence that t he entire lithosphere moves, and as it do es, continents split and the fragments

drift thousands of kilometers across Earths surface perchance to collide with one another.

The great Appala chian Mounta in chain, visible here as a series of para llel ridges and val-


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leys, formed w hen two continents collided hundreds of millions of years ago. The folded

and crumpled rock layers formed a high mountain belt that wa s slowly eroded aw ay by

streams. Subsequently, the margin of North America formed a s it rifted aw ay from Africa.

In fa ct, all of the structural feat ures of our planet a re the result of a simple system of mov-

ing lithospheric plates. Movement in this plate tectonic system is driven by the loss of inter-

nal heat energy.The concept of a natura l system, as developed for the study of geology, provides a frame-

work for understanding how each part of E arth works and why it is constantly changing. G eo-

logic systems are governed by natural laws that provide the keys to understanding Earth and

all of its varied landscapes and processes.

In this chapter, we will consider the fundamenta ls of na tural systems. We also explore

the ba sics of the hydro logic system and t he tectonic system as the ultimate causes of

geologic change.

Im age provided by O rbimage; O rbital Imaging Corporation

AppalachianMountainsGreat Lakes

CanadianShield

Florida

Bahamas


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GEOLOGIC SYSTEMS

A system is a group of interdependent materials that interact with energy to

form a unified whole. Most geologic systems are open; that is, they can

exchange matter and energy across their boundaries.

As w e emphasized in the preceding chapter, the world is a unified whole. Nothing

in or on it exists as an isolat ed entity. E verything is interconnected. We may know

myriad details about the many separate items found on or inside Earth, but most

of us do not understand how the pieces are interrelated a nd fit t ogether. Without

some concept of how the world functions as a who le, we easily miss important re-

lationships between seemingly isolat ed phenomena, such as the critical connec-

tions among rainfall, temperature, and landslides. To understand E arth a nd how it

functions, we need a model or framework,a plan or map that shows how things are

interrelated and how things operate. Such a framework is provided by the con-

cept of the system.There are many different kinds of systems. You are undoubted ly familiar with

many nat ural and a rtificial systems. An engineer may think of a system as a group

of interacting devices that w ork together to a ccomplish a specific task. In yourhome,t here is an electrical system, a plumbing system, and a heating system. E ach

functions as an independent unit in some way. E ach transfers material or energy

from one place to anot her,a nd each has a driving force that makes the system op-

erate. In a nother example, a biologist conceives of a similar kind of system, but it

is one composed of separa te organs ma de of living tissues that w ork together. The

circulatory system is composed of the heart, blood vessels, and o ther organs that

together move blood through the body.

In the physical sciences, we speak of systems in very general terms; a system is

that pa rt of space in which we are interested. The space may conta in various ma-

terials acted on by energy in different wa ys. B y defining a system, we identify the

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1. A natural system is a group of interdependent components that interact toform a unified w hole and a re under the influence of relat ed forces. The ma-

terials in a system change in an effort to reach and maintain equilibrium.

2. E arths system of moving water, the hydrologic system, involves the move-ment of wat erin rivers, as groundwater, in glaciers, in oceans,a nd as wa ter

vapor in the atmosphere.A s water moves, it erodes, transports, and deposits

sediment, creating distinctive landf orms and rock bod ies.

3. R adiat ion from the Sun is the source of energy for E arths hydrologic system.4. A system of moving lithospheric platesthe plate tectonic systemexplainsE arths major structural features. It o perates from Ea rths internal heat.

5. Where plates move apart, hot mat erial from the mantle wells up to fill thevoid and crea tes new lithosphere. The major features formed where plates

spread a part are continental rifts, oceanic ridges, and new ocean basins.

6. Where plates converge, one slides benea th the other a nd plunges dow n intothe mantle. The major features formed a t convergent plate margins are fold-

ed mountain belts, volcanic arcs, and deep-sea trenches.

7. Where plat es slip horizonta lly past one anot her, transfo rm plate bounda riesdevelop on long, straight fa ults. Shallow earthq uakes are common.

8. Far from plate m argins, plumes of less-dense mantle ma terial rise to shallowlevels, feeding within-plate volcano es and producing minor flexures of the

lithosphere.9. E arths crust floats on the denser mantle benea th. The crust rises and sinks

in attempts to maintain isostatic equilibrium.

MAJ OR CONCEPTS

What is a geologic system?


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G e o l o g i c S y s t e m s 3

extent of the material being considered and the energy involved so tha t w e can

more clearly understand any changes.

In ea ch of these cases, a system is composed of individua l items or components

that work together to ma ke a unified whole. In a ccomplishing specific tasks, ma-

terial and energy move about a nd change from one form to another. Such a sys-

tem is dynamic, in motion, rather than stat ic or unchanging.A natural system is a bit more complicated than a typical engineering system.

For example, a geologic system may have real bounda ries, such as the top and bot-

tom of a flowing stream of water or the walls of a body of molten rock (Figure2.1). Or it ma y have a rbitrary bo undaries defined for the specific purpose of study.

E verything outside of the systems boundaries is the surroundings or environment

and is not considered part of the system.

E arths systems obviously are so broad in scope, and cover so many phe-

nomena in the natural world, that w e should be careful about how we use the term.

Two types of systems are important in geology: (1) a closed systemexchanges onlyheat (no matter); and (2) an open systemexchanges both heat and matter with itssurroundings. In a closed system, such as a cooling lava flow, heat is lost, but

new material is neither added nor lost (Figure 2.1A). H owever, most geologic

systems are open systems, in which matter a nd energy freely flow a cross the sys-

tems boundaries. A river system, for example (Figure 2.1B ), gains water from

springs, snowmelt, and rainfall as it flows toward the ocean.

E arth itself is a system. It is a sphere of matter w ith distinct boundaries. E arth

has been an essentially closed system since the end of the heavy meteorite bom-

bardment some 4 billion years ago. Since then, no significant new ma terial has en-

tered the system (except meteorites and space dust), and, just as important, sig-

nificant q uantities have not left the system. Since the planet formed, however, its

mat erials have experienced tremendo us change. Sola r energy enters this nearly

closed system and causes matter (air and water) to move and flow in distinctive pat-

terns. H eat energy from within E arth a lso causes motion resulting in earthq uakes,

volcanism, and shifting continents. Thus, a space photograph of our planetary ho me

seen as a w hole is a powerful image of a natural systeman image that imparts a

sense of the oneness in a na tural system.

