Ch 13

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Title Page Photo

“Come forth into the light of things, Let Nature be your teacher.”—William Wordsworth

Vocabulary• asthenosphere (p. 388)• basalt (p. 392)• contact metamorphism (p. 398)• crust (p. 387)• external (geomorphic) processes• (p. 404)• geomorphology (p. 402)• granite (p. 392)• igneous rock (p. 391)• inner core (p. 388)• internal (geomorphic) processes• (p. 404)• landform (p. 402)• lava (p. 391)• lithosphere (p. 388)• magma (p. 391)• mantle (p. 388)• metamorphic rock (p. 398)

• mineral (p. 389)• Mohoroviˇci´c discontinuity (Moho)• (p. 387)• outcrop (p. 391)• outer core (p. 388)• plutonic (intrusive) igneous rock• (p. 392)• pyroclastics (p. 391)• relief (p. 403)• rock (p. 391)• rock cycle (p. 400)• sedimentary rock (p. 394)• silicate (silicate minerals) (p. 389)• strata (p. 396)• topography (p. 401)• uniformitarianism (p. 404)• volcanic (extrusive) igneous rock• (p. 392)

The Structure of Earth• Humans have not penetrated more than one-

thousandth of Earth radius.• Have inferential knowledge of interior, through

monitoring shock waves transmitted through Earth from earthquakes or from human-made explosions.– Knowledge is incomplete.– Deduced that it has a heavy inner core surrounded by

three concentric layers of various composition and density.

• Four regions are the crust, mantle, outer core, and inner core

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The Structure of Earth• Introduction

– Core• Inner core• Outer core

– Mantle– Crust

– Fig. 13-1

The Crust• Crust—outermost solid layer

of Earth, consisting of broad mixture of rock types.– On average, crust three times as

thick under continents as under ocean.

– Makes up less than 1 percent of Earth’s volume.

– Mohorovičić discontinuity(Moho)—the boundary between Earth’s crust and mantle.

• Thought to be a narrow zone with significant change in mineral composition.

The Mantle• The Mantle is that portion of Earth

beneath the crust and surrounding the outer core, about to depth of 2,900 kilometers (1,800 miles).– Largest volume of all four shells.– Scientists believe three zones within

mantle: lithosphere, asthenosphere, mesosphere.

• Lithosphere—the uppermost zone of mantle and the crust together. (Also, tectonic plates consisting of the crust and upper rigid mantle. Sometimes used as a general term for the entire solid Earth.)

• Asthenosphere—plastic layer of the upper mantle that underlies the lithosphere. Its rock is very hot and therefore weak and easily deformed.

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The Inner and Outer Core• Outer core—the (molten) liquid

shell beneath the mantle that encloses Earth’s inner core.– Responsible for generating Earth’s

magnetic field.– The north magnetic pole migrates.– The magnetic field also weakens

and undergoes magnetic reversals at regular intervals.

• Inner core—the supposedly solid, dense, innermost portion of Earth, believed to consist largely of iron/nickel or iron/silicate.– Understanding of crust and upper

mantle has fundamentally changed in last three decades.

Plate Tectonics and the Structure of Earth

• Continental drift—theory that proposes that the present continents were originally connected as one or two large landmasses that have broken up and literally drifted apart over the last several million years.

• Plate tectonics—a coherent theory of massive crustalrearrangement based on the movement of continent-sized lithospheric plates.

Composition of Earth• Mineral—a naturally

formed inorganic solid substance that has an unvarying chemical composition and characteristic crystal structure.

• About 4,400 identified, with new types being found each year.

Fig. 13-3. Quartz crystal, pure silica (SiO2)

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Minerals• In order for a substance

to be considered a mineral it must be

1. Solid2. Naturally found in nature3. Inorganic4. Possess a specific

chemical composition 5. Contain atoms arranged in

a regular pattern to form solid crystals.

Fig. 13-4. Iron pyrite crystals(FeS2)

Minerals• Seven principal categories of rock-forming minerals (on

basis of chemical properties and internal structure).1. Silicates—a category of minerals composed of silicon and

oxygen combined with another element or elements.• Largest and most important group; most are hard and durable.

2. Oxides—a category of minerals composed of oxygen combined with another element.• Quartz has chemical composition of oxide, but classified as silicate

because of its internal structure.3. Sulfides—a category of minerals composed of sulfur, combined

with another element or elements. • Includes many of the most important ore minerals.

Minerals4. Sulfates—a category of minerals composed of

sulfur and oxygen, combined with another element or elements. – Calcium is the principal combining element.

5. Carbonates—mineral that is a carbonate compound of calcium or magnesium.

6. Halides—a category of minerals that is notably salty.– Least widespread.

7. Native elements—those minerals that aren’t combined chemically with others, but appearing as discrete elements (e.g., gold and silver).

