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Title Page Photo

““Zeal is a volcano, the peak of which the Zeal is a volcano, the peak of which the grass of indecisiveness does not grow.grass of indecisiveness does not grow.””

——KahlilKahlil Gibran (Gibran (Brainquote.comBrainquote.com))

Vocabularypyroclastic flow (p. 440)pyroclastics (p. 429)reverse fault (p. 449sag pond (p. 450)seafloor spreading (p. 416)shield volcano (p. 433)strike-slip fault (p. 449)subduction (p. 416)syncline (p. 446)thrust fault (p. 449)tilted fault block mountain (p. 449)transform boundary (p. 422)volcanic island arc (p. 422)volcanic mudflow (lahar) (p. 442)vulcanism (p. 428)

graben (p. 449)horst (p. 449)igneous intrusion (p. 442)lava (p. 429)liquefaction (p. 452)magma (p. 429)magnitude (of earthquake) (p. 451)mantle plume (hot spot) (p. 426)midocean ridge (p. 420)normal fault (p. 449)oceanic trench (p. 421)overturned fold (p. 446)Pacific ring of fire (p. 424)paleomagnetism (p. 416)Pangaea (p. 413)plate tectonics (p. 419)

anticline (p. 446)batholith (p. 443)caldera (p. 437)cinder cone (p. 437)composite volcano (p. 434)continental drift (p. 413)continental rift valley (p. 420)convergent boundary (p. 420)divergent boundary (p. 420)earthquake (p. 450)epicenter (of earthquake) (p. 451)faulting (p. 447)fault scarp (p. 448)flood basalt (p. 433)folding (p. 445)

The Impact of Internal Processes on the Landscape

• Internal processes are fundamentally responsible for the gross shape of lithospheric landscape.– Do not always act independently and

separately.

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From Rigid Earth to Plate Tectonics

• Variety of recent discoveries and hypotheses called into question rigid-Earth theory.– The idea that all of the

continents had at one time been united has been around for a considerable period of time, but not until recently was a plausible theory formulated.

Wegener’s Continental Drift• When looked at in geological time scale, continents very mobile.• Theory of continental drift proposes that continents were originally

all connected, but broke up and are still drifting apart, so will continue to change position.– Pangaea—the massive supercontinent that Alfred Wegener postulated

to have existed about 250 million years ago.• Evidence includes remarkable number of close affinities of geologic features

on both sides of Atlantic Ocean.• Continental margins of subequatorial portions of Africa and South America fit

together. – Petrologic and paleontologic records on both sides of Atlantic show many

distributions would be continuous if no ocean.

• Fossils, e.g., the mesosaurus fossils

– Fig. 14-3

– Wegener’s flaw – no plausible propelling mechanism

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Plate Tectonics• The Evidence

– Oceans have a continuous system of large ridges located some distance from continents, often midocean. Also, deep trenches occur at many places in the ocean floors, often around margins of ocean basins.

– Seafloor Spreading—theory proposing that oceanic ridges are formed by currents of deep-seated magma rising up from the mantle (often during volcanic eruptions), creating new crust on the ridges (the newest crust formed on the planet).

• Subduction — process proposed to explain trenches, making them the site where older crust descends into the interior of Earth, where it is presumably melted and recycled into the convective cycle that operates in Earth.

Plate Tectonics• Convection: The missing propelling mechanism

– Movement of mass due to changes in its density caused by gain or loss of heat

heat

H-

H+

heatCore

Mantle

Lithosphere

Plate Tectonics

• Verification of Seafloor Spreading --– Theory of seafloor spreading supported by two sets of evidence:

• Paleomagnetism– Seafloor has a relative symmetrical pattern of magnetic orientation on both sides

of ridges, indicating that it has spread laterally by addition of new crust (displaying how the magnetic field has reversed itself [more than 170 times]).

• Ocean floor core sampling– Sediment age and thickness increase with increasing distance from the ridges,

indicating that sediments farthest from ridges are oldest

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• Age of the ocean floors based on paleomagnetism

– Fig. 14-9

Plate Tectonics• By 1968, the great body of

evidence convinced many scientists that the theory of plate tectonics was viable. – Number of plates and their

boundaries are not completely clear.

– About a dozen major plates and smaller ones are postulated; thought to be about 100 kilometers (60 miles) thick, and consist of both oceanic and continental crust.

– Driving mechanism for plate tectonics is convection within Earth’s mantle.

– Plates move at varying rates from 1 centimeter to 10 centimeters per year.