On a much smaller scale, a river and a ll its branching tributaries is a natura l sys-

tem. The floor of t he stream and t he upper surface of the flowing wa ter form someof its boundaries. Mat ter enters this system from the atmosphere as rain, snow, or

groundwa ter and t hen flows through the river channel and leaves the system as it

enters the sea. As long as rain falls, the system will be supplied with matter, gravi-

tat ional potential energy, and kinetic energy. The ultimate energy source for a river

system is energy from the Sun. Its energy heats water in the ocean, evaporat ing it

What is a dynamic system? Can yougive an example?

FIGURE 2.1 Natural systemsmay closed or open.

(A)A closed system,such as a cooling lava

flow, exchanges only radiant heat. Here, heat

from the lava is lost to the atmosphere.

(B)Open systems, such as a river, exchange energy

and matter. In a river,wa ter and sediment are

collected from the drainage a rea and flow through

the system to the sea. Most geologic systems are

open systems.


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Why is Planet Earth considered to be anatural system?

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and lifting it into the atmo sphere, and tra nsporting it to the continents. The force

of gravity causes the water to flow d ownslope to the sea.

Most o ther geologic systems are complex open systems like river systems. One

type of complexity results from subsystems. For example, a river system is only

part of the much larger hydrologic system tha t includes all possible paths of world-

wide wa ter movement.A tmospheric circulation of w ater va por is another impor-

tant subsystem of the hydrologic system. Oceanic currents are another; glaciers

and groundwa ter are others. E ach is a subset of the overall circulation of w ater and

energy a t E arths surface.

DIRECT ION OF CHANGE IN GEOLOGIC SYSTEMS

In a ll natura l systems change occurs in the direction necessary t o estab lish

and mainta in equilibriuma cond ition of the low est possible energy.

The very essence of Earths geologic systems is the flow of energy and the

movement of matter. As a result , materials on and in Earth are changed or

rearranged. Yet, this change does not occur at random. It occurs in a definite,

predictable way. B y carefully examining a system, we can see how one component

is connected to ano ther in an invisible web. The individual threads of t his network

are so tightly interdependent that a change in any component, even a small

change, causes change in the rest of the system. Predicting and understanding

these changes is an importa nt reason fo r using the system approach.

What determines the direction of change in a dynamic geologic system? For

example, does water flow do wnhill or uphill? D oes hot air rise or sink? Although

the answ ers to these questions seem self-evident, they seem simple only because

of your experience with na tural systems driven by gravity. You have thousand s of

experiences each da y tha t reveal ma ny of t he principles of gravity. These experi-

ences allow you to predict what will happen in many different situations.

H owever, because you lack experience with other natural systems, there are

many q uestions regarding direction of change tha t are more difficult for you to an-

swer. For example, under what conditions of temperature or pressure does one

mineral convert to different mineral? At what temperature does rock melt? O rwater f reeze? Why does heat flow from one rock to a nother or from one region to

anot her? When will solid rock break to cause an earthqua ke? In short, how can we

predict the direction of change in any natural system?

Most of these q uestions can b e answered, or a t least better understood, because

of one very simple principle. Cha nges in natural systems have a universal tenden-

cy to move toward a state of equilibriuma condition of the lowest possible ener-gy. This pattern holds for the landscape, earthq uakes, volcanoes,f lowing water, and

many other geologic phenomena. This governing principle has been clearly estab -

lished through painstaking experimentat ion by thousands of scientists working over

several centuries. Thus, if we ca n deduce which of several possible conditions is low-

est in energy, we can predict the direction of change in a nat ural system.

Anot her way to t hink abo ut equilibrium is to consider it a condition in which

the net result of the forces acting on a system is zero. It is a state of no permanentchange in any characteristic of the system. Systems not in equilibrium tend to

change in a direction to rea ch equilibrium. To better understand this idea, think

of tw o boulders on a hillside. One sits high on the side of the hill and has much

gravitational potentia l energy. Another sits on the valley floor and has very little

gravitationa l potential energy.Which boulder is more likely to cha nge its position?

Ob viously, only a small perturbation could send the first boulder rolling down

the hillside. B ut any force exerted on the second boulder would cause only a mod-

est and temporary change in position, and it w ould then roll back to its original

position. The second bo ulder has low gravitational potential energy and is at an

equilibrium position.


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Now, imagine a third bo ulder that sits in a slight depression on the hillside. It is

in a metastableposition.A very small force would be insufficient to change its po-sition permanent ly, but a larger force wo uld push it over the brink and allow it to

crash down the hill to a stable position.

A hot lava flow cools for similar reasons. It loses heat energy to its surround-

ings in order to reach equilibrium with its environment. If a change upsets this

equilibrium, the system will natura lly change in such a direction as to reestablish

equilibrium under the new conditions.

In all such transforma tions, some energy is lost to the environment, gener-ally as heat. Of ten, the lost heat energy is no longer ava ilable to cause change.

A fundamental natural law holds that any system tends to run down, mean-

ing that it grad ually loses energy of the sort that can cause change.

If you look carefully at a geologic system, you should be able to identify its

equilibrium state. For example, what would be t he equilibrium landscape formed

by a river system? The state that would provide the very least gravitational po-

tential agency would be one of flatness. Thus, a perfectly flat landscape with no

hills, ridges, or valleys would be the equilibrium landscape. Of course, that stat e may

never be perfectly achieved, because of the inability of erosion to keep up with

other cha nges imposed on the river system.

In summary, the tota l energy of a system must decrease for a sponta neous change

to occur. The change will proceed until equilibrium is atta ined and the energy is at a

minimum. The most stable state is always the one w ith the lowest energy. In other

words,all materials attempt to a chieve a ba lance with the chemical and physical forces

exerted upon them, and they will change to arrive eventually at eq uilibrium. This ef-

fort results in progressive changes in any planeta ry material that is exposed to a n en-

vironment different from tha t in which it formed. Although this equilibrium state is

the preferred stat e of all systems, there are many intermediate or metastable states,

adding to the complex problem of understanding Earths dynamic systems.

Systems, Equilibrium, and Geology

The dual concepts of systems and eq uilibrium, as developed for the study of geolo-

gy, provide a framework for understanding how each part of E arth works and w hy it

is constantly changing. Ord er can be seen in all scales of time and space. Nothing is

random. Everything, from a grain of sand on a beach to a lake, mountain range, orcanyon, is there because it was fo rmed in a systematic way by a n organized interac-

tion of ma tter and energy. D ynamic geologic systems are governed by na tural laws,

which provide the keys to understanding Ea rth and all its landscapes and processes.

The major geologic systems are the hyd rologic system and t he tectonic system.

Perhaps no ot her place on E arth illustrates the operation of these two grand sys-

tems as well a s the Middle E ast (Figure 2.2). This satellite photo graph ma y at first

appear to be a chaotic jumble of colors and textures, but careful examination shows

that every feature is a product of these two dynamic systems. Ca refully read the

figure caption to understand this important point.