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Rocks• Rock—solid material

composed of aggregated mineral particles (in lithosphere).

• Outcrop—surface exposure of bedrock.

• Bedrock—buried layer of residual rock that has not experienced erosion.

– Three types of rocks• Igneous• Sedimentary• Metamorphic

- Fig. 13-6

Igneous Rocks• Igneous rock—rock

formed by solidification of molten magma.– Many kinds, but principal

shared trait is crystalline structure.

• Magma—molten material in Earth’s interior.

• Pyroclastics – rocks formed from the “welding”together of tiny pieces of volcanic rock.

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Igneous Rocks

• Classification of igneous rocks based on mineral composition and texture.– Quantity of silica is one of the most important

variables.• Felsic—contain large portions of light-colored

silicate minerals.– Quartz and feldspar

• Mafic—contain low amounts of silicate.– Contain a large portion of dark-colored silicate minerals

such as olivine and pyroxene.

Igneous Rocks• Plutonic (Intrusive) Rocks—rocks that cool and solidify

beneath Earth’s surface (may be pushed up to surface or exposed through erosion). – Granite is most common and well known.– Large mineral crystals because of slow rate of cooling.

• Volcanic (Extrusive) Rocks—molten rock ejected onto Earth’s surface, solidifying quickly in the open air.– Basalt is most common.– Also obsidian, tuff, and pumice.

• Small mineral crystals because of rapid rate of cooling.

– Fig. 13-8a. Sylvan Lake, Black Hills, SD

Granite

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– Volcanic (Extrusive) Rocks• Small mineral structure (fine-grained)• Dark-colored, generally (mafic igneous rock)• Basalt most common (extensive seafloor bedrock)

– Fig. 13-17

– Fig. 13-19. Snake River Canyon, Idaho.

Basalt

Sedimentary Rocks• Sediment—small particles of rock

debris or organic material deposited by water, wind, or ice.

• Sedimentary rock—rock formed of sediment that is consolidated by the combination of pressure and cementation.– During sedimentation, materials

sorted roughly by size (the finer particles carried farther than heavier particles).

• Strata (plural; stratum, singular)—distinct layers of sediment.– Results in parallel structure

(stratification), with layers varying in thickness and composition.

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Sedimentary Rocks

• Clastic Sedimentary Rocks– Composed of fragments of preexisting rocks.

• AKA clastic or detrital• For example, shale and sandstone.• When sedimentary rock is composed of rounded

particles, it is called conglomerate.

Sedimentary Rocks

• Chemical and Organic Sedimentary Rock– Chemically accumulated: precipitation of

soluble materials or chemical reactions.• For example, calcium carbonate and limestone.

– Organically accumulated: remains of dead plants or animals.• For example, coal and limestone.

– Nearly horizontal layers of limestone and shale

– Fig. 13-11Limestone

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– Tilted sedimentary strata (limestone and shale, mostly)

– Fig. 13-12

– Lithification• Compaction (a)• Cementation (b)

– Fig. 13-10

– Clastic Sedimentary Rocks

– Fig. 13-13

Sandstone

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– Chemical and Organic Sedimentary Rocks• Chemical Precipitation (Limestone most common result)• Compaction of organic sediments (e.g., limestone and coal

formations)

– Fig. 13-11. (a) Layers of limestone and shale. (b) Limestone with fossil mollusks.

– Relative Abundance of Sedimentary Rock Types

– Fig. 13-14

Metamorphic Rocks• Metamorphic rock—rock that was originally something

else (igneous or sedimentary) but has been drastically changed by massive forces of heat and/or pressure working on it from within Earth.– Contact metamorphism

• When magma comes in contact with surrounding rocks.– Regional metamorphism

• Large volumes of rock are subject to heat and pressure.• Process recrystallizes and rearranges mineral components.• Foliation—Prominent alignment of minerals.• Some predictability, such as limestone metamorphized becomes

marble.• Sometimes metamorphosis so great, can’t determine nature of

original rock. • Most common are schist and gneiss.

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Metamorphic Rocks

– Contact Metamorphism

– Fig. 13-15

– Regional Metamorphism and Foliation

– Fig. 13-17. Outcrop of banded gneiss.

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– Common Metamorphic Rocks and Their Counterparts

• Marble limestone• Quartzite sandstone• Slate ShaleMetamorphic rocks with

multiple counterparts:• Gneiss• Schist

– Slate, Northampton County, PA (Source: Richard A. Crooker)

The Rock Cycle

• There is an ongoing recycling of lithographic material via the rock cycle (Figure 13-18 on page 400).

Continental and Ocean Floor Rocks

• Lithosphere has very uneven distribution of sedimentary, igneous, and metamorphic rocks.– Sedimentary rocks dominant

on surface of Earth, both in United States and rest of world.