Plate Boundaries• Divergent boundary—type of plate association in which

two plates are moving away from each other because of magma welling up from asthenosphere. – Most common is midocean ridge, but also occurs within a

continent (continental rift valley), as in East African Rift Valley.

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– Divergent Plate Boundaries: Origin of Ocean Basins

– Fig. 14-12

• Rift Valley Formation– Begins on a continent

» East African Rift Valley– Grows to become linear sea (“proto-ocean”)

» Red Sea• Constructive boundary (rock is created)

– Fig. 14-13

Plate Boundaries• Convergent boundary—type of plate association in which two plates are

colliding.– Normal result is one plate being subducted, but showing crumpling at the

edges where they meet (often resulting in massive and spectacular landforms).– Three types:

1. Oceanic–continental convergence2. Oceanic–oceanic convergence3. Continental–continental convergence

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– Convergent Plate Boundary: Continent-to-Continent• No subduction• Conservative boundary: Rock is neither created nor

destroyed• Folded Mountains: e.g., Himalayas

– Fig. 14-14c

Plate Boundaries• Convergent boundary (con’t)• Oceanic–continental convergence—denser oceanic

plate is subducted, and oceanic trench and coastal mountains are usually created (e.g., Andes).– Accompanied by earthquakes, and volcanoes develop.

• Oceanic–oceanic convergence—creates oceanic trench and volcanoes on ocean floor, which initiate volcanic island arc (e.g., Aleutians and Japan).

• Continental–continental convergence—no subductionoccurs, so huge mountain ranges are built up (e.g., Alps and Himalayas). – Volcanoes rare, but shallow-focus earthquakes common.

Plate Boundaries• Transform boundary—plate association

in which two plates slip past one another laterally in a typical fault structure.– Associated with a great deal of seismic

activity (e.g., San Andreas in California).

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• California and San Andreas Fault system

– Fig. 14-16

The Rearrangement1. Evidence points to five continents existing 450 million

years ago.2. These converged into single continent of Pangaea,

which then began to break up about 250 million years ago into two pieces: Laurasia (Northern Hemisphere) and Gondwanaland (Southern Hemisphere).

3. Look at rocks, fossils, and magnetic patterns to determine relative positions.

4. Numbers of plates appear to have changed through time, with some eras having more than today, others having less. Sizes and shapes also have changed through time.

• The RearrangementAnimation (Assembly and Breakup of Pangaea)

– Fig. 14-17

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The Pacific Ring of Fire

• Plate boundaries are found all around the Pacific basin—primarily subductionzones.– Along these plate

boundaries many volcanoes have formed giving this region the name the Pacific Ring of Fire

Additions to the Basic Plate Tectonic Theory

• Mantle plumes (hot spots)—a location where molten mantle magma rises to, or almost to, Earth’s surface.– Cause is unknown; creates volcanoes and/or hydrothermal (hot

water) features. – Recent research indicates that explanation of hot spots may be

more complex than previous assumed.– Seismic tomography suggests that some mantle plumes may be

shallow and that some may be mobile.

• Hawaiian Islands

– Fig. 14-20

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Accreted Terranes

• Terrane—a small-to-medium mass of lithosphere that is too buoyant to be subducted so instead is fused to a plate. It is often very different from the plate. – For example, most of Alaska, western

Canada, and western U.S. are a mosaic of several dozen accreted terranes.

– Accreted Terranes

– Fig. 14-21, with Fig. 14-22 overlay

The Questions

• Several unanswered questions regarding plate tectonics include the following:1. Is the contemporary pattern of plates adequate even

though it does not explain all mountain belts? 2. Why does the number and the size and shape of

plates seem to vary through geologic time?3. Why does plate size vary so much?4. What determines zones of weakness where plate

boundaries first develop?5. What explains tectonic activity in the middle of

plates?6. What is ultimate cause of plate movement?

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Vulcanism• Other internal processes

are directly associated with tectonic movement; one is vulcanism.

• Vulcanism—general term that refers to all phenomena connected with origin and movement of magma from the interior of Earth to or near the surface

– Volcano Distribution

– Fig. 14-24

– Magma Chemistry, Styles of Eruption, Etc.