THE HYDROLOGIC SYSTEM

The hydro logic system is the complex cycle through w hich wa ter moves

from the oceans, to the atmosphere, over the land, and ba ck to the oceans

aga in. Water in the hydrologic systemmoving as surface runoff, ground-

water, glaciers, waves, and currentserodes, transports, and deposits

surface rock material.

The complex motion of Earths surface waterthe hydrologic systemoperateson a global scale. It unites all possible paths of wa ter into a single, grand system of

motion. The term hydrologic is rooted in the G reek term hydorfor w ater. The

What is equilibrium in a naturalsystem?


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FIGURE 2.2 Earths geologic systemsare evident in this space photo graph. The effects of the hydrologic system are revealed by river systems,even in this desert region of the Middle E ast. The Sinai P eninsula (right page) is etched by delicate networks of stream valleys, which disappear into

the sandy desert along the shores of the Mediterranea n Sea. E lsewhere, stream erosion has etched out the fractures in the rocks, but the drainage

patterns are ma sked by wind-blown sand. The Nile River is flanked by farmlands (red). The Nile carries a tremendous volume of sediment to t he sea,

where it is deposited in a huge delta (da rk red because of vegetation). The Nile River and its delta a re a dra matic expression of the hydrologic system

as running water erodes the highlands of central Africa and transports the sediment to the sea. An a dditional expression of the hyd rologic system is the

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wave a ction along the delta front tha t reworks the sediment brought to the sea by the Nile and redeposits it as beaches and barrier bars.I n this arid

region, linear windblown sand d unes have developed on either side of the Nile D elta. The tectonic system is expressed by the rift o f the R ed Sea a nd

the fracture system extending northward up the G ulf of Aqa ba and into the D ead SeaJorda n R iver valley. The Arabian P eninsula is moving to the

northeast and, as it splits and moves away from Af rica, a new ocean b asin (the Red Sea) is born. The movement of tectonic plates is a clear expressio

of the fundamental dyna mics of E arths interior. (Cour tesy of Earth Satellite Corp oration)

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36 C h a p t e r 2

basic elements of the system can be seen from space (Figure 2.3) and a re dia-

grammed in Figure 2.4. The system operates as energy from the Sun hea ts wa ter

in the oceans, the principal reservoir for E arths water. As it is heated, the water

evaporates. Most of the wa ter vapor condenses and returns directly to the oceansas rain. Atmo spheric circulation carries the rest over the continents, where it is

precipitated as rain, sleet, hail, or snow.

Water tha t falls on the land can take a va riety of paths. The greatest quantity re-

turns to the atmosphere by evaporat ion, but the most visible return is to the oceans

by surface runoff in river systems, which funnel wa ter back to the oceans. Some wa ter

also seeps into the ground a nd moves slowly through pore spaces in the soil and rocks,

where it is available to plants. Pa rt of the w ater is used by the plants, which exhale

it into the atmo sphere, but much of it slowly seeps into streams and lakes. In polar re-

gions, or in high mountains, wa ter can be temporarily trapped on a continent as glacial

ice, but the glacial ice gradually moves from cold centers of accumulation into warmer

area s, where melting occurs and the wa ter returns to the oceans as surface runoff.

In short, wa ter in the hydrologic system is constantly moving as vapor, rain,

snow, surface runoff, groundwater, and glaciers, or even in ocean waves and cur-rents.As it moves across the surface, it erodes and transports rock material and then

deposits it as deltas, beaches, and other accumulations of sediment. C onsequent-

ly, the surface materials, as well as the water, are in motionmotion that results

in a continuously changing land scape.

One o f the best wa ys to gain an accurate conception of the magnitude of the hy-

drologic system is to study space photogra phy. These photographs provide a view of

the system in operation on a globa l scale. A tra veler arriving from space would see

that the surface of E arth is predominantly wa ter (Figure 2.3).The movement of w ater

from the oceans to the a tmosphere is expressed in the flow patterns of the clouds.The

atmosphere and moving cloud cover are among the most distinctive features of E arth

FIGURE 2.3 The hydrosphere,thethin film of wa ter that makes Planet Earth

unique, is essential for life.E arth is just the

right distance from the Sun for wa ter to

exist as a liquid,solid,a nd gas. If it were

closer to the Sun, our oceans wouldevaporate;if it were farther from the Sun,

the oceans would freeze solid.D rawing

energy from the Sun, it moves in great

cycles from the o ceans to the a tmosphere

and over the landscape in river systems,

ultimately returning to the oceans. (Image

provided by Orbim age, O rbital Imaging

Corporation)

Hurricanes


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What are the major components of

the hydrologic system and how do theyoperate?

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A tmosphere-O cean System. E arths oceans are vast reservoirs of liquid wat er

that together with the gases in the atmosphere create the climate system.Circulation in these envelopes of fluid is driven by heat from the S un. The uneven

heating of E arths surface causes the atmo sphere to convect, winds to blow, causes

evaporation of huge quantities of wa ter vapor into the atmosphere, and drives

ocean currents. In addition, variations in this convection system sets up a regular

pattern for the distribution of precipitation a nd temperature around t he entire

globe. Thus, the climate is controlled by the mat erials and energy in his system. In

turn, the climate controls how the hydrologic system operates in a local a rea.

R i ver Systems. Most water precipitated onto the land returns directly to the

oceans through surface drainage systemsriver systems.The amount o f wa ter inE arths rivers appears vast, but in fa ct it is startlingly small; it is only abo ut 0.0001%

of the to tal wa ter on E arth, or 0.005% of the wa ter not in the oceans. Water flows

through rivers very rapidly, at a n average rate of 3 m per second. At this rate, wa ter

can tra vel through the entire length of a long river in a few weeks. This means that,

although the volume of the wa ter in rivers at any given time is small, the tota l

volume passing through river systems in a given period ca n be enormous. As a

result, most of the landscape is dominated by fea tures formed by running wat er.

From viewpoints on the ground, we cannot a ppreciate the prevalence of stream

channels on the surface of Ea rth. From space,how ever, we readily see that stream

valleys are the most abundant landf orms on the continents. In a rid regions, where

vegetation and soil cover do not obscure our view, the intricate netw ork of stream

valleys is most impressive (Figure 2.5). Most o f the surfa ce of every cont inent is

somehow related to the slope of a stream valley, which collects and f unnels surface

runoff toward the ocean.

