1. Title Page Photo
“Zeal is a volcano, the peak of which the
grass of indecisiveness does not grow.”
grow.”
—Kahlil Gibran (Brainquote.com)
(Brainquote.com)
Vocabulary
anticline (p. 446) graben (p. 449) pyroclastic flow (p. 440)
batholith (p. 443) horst (p. 449) pyroclastics (p. 429)
caldera (p. 437) igneous intrusion (p. 442) reverse fault (p. 449
cinder cone (p. 437) lava (p. 429) sag pond (p. 450)
composite volcano (p. 434) liquefaction (p. 452) seafloor spreading (p. 416)
continental drift (p. 413) magma (p. 429) shield volcano (p. 433)
continental rift valley (p. magnitude (of earthquake) strike-slip fault (p. 449)
420) (p. 451) subduction (p. 416)
convergent boundary (p. mantle plume (hot spot) (p. syncline (p. 446)
420) 426) thrust fault (p. 449)
divergent boundary (p. 420) midocean ridge (p. 420) tilted fault block mountain
earthquake (p. 450) normal fault (p. 449) (p. 449)
epicenter (of earthquake) oceanic trench (p. 421) transform boundary (p. 422)
(p. 451) overturned fold (p. 446) volcanic island arc (p. 422)
faulting (p. 447) Pacific ring of fire (p. 424) volcanic mudflow (lahar) (p.
fault scarp (p. 448) paleomagnetism (p. 416) 442)
flood basalt (p. 433) Pangaea (p. 413) vulcanism (p. 428)
folding (p. 445) plate tectonics (p. 419)
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.
1
2. 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
2
3. 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
Lithosphere H-
Mantle
H+
Core heat heat
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
3
4. • 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.
4
5. – 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 convergence
2. Oceanic–oceanic convergence
3. Continental–continental convergence
5
6. – 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 subduction
occurs, 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).
6
7. • California and
San Andreas
Fault system
– Fig. 14-16
The Rearrangement
1. 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 Rearrangement
Animation (Assembly and Breakup of Pangaea)
– Fig. 14-17
7
8. The Pacific Ring of Fire
• Plate boundaries are
found all around the
Pacific basin—
primarily subduction
zones.
– 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
8
9. 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?
9
10. 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.
Characteristics Explosive Effusive (mild)
Magma chemistry High silica content Low silica content
(felsic magma) (mafic magma)
More polymerization Less polymerization
Thick magma Thin magma
More gas pressure Less gas pressure
build up build up
High explosivity index Low explosivity index
Common type of Lava Andesite Basalt
Type of Volcano Composite volcano Shield volcano
General locations Subduction areas Mid-ocean ridge and
hot spots
– Table comparing two eruptive styles and their characteristics
10
11. 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.
11
12. 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.
12
13. 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).
13
15. – 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
– Photographs of Volcanic Peaks
Composite volcano
– Fig. 14-32 Volcán
Popocatépetl, Mexico
15
16. Lava (Plug) Dome
– Fig. 14-33. Crater Mountain, Mono Craters, CA
Cinder Cone
– Fig. 14-34 Sunset Crater, AZ
Calderas: Massive crater, Lava dome (plug),
Volcano “blows its top”
16
17. – 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, pyroclastic
flows, volcanic
mudflows (lahars).
17
18. Volcanic Hazards
1. 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 Hazards
4. 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.
18
19. Igneous Features
1. 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 Features
5. Laccoliths—an igneous intrusion produced when slow-moving
viscous magma is forced between horizontal layers of preexisting
rock. 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.
19
20. 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
20
21. – 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.
21
22. 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 upthrown
block 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
22
23. – Reverse
– Fig. 14-52
– Strike-slip
– Fig. 14-52
• Example of a strike-slip
fault.
– Fig. 14-53. Calaveras
fault, Hollister, CA. Offset
wall and sidewalk.
23
24. 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.
24
25. 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.
25
26. 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.
26