– Table comparing two eruptive styles and their characteristics

BasaltAndesiteCommon type of Lava

Shield volcanoComposite volcanoType of Volcano

Subduction areas

High silica content (felsic magma)

More polymerizationThick magmaMore gas pressure

build upHigh explosivity index

ExplosiveLow silica content

(mafic magma)Less polymerizationThin magmaLess gas pressure

build upLow explosivity index

Magma chemistry

Mid-ocean ridge and hot spots

General locations

Effusive (mild)Characteristics

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Vulcanism• Three types of vulcanism: volcanism,

intrusive vulcanism, and plutonic activity.1. Volcanism—extrusive vulcanism, in that the

magma is expelled onto Earth’s surface while still molten.

2. Intrusive vulcanism—occurs where magma solidifies in shallow crust near surface.

3. Plutonic activity—occurs where magma solidifies very deep inside Earth, far below surface.

Vulcanism• Lava—molten magma that is extruded onto the

surface of Earth, where it cools and solidifies; affects landscape whether gentle or explosive.

• Pyroclastic material—solid material such as rock fragments, solidified lava blobs, and dust thrown into the air by volcanic explosions.– For example, the Krakatau explosion in 1883 ejected

20 cubic kilometers (6 cubic miles) of material into air.

Vulcanism• Active volcanoes—

those that have erupted at least once in recorded history.– U.S. has 10% of about

550 active volcanoes in world.

– Pacific Ring of Fire or Andesite Line has some 80% of world’s volcanoes.

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Vulcanism

• Volcanic activity is primarily associated with plate boundaries. – At divergent boundaries, magma wells up by

volcanic eruption and flooding from fissures.– At convergent boundaries, volcanoes form

because of turbulent descent and melting of crust that occurs with subduction.

Vulcanism

• Magma Chemistry and Styles of Eruption– Chemistry of magma largely determines

nature of eruption.• Critical component appears to be relative amount

of silica (Si02). – High silica content can result in explosive eruption.

– Strength of surface crust and degree of confining pressure can also play role.

Vulcanism

• Volcanic Activity– Benefit of volcanoes: magma provides

essential nutrients for plant growth. Volcanoes provide fertile environments.

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Vulcanism• Lava Flows

– Flood basalt —a large-scale outpouring of basaltic lava that may cover an extensive area of Earth’s surface.

• Can build up, layer upon layer, to depths of many hundreds of feet and cover tens of thousands of square kilometers, like Columbia Plateau in United States.

Volcanic Peaks

• Volcanic Peaks often starts small, can grow into hill or mountain. Most have crater set at apex of cone.

Volcanic Peaks• Shield Volcanoes—never steep-sided, though can be very high (e.g.,

Hawaiian Islands).• Composite Volcanoes—steep-sided, large, symmetrical cones (e.g., Mt.

Fuji, Japan).• Lava Domes—usually small, with irregular shape (e.g., Lassen Peak,

California).• Cinder Cones—smallest of volcanic mountains (e.g., Sunset Crater in

Arizona).• Calderas—uncommon, but large, steep-sided, roughly circular depression

resulting from the explosion and subsidence of a large volcano (e.g., Oregon’s Crater Lake).

• Volcanic Necks—rare but prominent sharp spire that rises abruptly above the surrounding land. It represents the pipe or throat of an old volcano, filled with solidified lava after its final eruption. The less resistant material that makes up the cone is eroded, leaving the harder, lava-choked neck as a remnant (e.g., Shiprock in New Mexico).

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• Volcanic PeaksAnimations– Shield Volcanoes

• Hawaiian Islands (Mauna Loa and Kilauea)

1. Volcanoes2. Formation of Crater Lake

– Fig. 14-29b

• Dome-shaped (broad base, gentle slopes)

– Fig. 14-30

– Composite Volcanoes (cone-shaped, steep slopes)• Mt. Fuji, Mt. Kilimanjaro, Mt. Rainier, Mt. Shasta

– Fig. 14-29d

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– Lava domes (plug domes)• Solitary volcanoes: Mono Craters, CA• Inside craters of composite volcanoes: Mt. St. Helens

– Fig. 14-29c

– Cinder Cones• Youthful volcanoes• Highly erodible slopes (loose pyroclasts)• Found in association with other volcanoes

– Fig. 14-29a

– Fig. 14-32 VolcánPopocatépetl, Mexico

– Photographs of Volcanic Peaks

Composite volcano

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– Fig. 14-33. Crater Mountain, Mono Craters, CA

Lava (Plug) Dome

– Fig. 14-34 Sunset Crater, AZ

Cinder Cone

Calderas: Massive crater, Lava dome (plug), Volcano “blows its top”

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– Fig. 14-35a and b– Crater Lake, OR

– Volcanic Necks

– Fig. 14-38 Shiprock, NM.