Ano ther important a spect of a river system is tha t it provides the fluid medium

that t ransports huge amounts of sand, silt, and mud to the ocea ns. These sediments

form the great deltas of the world, which are records of the amount of material

wa shed off t he continents by rivers. The Nile D elta is a classic example (Figure

2.2). The Nile River is confined to a single channel far upstream from C airo. It

then splits into a series of bra nching channels, from w hich the sediment carried

by the river is eventually deposited a s new land in the M editerranean Sea. The

main channels slowly shift their courses back and forth a cross the delta, and theolder extensions of the delta are eroded back by ocean waves and currents.

FIGURE 2.5 River systemsare clearrecords of ho w t he hydrologic system sculpts

the land. They testify to the magnitude of

this vast interconnected system of moving

water, for few a reas on land are untouched

by stream erosion. In this photogra ph of a

desert region, details of the delicate network

of tributaries are clearly shown. On the

Moon, Mercury, and Mars, craters dominate

the landscape, but on the continents of

Ea rth, stream valleys are the most abundant

landforms. (Courtesy of N A SA )

Hydrologic System


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Glacial Systems. In cold climates, precipitation fa lls as snow, most of which

remains frozen and d oes not return immediately to the ocean as surface runoff. If

more snow fa lls each year tha n melts during the summer months, huge bodies of

ice build up to fo rm glaciers (Figure 2.6). La rge valley glaciers originate fro m

snowfall in high mountains and slowly flow down valleys as rivers of ice. Glaciersystemsgreatly modify the norma l hydrologic system because the wa ter that fallsupon the land does not return immediately to the ocean as surface runoff. It is not

until the glaciers melt at their lower end that w ater flow s back to the sea, seeps into

the ground, or evaporates.

At present, the continent of Anta rctica is almost entirely covered with a conti-

nental glacier, a sheet o f ice from 2.0 to 2.5 km thick. It covers an area of 13 million

km2an area larger than the U nited States and Mexico combined. An ice sheet

similar to that now on Antarctica covered a large part of North America and E u-rope during the last ice age, and it retreated only within the last 18,000 years. As the

ice moved, it modified the landscape by creating many lakes and other landf orms

in Cana da a nd the northern U nited States, including the G reat La kes.

Wat er in the form of ice constitutes abo ut 80% of the wa ter not in the o ceans,

or a bout 2% of E arths total w aterfar more than is in our streams and rivers.

Water in glaciers moves very slowly a nd may remain in a glacier for tho usands or

even millions of yea rs. P resent estimates suggest that w ater resides in a glacier for

ab out 10,000 years on avera ge.

G rou ndwat er Systems. Another segment of the hydrologic system is the

groundwater systemthe water that seeps into the ground and moves slowlythrough the pore spaces in soil and rocks. Surprisingly, ab out 20% of the wa ter

not in the oceans occurs as groundwater. As it slowly moves, groundwa terdissolves soluble rocks (such as limestone) and creates caverns and caves that can

enlarge a nd collapse to form surface depressions called sinkholes. This type of

dissolution-generated landform is common in Kentucky, Florida, Indiana, and

western Texas and is easily recognized from t he a ir (Figure 2.7). Sinkholes

commonly create a pockmarked surface somewhat resembling the cratered

surface of the Moon. They may also become filled with water and f orm a series

of circular lakes.

Shorel i ne Systems. The hydro logic system also o perates in shoreline systemsalong the shores of all continents, islands, and inland lakes through the unceasing

FIGURE 2.6 Valley glaciers, such asthese in Alaska, occur where more snow

accumulates each yea r than is melted in th

summer. Over many years, this cycle allows

accumulating snow to build the glaciers.

Valley glaciers originate in the snowfields o

high mountain ranges and slowly flow as

large tongues of ice down preexisting strea

valleys. The moving ice is a powerful agent

erosion and mo difies the valleys in which i

flows.The dark lines on the glaciers are roc

debris derived from the valley wa lls.

(Cour tesy of U.S. D epartment of Agricultur

Why are glaciers considered to be par

of the hydrologic system?


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FIGURE 2.7 Groundwateris a largely

invisible part o f the hydrologic systembecause it occupies small pore spaces in the

soil and rocks beneath the surface. It can,

however, dissolve soluble rocks, such as

limestone, to form complex networks of

caves and subterranean passagewa ys. As the

caverns enlarge, their roofs may collapse,

forming circular depressions called

sinkholes.The hundred s of lakes shown in

this false-color photograph o f the area west

of Cape Canaveral, Florida, occupy

sinkholes and testify to the effectiveness of

groundwa ter as a geologic agent. (Courtesy

of U.S. D epartment of Agricultur e)

40 C h a p t e r 2

work of wa ves. The oceans and lakes are bo dies of mobile water subject to a va rietyof movementswaves, tides, and currents. All of these movements erode the coast

and transport vast quantities of sediment (for example, the Nile D elta in Figure

2.2).The effects of shoreline processes are seen in w ave-cut cliffs, shoreline terraces,

deltas, beaches, bars, and lagoons.

Eo li an (Wi nd) Systems. The hydrologic system also operates in the arid regions

of the w orld. In ma ny deserts, river valleys are still the dominant landf orm. There is

no completely dry spot on E arth. E ven in the most arid regions some rain falls, and

climate patterns change over the years. R iver valleys can be obliterated, however,b y

dunes of wind-blown sand that cover parts of the desert landscape (Figure 2.2).

The circulat ion of t he at mosphere forms the eolian system.Wind can t ransportenormous quantities of loose sand and d ust, leaving a distinctive record of the

winds activity. In the broadest sense, the wind itself is part of the hydrologic sys-tem, a moving fluid on t he planets surface.

THE TECT ONIC SYSTEM

The tectonic system involves the movement of the lithosphere, which is

broken into a mosaic of separa te plates. These plates move independently,

separating, colliding, and sliding past one another. The margins of the

plates are sites of considera ble geologic activity, such as seafloor spread ing,

continental rifting, mountain building, volcanism, and ea rthquakes.

G eologists have long recognized that E arth ha s its own source of internal energy.It is repeatedly manifested by earthq uakes, volcanic activity, and folded mountain

belts. B ut it was not until the middle 1960s that a unifying theory d eveloped to ex-

plain Ea rths dynamics. This theory, known as plate tectonics,provides a master planof E arths internal dyna mics. The term tectonics, like the related word architec-

ture, comes from the G reek tektonikos and refers to building or construction. In

geology, tectonics is the study of t he formation a nd deformation o f E arths crust

that results in large-scale fea tures.

Evidence for this revolutionary theory of lithospheric movement comes from

many sources. It includes data on the structure,t opography, and magnetic patterns

of the ocean floor; the locations of earthquakes; the patterns of heat flow in the

Plate Tectonic System


14/24


crust; the locations of volcanic activity; the structure and geographic fit of the con-

tinents; and t he nature and history of mountain belts.