– This dominance, however, is only on surface, as sedimentary cover is not thick.

• Averages less than 2.4 kilometers (1.5 miles).

• Assume that igneous make up the bulk of the crust, but metamorphic rocks might because of enormous pressures at play beneath crustal surface.

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Continental and Ocean Floor Rocks

• Continental crust is primarily made up of silica and aluminum.– Sial for short.– 2.7 grams per cubic centimeter

Continental and Ocean Floor Rocks

• Oceanic crust is primarily made up of silica and magnesium.– Sima for short. – 3.0 grams per cubic centimeter.

Continental and Ocean Floor Rocks

• Isostasy—maintenance of the hydrostatic equilibrium of Earth’s crust.– Basically, where material is added, crust will sink, but

it will rise when material is removed.– Variety of causes result in isostatic reactions.

• For example, deposition of sediment or accumulation of glacial ice vs. erosion as ice sheet melts or large body of water drains.

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The Study of Landforms• Focus on topography—the surface

configuration of Earth.• Landform—an individual topographic feature.• Geomorphology—the study of the

characteristics, origin, and development of landforms.

• Focusing just on land surfaces, study must encompass 150 million square kilometers (58 million square miles) scattered over seven continents and innumerable islands.

The Study of Landforms• Such study is complex endeavor and requires

organized approach, including examining following four elements.1. Structure—nature, arrangement, and orientation of the

materials in feature being studied. • Geologic underpinning of landform.

2. Process—the actions that have combined to produce the landform.• Encompasses interaction of forces such as geologic, hydrologic,

atmospheric and biotic.3. Slope—fundamental aspect of shape for any landform.4. Drainage—refers to movement of water (from rainfall and

snowmelt).

The Study of Landforms• After identifying the previous four basic

elements, geographer can analyze topography.• Fundamental questions of geographic inquiry:

1. What? (the form of features)2. Where? (the distribution and pattern of landform

assemblage)3. Why? (an explanation of origin and development) 4. So what? (the significance of the topography in

relationship to other elements of environment and to human life and activities)

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The Study of Landforms

• Some Critical Concepts– Basic Terms

• Relief—the difference in elevation between the highest and lowest points in an area (e.g., the vertical variation from mountaintop to valley bottom).

Internal and External Geomorphic Processes

• Variety of topography reflects complexity of interactions between process and structure.– Internal Processes

• Internal processes operate within Earth, drawing energy from heat.

• In general, they are building forces, increasing relief of land surface.

– External Processes • External processes operate at base of atmosphere

(subaerial), drawing energy from sources above lithosphere (atmosphere or oceans).

• Better understood than internal processes; behavior is predictable.

• In general, they are wearing-down or destructive forces, diminishing relief of land surface.

Internal and External Geomorphic Processes

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Uniformitarianism

• Uniformitarianism—the concept that “the present is the key to the past” in geomorphic processes.

• The processes now in operation have also operated in the same way in the past and should also operate in future.

Geologic Time• Geologic time

enumerates temporal expanses of almost unfathomable scope.– Geologic time

encompasses millions and hundreds of millions of years.

• Four eras, three most recent being subdivided into 12 periods.

• Two most recent periods divided into 7 epochs.

Geologic Time• Chapter 13 offers chart of geologic time expressed in equivalent 1-year

scale.– On one-year scale, first 4 months, planet was lifeless.– One-celled life appeared in early May.– Multicelled organisms began evolving in early November.– Antediluvian fishes, the first vertebrates, appeared about November 21.– Amphibians appeared late November.– Vascular plants appeared about November 27.– Reptiles began era of dominance about December 7.– Mammals arrived about December 14; birds arrived December 15.– Flowering plants arrived December 21.– December 24 came the first grasses and first primates.– First hominids came New Year’s Eve.– Homo Sapiens arrived one hour before midnight.– Age of written history equals last minute of the year.

• If geologic time examined in context of cliff one kilometer high (3300 ft.), an individual’s existence would equal less than the thickness of the finest hair.

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Scale and Pattern• An Example of Scale

– Text offers an example of different perspectives of scale, from largest scale of ordinary human experience, walking though a landscape, to driving thorough it, to flying over it, to satellite viewpoint, then to smallest scale, from perspective of spacecraft.

• The Pursuit of Pattern1. Orderly patterns of distribution are much more difficult to discern

in geomorphology than in other geographic elements, such as climate.

2. Overall, global distribution of topography is very disordered and irregular.

3. Comprehending process (dynamics of topographic development) more important than study of landform distribution.

• The Pursuit of Pattern– Patterns in the

distribution of landforms are difficult to discern.

– Geomorphology concentrates less on distribution and more on process.

– Fig. 13-27

Scale and Pattern