Volcanic Hazards

• Wide range of hazards can affect people living near volcanoes:– Volcanic gases, lava

flows, eruption column and clouds, pyroclasticflows, volcanic mudflows (lahars).

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Volcanic Hazards1. Volcanic Gases

– Volcanic gases include carbon dioxide, sulfur dioxide, hydrogen sulfide, and fluorine. Affects can range from acid rain that destroys local vegetation to droplets affecting solar insolation and lowering global temperatures (e.g., Mount Pinatubo in 1991 lowered temperatures for more than a year).

2. Lava Flows– Lava flows cause more property damage than loss of

life.3. Eruption Column and Clouds

– Violent ejection of pyroclastic material and gases; can reach elevations of 16 kilometers (10 miles) or more.

Volcanic Hazards4. Pyroclastic Flows

– Pyroclastic flows —terrifying high-speed avalanche of searing hot gases, ash, and rock fragments (e.g., one on Martinique in the Caribbean killed nearly 28,000 inhabitants of one town in a matter of moments in 1902).

5. Volcanic Mudflows(Lahars)—fast moving, and sometimes hot, slurry of mud and boulders; one of most common volcanic hazards. (e.g., Nevada del Ruiz volcano in Columbia produced a mudflow that killed more than 20,000 people in a town nearly 50 kilometers [30 miles] away).

Fig. 14-40 Eruption of Mount St. Augustine, Alaska. A pyroclastic flow is moving down the slope to the left.

– Fig. 14-41 Unzen volcano in Japan showing path of pyroclastic flows.

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Igneous Features1. Intrusions can rise high enough to

deform overlying material and affect landscape, or be exposed at surface.

2. Can take on almost infinite variety of forms, but are classified into scheme of six types (the largest three—batholiths, stocks, and laccoliths—are subgrouped as plutons).

3. Batholiths—the largest and most amorphous of igneous intrusions; subterranean igneous body of indefinite depth and enormous size. Often form the core of major mountain ranges.

4. Stocks—a small body of igneous rock intruded into older rock, amorphous in shape and indefinite in depth. Many are offshoots of batholiths.

Igneous Features5. Laccoliths—an igneous intrusion produced when slow-moving

viscous magma is forced between horizontal layers of preexistingrock. The magma resists flowing and builds up into a mushroom-shaped mass that domes the overlying strata. If near enough to Earth’s surface, a rounded hill will rise above the surrounding area (e.g., South Dakota’s Black Hills).

6. Dikes—a vertical or nearly vertical sheet of magma that is thrust upward into preexisting rock; probably most widespread.

7. Sills—a long, thin intrusive body that is formed when magma is forced between parallel layers of preexisting rock to solidify eventually in a sheet.

8. Veins—small igneous intrusions, usually with vertical orientation.

Diastrophism

• Diastrophism—a general term that refers to the deformation of Earth’s crust, and implies that the material is solid and not molten.– Two types of diastrophic movements: folding

and faulting.– Separation of two not always discrete and

clear-cut.

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Folding• Folding—the bending of crustal rocks by compression

and/or uplift (with great pressure being applied for long periods).– Can vary from centimeters to tens of kilometers, from simple and

symmetrical forms to complex, asymmetrical features.• Figure 14–33 shows the various main types of folds:

Folding• Monocline—a one-sided slope connecting two horizontal or gently

inclined strata.• Anticline—a simple symmetrical upfold.• Syncline—a simple downfold.• Overturned fold—an upfold that has been pushed so vigorously

from one side that it becomes oversteepened enough to have a reverse orientation on the other side.

• Overthrust fold—a fold in which the pressure was great enough to break the oversteepened limb and cause a shearing movement, so older rock rides above younger.

• Formation of Anticlinal Valleys and Synclinal Ridges

– Fig. 14-47a and b

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– Fig. 14-48 Intensely folded Appalachian topography in the eastern United States.

– Fig. 14-49 Cross section through the Swiss Alps, showing the enormous complexity of fold structures.

Faulting• Faulting—the breaking apart of crustal rocks

with accompanying displacement (vertical, horizontal, or both).– Can vary in time (slow or sudden) and in size

(centimeter to 20 or 30 feet [in sudden slippage] up to hundreds of kilometers horizontally and tens of kilometers vertically [over millions of years]).

– Earthquakes usually but not exclusively associated with faults.

• Fault lines often marked by other prominent topographic features.

• Fault scarp—steep cliff formed by faulting; represent the edge of a vertically displaced block.