The basic elements of the tectonic system are simple and can be easily understood

by carefully studying Figure 2.8. The lithosphere, which includes E arths crust a nd

part of the upper mantle, is rigid, but the underlying asthenosphere slowly flows.

A funda mental tenet of plate tectonics is that the segments, or plates,of the rigidlithosphere are in constant motion relative to one another and carry the lighter

continents with them.

Pla tes of oceanic lithosphere form as hot mantle material rises along mid-

oceanic ridges; they a re consumed in subduction zones, where one converging plate

plunges dow nwa rd into the hotter ma ntle below (Figure 2.8). The descent of these

plates is marked by deep-sea trenches that border island a rcs and some continents.

Where plates slide by one another, large fractures form. The movement and colli-sion of plates accounts for most of E arths earthquakes, volcanoes, and folded

mounta in belts, as well as for the drift of its continents.

From the standpoint of E arths dynamics, the boundaries of plates are where the

action is. As seen in Figure 2.9, plate bo undaries do not necessarily coincide with

continenta l boundaries, altho ugh some do. There are seven very large plates and

a do zen or more small plates (not all of which are shown in Figure 2.9). E ach plate

is as much as a few hundred kilometers thick. P lates slide over the more mo bile as-

thenosphere below, generally at rates between 1 and 10 cm per year. B ecause the

plates are internally quite rigid, they become most deformed a long their edges.

The ba sic source of energy for tecto nic movement is believed to be E arths in-

ternal heat, which is transferred by convection. In a simple model of Ea rths con-vecting interior, hot ma ntle material rises to the lithospheres base, where it then

moves latera lly, cools, and eventually descends to become reheated, continuing

the cycle. A fa miliar example of convection can be seen while heating a pot of

soup (Figure 2.10). H eat a pplied to the b ase of the pot wa rms the soup at the bot-

tom, which therefore expands a nd becomes less dense.This warm fluid rises to the

top a nd is forced to move lat erally while it cools. Co nsequently, it becomes denser

and sinks, setting up a continuing cycle of convection.

Major Subsystems of the Tectonic System

Many features of the ocean basins and continents can be nicely explained by the

plate tectonic system.We will consider many of them in detail in subsequent chapters, but

What is the source of energy for thetectonic system?

Oceanic ridge

Ocean floor

South America

Lithosphere

Mesosphere(Lower mantle)

Plume

Africa

Trench

Outercore

(Mantle and crust)

Asthenosphere(Upper mantle)

FIGURE 2.8 The tectonic systemispowered by E arths internal heat. The

asthenosphere is more plastic than either th

overlying lithosphere or the underlying

lower mantle.Above the plastic

asthenosphere, relatively cool and rigid

lithospheric plates split and move a part a s

single mechanical units (along the o cean

ridges). As this happens, molten rock from

the asthenosphere wells up to fill the vo id

between the lithospheric plates and thus

creates new lithosphere.Very slow

convection occurs in the mantle. Some plat

contain blocks of thick, lower-density

continental crust, which cannot sink into thdenser mantle.A s a result, where a plate

carrying continental crust collides with

another plate, the continental margins are

deformed into mountain ranges. Plate

margins are the most active areas on Eart h

the sites of the mo st intense volcanism,

seismic activity, and crustal deforma tion.

Locally, convection in the deep mantle

creates rising mantle plumes. (A fter P. J.

Wyllie)


15/24

42 C h a p t e r 2

let us look briefly at the major features of the planet and how they fit into the

tectonic system.The different types of plate boundaries are, in effect, subsystems

of the tectonic system. E ach creates specific geologic phenomena. We have

illustrated each boundary type with an example from the continents.

D ivergent Pl ate Boundari es. The plates move apart at divergent plateboundaries,which coincide w ith mido ceanic ridges (Figure 2.9 and Figure 2.12).H ot molten material from the deeper mantle wells up to fill the void. Some of

this material erupts on the seafloor as lava . The molten rock solidifies and forms

new lithosphere. The midoceanic ridges stand high because their materia l is hot

and, therefore, less dense than t he colder adjacent oceanic crust.

The most intense volcanism on E arth occurs at divergent plate bound aries, but

it is largely concealed below sea level. When oceanic earthqua ke locations are

plotted on a ma p, they outline with drama tic clarity the divergent plate bound-

aries (Figure 2.11). Most o f these a re shallow ea rthquakes,q uite unlike those found

where plat es converge.

Most divergent boundaries occur on the seafloor, but continental rifts alsodevelop where divergent bounda ries form on the continents. Such a continental rift

eventually creates a new o cean ba sin. The great rift of the R ed Sea (Figure 2.2)

displays many feat ures of a continental rift. The R ed Sea is an extension of the

midoceanic ridge of the I ndian O cean, which splits the Sinai and A rabian penin-

sulas from Africa. Take the time to locate the a rea shown in this remarkable

photogra ph (Figure 2.2) on the topogra phic map on the inside covers of this book.

The structure of the a rea is dominated by the long, linear fa ult valley that forms

the north end of the Red Sea and G ulf of Suez. Note the sharp contrast where

faults have juxtaposed young, light-colored sediments against the ancient shields

as this region is slowly ripping asunder. New seafloo r is forming on the floor o f the

Convergent plate boundaries Divergent plate boundaries

Antarctic plate

Nazcaplate

Pacific plate

Philippineplate

China

subplate

Eurasian plate

AustralianIndianplate

Cocosplate

North American

plate

Juan deFuca plate

SouthAmerican

plate

Caribbeanplate

Scotia plate

Africanplate

Somaliansubplate

Arabianplate

Eurasian plate

Transform plate boundaries

FIGURE 2.9 A mosaic of platesforms E arths lithosphere, or outer shell.The plates are rigid, and ea ch moves as a single unit.Thereare three types of plate bo undaries: (1) the axis of the oceanic ridge, where the plates are diverging and new o ceanic crust is generated (red

lines); (2) transform faults, where the plates slide past each other (the short lines slicing across the divergent boundaries); and (3)

subduction zones, where the plates are converging and one descends into the a sthenosphere (blue lines).

What are the major subsystems of thetectonic system and how do theyoperate?


16/24


R ed Sea . This rift expresses dra matica lly the tensional stresses in the lithosphere

and the wa y these stresses affect E arths surface.

Transform Pl ate Boundari es. The oceanic ridges are commonly broken a nd

offset a long lines perpendicular to the ridges. These offsets are large f aults

expressed by their own high ridges a nd deep va lleys. Transform plate boundariesoccur where plates horizontally slide past one another (Figure 2.13). Sha llow

earthq uakes are common along all transform bounda ries (Figure 2.11), but

volcanic eruptions a re uncommon.

Most transform plate boundaries are on the seafloor, but the best-known ex-

ample of this type of fa ult on a continent is the great Sa n Andreas Fault system inC alifornia (Figure 2.13).The fa ult zone is marked by sha rp linear landfo rms, such

as straight and narrow valleys, straight and narrow ridges, and of fset stream valleys.

The San Andreas Fault system is an active boundary between the Pacific plate to

the west and the North American plate t o the ea st. The Pa cific plate is moving at

abo ut 6 cm per year, relative to the North American plate. As stress builds be-

tween the plates, the rock bodies deform until they break. This sudden release

along the fault causes earthqua kes like those so common in California. Another

FIGURE 2.10 Convection in themantlecan be compared to convection in a

pot of soup. Hea t from below causes the

material to expand and thus become less

dense. The wa rm mat erial rises by

convection and spreads laterally. It then

cools, and thus becomes denser,a nd sinks.

is reheated as it descends, and the cycle is

repeated.

Hot

Heat Source

High heat flowOcean

Cooling Cooling

Cool Cool

Earthquakes Active volcanoesDivergent boundary Convergent boundary Transform boundary

FIGURE 2.11 Earthquakes and active volcanoesoutline plate margins with remarka ble fidelity. At divergent plate boundaries,shallow earthq uakes, submarine volcanic eruptions, and tensional fractures occur.Transform bounda ries have shallow ea rthquakes

but generally lack active volcanoes.Alo ng convergent margins, there are deep earthqua kes, volcanic eruptions, trenches on the

seafloor, and folded mo untain belts. Isolated area s of volcanism and earthq uakes reveal the locations of active mantle plumes.

Convection


17/24

FIGURE 2.12 The Mid-Atlantic Ridgeisa divergent plate boundary a nd marks the

spot where new lithosphere is forming and

where two plates are separating.The North

American plate is slowly mo ving west and

Africa on the E urasian plate is moving east.

Earthquakes and volcanoes are

concentrated along the crest of the ridge.

Transform fa ults cut the ridge and offset it.

(Cour tesy of K en Perry, Chalk B utte, In c.)

44 C h a p t e r 2

transform boundary cuts the Asian continent from the G ulf of Aq aba to the D ead

Sea a nd creat es a va lley obvious from space (Figure 2.2).

Convergent Pl ate Boundar ies. Plates move toward o ne another along convergentplate boundaries.Along such plate margins, geologic activity is far more variedand complicated than a t transform plate bo undaries (Figure 2.14). Intense

compression ultimat ely rumples the lithosphere and b uilds high folded mounta in

belts. P reexisting rocks become altered by hea t and pressure.The net result is the

growth o f continents. Where two plat es converge, one tips down a nd slides beneath

the other in a process known a s subduction.It is clear tha t earthq uakes and volcanoes drama tically outline convergent

plat e ma rgins (Figure 2.11).The simplest form o f convergence involves two plat es

with ocea nic crust. Such subduction zones in the western and northern P acific re-

gion lie along the volcanic islands of Tonga, the Ma rianas, and the Aleutians.Trench-

es form where the d owngoing plate plunges into the mantle. These are long, nar-

row troughs, normally 5 to 8 km deep, and a re the lowest features on E arth. As a

plate of lithosphere slips into the mantle, it becomes heated and d ehydrated.

Some rock material melts, becomes less dense, and rises, and some erupts to

form a string of volcanic islands called an island arc .

If the oceanic plate dives beneath a continent, the molten rock may form a

chain of volcanoes on the continental margin; the Ca scades of C alifornia-Ore-

gon-Washington a re an exa mple. The remarka ble series of deep-sea trenches

and a ssociated volcanic arcs make the ring of fire that a lmost surrounds theP acific O cean (Figure 2.11).

As each subducting plate grinds its way downw ard, earthqua kes are produced.

The deepest of all earthq uakes, almost 700 km deep,occur at convergent plate bound-

aries. Pla te tectonics can thus readily explain why the Andes mountains of South

America are tormented by repeated volcanic eruptions and earthquakes (Figure

2.14). They a re forming w here two tectonic plates converge. The same is true for the

western coasts of Central America. It is equally clear that the ea rthquakes and vol-

canic eruptions in the Mediterranean area occur at a convergent plate margin.

Where moving plates converge, the rocks in the crust may also become d e-

formed. The crust in continents and in island a rcs is buoya nt (it is less dense than


18/24


oceanic crust) a nd resists subduction b ack into the dense ma ntle. Consequently, this

kind of crust becomes intensely compressed and folded at some convergent plat e

margins. The structures of the And es Mounta ins (Figure 2.14) of So uth America

vividly express this type of def orma tion. The complex system of ridges and va l-

leys in the eastern And es is produced by fo lded sedimentary rock layers deformed

by the collision of tw o plates. The folded layers now a ppear like wrinkles in a rug.

The Appala chians were formed in a similar manner (Figure 1.10).

A younger mountain belt that extends from Alaska through the R ockies and

C entral America a nd into the Andes of South America wa s produced by t he en-

counter of the American plates with the P acific, C ocos, and Nazca plates.This is a

geologically young mountain system, with many parts still being deformed a s the

plates continue to move.

Wi thi n-Pl ate Tectonics and M antl e Pl umes. Within the moving plates, the

continenta l and ocea nic crust experience little tectonic or volcanic activity a s they

move awa y from midoceanic ridges. H owever, plumesof hot ro ck rising from thedeep mant le (Figure 2.8) may crea te isolated vo lcanoes and gent ly warp the interior

of a plate.An excellent example is the Ha waiian I sland chain in the P acific Ocean

(Figure 2.15). The huge volcanoes and geysers of Yellowsto ne Nat ional P ark inwestern North America may a lso overlie a mantle plume. E arthqua kes related to

the volcano es in these areas are also common (Figure 2.11), but large deep

earthq uakes are rarely felt in such within-plate regions.

Plates and Plate Motion

Take a mo ment to study Figure 2.9 aga in. You will want t o become very fa miliar

with this map b ecause it shows a new geographythe geography of tectonic plates.

As you have seen, most of E arths major features can be understood from the

interactions of these plates in the tectonic system.

North American Plate

FIGURE 2.13 The San Andreas Faultsystemin California is part of a long

transform plate boundary that separates th

North American plate from t he Pa cific plat

It connects a divergent boundary in the G u

of California with the Mendocino transform

fault and the Juan de Fuca ridge. At least a

dozen major fa ult systems can be seen as

linear mountain trends. Movement along th

San Andreas Fault is horizontal; that is, one

block of E arths crust slides laterally past

another. (Cour tesy of K en Perry, Chalk Bu t

Inc.)