– Linear erosional valleys and sag ponds.

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Types of Faults

• Two dozen types of faults can be generalized into four principal types on basis of direction and angle of movement:– Normal fault, reverse

fault, strike-slip fault, and overthrust fault.

Types of Faults• Normal fault—the result of tension producing a steeply inclined fault

plain, with the block of land on one side being pushed up, or upthrown, in relation to the block on the other side, which is downthrown (displacement is mostly vertical).

• Reverse fault—a fault produced from compression, with the upthrown block rising steeply above the downthrown block, so that the fault scarp would be severely oversteepened if erosion did not act to smooth the slope (displacement is mostly vertical).

• Thrust fault—a fault created by compression forcing the upthrownblock to override the downthrown block at a relatively low angle; complicated in structure.

• Strike-slip fault—a fault produced by shearing, with adjacent blocks being displaced laterally with respect to one another (displacement is entirely horizontal).

• Types of Faults– Normal

– Fig. 14-52

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– Reverse

– Fig. 14-52

– Strike-slip

– Fig. 14-52

– Fig. 14-53. Calaveras fault, Hollister, CA. Offset wall and sidewalk.

• Example of a strike-slip fault.

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Landforms Associated with Normal Faulting

• Tilted Fault-block Mountains– A mountain formed under certain conditions of

crustal stress, whereby a surface block may be severely faulted and upthrown on one side without any faulting or uplift on the other side. The block is tilted asymmetrically, producing a steep slope along the fault scarp and a relatively gentle slope on the other side of the block.

Landforms Associated with Normal Faulting

• Horst and Graben– Horst—an uplifted block of land between two parallel faults.– Graben—a block of land bounded by parallel faults in which the

block has been downthrown, producing a distinctive structural valley with a straight, steep-sided fault scarp on either side.

Landforms Associated with Normal Faulting

• Rift Valleys—a downfaulted graben structure extended for extraordinary distances as linear structural valleys enclosed between typically steep fault scarps.

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Landforms Associated with Strike-Slip Faulting

• Can result in wide variety of landforms, including– Linear fault trough—a valley marking a strike-slip fault; occurs by

repeated movement and fracturing of rock.– Sag pond—a pond caused by the collection of water from springs

and/or runoff into sunken ground, resulting from the jostling of Earth in the area of fault movement.

– Offset stream—offset drainage channel; perhaps most conspicuous landform produced by strike-slip faulting.

– Shutter ridge—displaces streams flowing across a fault.

Earthquakes• When there is a sudden

displacement of a fault, shock waves produce vibrations we call earthquakes.

• Earthquake Waves– The focus, or center, of the

fault motion produces several kinds of shock or seismic waves that travel outward in widening circles.

• Epicenter—the location on the ground directly above the focus of an earthquake, where the strongest shocks and greatest crustal vibrations are often felt.

Earthquakes• Primary or P waves are the fastest moving waves,

moving through Earth like sound waves, alternately compressing and relaxing the material they pass through.

• Secondary or S waves are slower moving waves, also passing through body of Earth, producing both strong side-to-side and up-and-down “shearing” motion.

• A third type of waves, surface, do not travel through Earth like P and S waves do, but only travel across surface, immediately after S waves, and produce strong side-to-side and up-and-down “rolling” motion.

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Earthquake Magnitude• Seismograph—instrument used to record earthquakes.

– Seismologists can pinpoint earthquake focus with great precision, measuring lag time between arrival of P and S waves.

• Magnitude—the relative amount of energy released during an earthquake. Calculated on a logarithmic scale, so there’s an energy increase from one magnitude to the next of about 32 times.

• Richter scale—a scale of earthquake magnitudes, devised by California seismologist Charles F. Richter in 1935, to describe the amount of energy released in a single earthquake.– Most commonly quoted magnitude, but is not ideal for comparing

very large earthquakes (those of magnitude 7 or higher).

Shaking Intensity

• Moment magnitude—now most commonly used to describe large earthquakes.

• Modified Mercalli intensity scale—a scale that assigns the strength of local shaking, based on observed effects and damage.

Earthquake Hazards• Ground shaking—causes most damage. Generally,

ground shaking diminishes with distance from epicenter, but local geology also modifies.

• For example, loose, unstable soils will amplify the shaking.

• Liquefaction—occurs when loose, water-saturated sediments turn fluid, resulting in subsidence, fracturing, and horizontal sliding of the ground surface.

• Landslides• Tsunami—very long sea wave generated by submarine

earthquake or volcanic eruption.