19/24


20/24

FIGURE 2.14 The Andes Mountainswere formed by the subduction of the Nazca plate beneath South America at a convergent plate margin.La yers of sedimentary rock, which were originally horizontal, have been elevated and compressed into folds that w ere subsequently eroded. The

resistant la yers appear as ridges in the eastern Andes. Folded mountain belts such as the Andes are one o f the mo st significant results of converging

plates, but if you look carefully you can a lso see the relatively smooth volcanic plains and isolated volcanic cones that show the ro le played by

volcanism at convergent plat e margins. (Cour tesy of K en Perry, Chalk B utte, In c.)

Volcanoes

Volcanoes


In Figure 2.9, you can see tha t seven major lithospheric plates are recognized

the North American, South American, Pa cific, Australian,A frican, Eurasian, and

Anta rctic platestogether with several smaller ones. Let us ta ke a b rief tour, so

you will know w hat to look for.

1. The divergent plate bounda ries are marked b y oceanic ridges, which extendfrom the Arctic south through the central Atlantic and into the Indian a nd P a-

cific oceans.Movement of the plates is away from the crest of the oceanic ridge.

2. The North American and South American plates are moving westwa rd a ndinteracting with the Pa cific, Juan de Fuca, Co cos, and Nazca plates along the

west coast of the Americas.

3. The Pacific plate is moving northwestward away from the oceanic ridge to-ward a system of deep trenches in the western Pacific basin.

4. The Australian plate includes Australia, India, and the northeastern Indi-

an O cean. It is moving northward, causing India to collide with the rest ofAsia to produce the high Himalaya Mountain ranges and the volcanic arc

of I ndonesia.

5. The African plate includes the continent of A frica, plus the southeastern At-lantic and western Indian oceans. It is moving northward a nd colliding with

the E urasian plate.

6. The Eurasian plate, which consists of E urope and most of Asia, moves eastward.7. The Antarctic plate includes the continent of Anta rctica, plus the floor of the

Anta rctic Ocean. It is unique in that it is nearly surrounded by oceanic ridges.

What are the major plates and how arthey moving?

Earth Systems


21/24

GRAVITY AND ISOSTASY

G ravity plays a fundamental role in Earths dynamics. It is intimately

involved with differentiatio n of the planets interior, isostatic adjustments

of the crusts elevation, plate tectonics, and dow nward flow of wa ter in the

hydrologic system.

G ravity is one of the great fundamental forces in the universe. It played a vitalrole in the formation of the solar system, the origin of the planets, and the impact

of meteorites that dominated their early history. Since then, gravity has been a

constant f orce in every phase of planetary dynamics, and it is a dominant fa ctor in

all geologic processes operating on and within Ea rthglaciers, rivers, wind, and

even volcano es.

G ravity also operates on a much grander scale within E arths crust. It causes

lighter (less dense) portions, such as continents, to stand higher than the rocks of

the heavier, denser ocean floor. Similarly, the loading of Ea rths crust at one place

with thick sediment in a river delta, or with glacial ice, or with wa ter in a deep lake

will cause that region to subside. Conversely, the removal of rock from a mountain

range by erosion will lighten the load, causing the deep crust to move upward to ta ke

its place. This gravita tional a djustment of E arths crust is isostasy (G reek isos,

equal ; stasis, standing). E arths lithosphere therefore continuously respondsto the force of gravity a s it tries to maintain a gravita tional ba lance.

Isostasy occurs because the crust is more buoyant than the denser mantle be-

neath it. E ach portion of the crust displaces the mant le according to its thickness

and d ensity (Figure 2.16). D enser crustal materia l sinks deeper into the mantle

than does less-dense crustal material. Alterna tively, thicker crustal material will

extend to greater depth tha n thin crust of the same density. Isosta tic adjustments

in Ea rths crust can be compared t o ad justments in a sheet of ice floating on a la ke

as you skate on it. The layer of ice bends down beneath yo u, displacing a volume

of wa ter with a weight equal to your weight.A s you move ahead, the ice rebounds

behind you, and the displaced wa ter flows back.

How is isostasy a reflection of an equi-

librium state?

FIGURE 2.15 The Hawaiian Islandsformed far from a ny plate boundary and are thought to lie above a plume of hotmaterial rising through the mantle.A s the lithosphere slowly moves northeast, it carries the older volcanoes awa y from the

hotspot. Volcanoes are still active on the large southern island of H awa ii, but not on the more eroded Maui, and other

islands to the no rtheast where the a ction of the hydro logic system is dominant. (Cour tesy of K en Perry, Chalk B utte, Inc.)

48 C h a p t e r 2


22/24


How do we know that isostatic adjustments occur?

As a result of isostatic adjustment, high mountain belts and plateaus are com-

monly underlain by thicker crust tha t extends deeper into the ma ntle than do areas

of low elevation. Any t hickness change in an area of the crustsuch as the re-

moval of material by erosion or the addition of material by sedimentation, vol-

canic eruption, or a ccumulations of large continental glacierscauses an

isostatic adjustment.The construction of H oover D am, on the C olorado R iver, is a well-documented

illustration of isostatic ad justment. The added weight of wa ter and sediment in the

reservoir was sufficient to cause measurable subsidence. From the time of the dams

construction in 1935,24 billion metric tons of w ater, plus an unknown amount of sed-

iment, accumulated in Lake Mead, behind the dam. In a mat ter of years, this added

weight caused the crust to subside in a roughly circular a rea a round the lake. Con-

tinental glaciers are a nother clear example of isostat ic adjustment of the crust. The

weight of an ice sheet several thousand meters thick disrupts the crustal bala nce and

depresses the crust beneath. In both Anta rctica and G reenland, the weight of the

ice has depressed the central pa rt of the land ma sses below sea level. A similar iso-

static ad justment occurred in E urope and No rth America during the last ice age,

when continental glaciers existed there.Parts of both continents, such as Hudson B ay

and the B altic Sea, are still below sea level. Now that the ice is gone, however, thecrust is rebo unding a t a rat e of 5 to 10 m/1000 yr.

Tilted shorelines of a ncient lakes provide another mea ns of documenting isostat -

ic rebound. Lake B onneville, for example, was a large lake in Uta h and Nevada dur-

ing the ice age but has since dried up, leaving such small remnants as U tah L ake and

G reat Salt L ake. Shorelines of Lake B onneville were level when they were formed

but have been tilted in response to unloading as the water was removed.

The concept of isostasy, therefore, is fundamenta l to studies of the crusts major

featurescontinents, ocean ba sins, and mountain ranges. It also is fundamental to

understanding the response of the crust to erosion, sedimentation, glaciation,

and the tectonic system.

(B)High mounta ins in low-density crust are b alanced by a deep

root tha t extends into the mantle.

(A)Low -density blocks float on a denser liquid. If the blocks

have eq ual densities, the thicker blocks rise higher and sink

deeper than the thinner blocks.

(C)Floating blocks of uneq ual density. The block with denser

(green) portions sinks and the surface is lower tha n the a djacent

blocks, even though the th ickness is the same.

Crust

Mantle

Denseblock

Dense rocks

Mantle

Crust

(D)A deep ba sin may form if the rocks beneath it a re denser

(green) than surrounding rocks (brown).

FIGURE 2.16 Isostasyis the universal tendency of segments of E arths crust to establish a condition of gravitationa l balance.D ifferences in bot h thickness and density can cause isostatic adjustments in E arths crust.


23/24

A view of planet Earth from space gives us a truly global

view of t he geologic systems that shape t he planet.

Observations

1.The continents and ocean b asins are Ea rths most promi-

nent fea tures.

2.The planets wa ter is seen in the vast blue oceans a nd the

white pola r ice caps. Water cycles through the a tmosphere

as shown b y the bright sw irling clouds.

3. C limate zones are expressed as regular patterns in the

distribution of green vegetation on land, of the amount

and shapes of clouds in the atmosphere, and ice at the

poles.

Interpretations

Two major geologic systems shape the E art hthe hydro-

logic system a nd the t ectonic system. The tectonic system

created t he lithosphere with its huge ocean b asins and high

continental platforms, which are underlain by rocks of

different compositions, structures, and ages. Locally, new

ocean ba sins are forming where continents are rifting apa rt

as shown by the separation of Africa from Arabia. E lse-

where, plates are colliding to form new continents and

mounta in belts like the one ba rely visible in southern Ira n.

The hydrologic system endlessly modifies the surface fea-

tures of the lithosphere with rain, wind, waves, flowingwat er, and ice.The role played b y the climate in the opera-

tion of the hyd rologic system is clear. In the cloud-spotted

tropics, air heated by the Sun rises in vertical convection

cells making abundant rainfall and fueling the growth of

vegetation in the b iosphere. Cloud-free deserts lie north a nd

south of the tropics. At the south pole, the cold climate ha s

created the vast Anta rctic glacier. C yclonic storms, which

appear a s clouds resembling huge commas, show the pre-

vailing wind patterns generated b y solar radiation a nd by

Earths rotation.

Tropics

Climate zones

Deserts

Temperate

Polar

30

20

0

20

30

60

FoldedMtnsAfrica

Antarctica

50

GeoLogic Earths Systems from Space

Courtesy of NA SA


24/24


KEY TERMS

climate system (p. 38)

closed system (p. 31)

convection (p. 41)

convergent plate boundary

(p. 44)

divergent plate boundary

(p. 42)

dyna mic system (p. 31)

eolian system (p. 40)

equilibrium (p. 32)

glacier system (p. 39)

groundwa ter system (p. 39)

hydrologic system (p. 33)

isostasy (p. 48)

metasta ble (p. 33)

open system (p. 31)

plate (p. 41)

plate t ectonics (p. 40)

plume (p. 45)

river system (p. 38)

shoreline system (p. 39)

subduction (p. 44)

system (p. 30)

transform plate boundary

(p. 43)

REVIEW QUESTIONS

1. Consider the gravitational interactions among E arth, theSun, and the Moo n. D oes this constitute a system? If so,

wha t are its bounda ries? Is it open or closed? What fo rms

of energy are involved?

2. D iagram the paths by which water circulates in the hydro-logic system.

3. What energy drives the hyd rologic system?4. Approximately how much wa ter evaporates from the ocean

each year?5. D escribe the ma jor landforms resulting from (a) rivers,

(b) groundwater, (c) glaciers, and (d) wind.

6. D raw a diagra m (cross section) showing (a) convergingplates and (b) diverging plates.

7. On a map such as the one in Figure 2.9, identify the threefundament al kinds of plate bound aries.

8. What surface fea tures distinguish each kind of plate boundary?

9. E xplain how the Alps, midocean ridges, deep-sea trenchesisland a rcs, and volcano es are related to plate tectonics.

10. D escribe the geologic processes that occur above a m antleplume.

11. Why do the ma terials inside E arth convect?12. Make a list of the ma ny roles played by gravity in geologic

systems.

13. E xplain isostasy, and give tw o examples of isostat ic adjust-ment of Earths crust in recent geologic time.

ADDITIONAL READINGS

Fothergill,A., Holmes, M.,Attenborough, D. , B yatt ,A . 2002.

The B lue Planet. New York: D K P ublishing.

Condie, K. C. 1998. Plate Tectonics and Crustal E vol ution, 4th ed.

New York:B utterworth-Heinemann.

Cox,A. , and R. B. Hart . 1986. Plate Tectonics: H ow I t Work s.Palo Alto, Calif. : Blackwell.

H allam,A . 1973. A Revolution i n the Sciences: From Continental

D ri ft to P late Tectonics. New York: Oxford U niversity P ress.

Kearey, P., and F. J.Vine. 1996. Global Tectoni cs, 2nd ed. Oxford:

Blackwell.

Leopold, L. B., and K. S.D avis. 1980. Water. Alexandria,Va.:

Time-Life B oo ks.

Peixit, J. P., and M. A. Kettani. 1973. The control of the water

cycle. Scientifi c A merican.

U.S. G eological Survey. 1990. National Water SummaryHyd ro

logi c Events and Water Suppl y and Use. U.S. G eological

Survey Water S upply Paper 2350, Washington, D.C .: U.S.G overnment P rinting O ffice.

Van Andel, T. H . 1994. New Views on an O ld Pl anet:A H istory o

Gl obal Change, 2nd ed. Ca mbridge:C ambridge U niversity

Press

Ear ths D ynami c Systems Websi te

The C ompanion Website a t www.prenhall.com/hamblinprovides you with an on-line study guide and a ddition-

al resources for each chapter, including:

On-line Quizzes (Chapter Review, Visualizing G eology,

Quick Review,Vocabulary Flash Cards) with instant feedback

Quantita t ive Problems

Critical Thinking Exercises

Web Resources

Ear ths Dynami c Systems CD

Examine the CD that came with your text.It is designto help you visualize and t hus understand the concep

in this chapter. It includes:

Animations of plate tectonics

Slide shows with examples of the action of the hydrologic

system and t ectonic system

An interactive map that allows you to explore Ea rths geolo

A direct link to the Companion Website

MULTIMEDIA TOOLS

Geologic Systems

Documents