Download Chapter 14: The Internal Processes

Chapter 14: The Internal Processes
I.
The Impact of Internal Processes on the Landscape
A. Internal processes are fundamentally responsible for the gross shape of
lithospheric landscape.
1. Do not always act independently and separately.
II. From Rigid Earth to Plate Tectonics
1. Variety of recent discoveries and hypotheses called into question rigid-Earth
theory.
a) Two layers of igneous rocks of upper crust are well differentiated in
several characteristics, especially density.
(1) sima
(2) sial
(a) Sima—the lower of two igneous rock layers that is continuous
and underlies both the ocean basin and the continents. “Sima” is
named for its two most prominent mineral compounds, silica and
magnesium. Also called oceanic crust.
(b) Sial—the upper layer of two igneous rock layers, which is
discontinuous, apparently underlying only the continental
masses, where it sits as immense bodies of rock on the sima
beneath. “Sial” is named for its common constituents of silica and
aluminum. Also called continental crust.
b) General crustal structure appears to be continents of lightweight granitic
material floating on a foundation of denser basaltic material.
B. Isostasy
1. Isostasy—maintenance of the hydrostatic equilibrium of Earth’s crust.
a) Basically, where material is added, crust will sink, but it will rise when
material is removed.
b) Variety of causes result in isostatic reactions.
(1) For example, deposition of sediment or accumulation of glacial ice vs.
erosion as ice sheet melts or large body of water drains.
C. Continental Drift
1. When looked at in geological time scale, continents very mobile.
2. 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.
a) Pangaea—the massive supercontinent that Alfred Wegener postulated to
have existed about 250 million years ago.
(1) Evidence includes remarkable number of close affinities of geologic
features on both sides of Atlantic Ocean.
(a) Continental margins of subequatorial portions of Africa and South
America fit together.
(b) Petrologic and paleontologic records on both sides of Atlantic
show many distributions would be continuous if no ocean.
III. Plate Tectonics
A. The Evidence
1. 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.
a) 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).
b) 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.
2. Theory of seafloor spreading supported by two sets of evidence:
a) Paleomagnetism
(1) 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]).
b) Core sampling
(1) Sediment age and thickness increase with increasing distance from
the ridges, indicating that sediments farthest from ridges are oldest.
c) Theory of plate tectonics now generally accepted.
d) Thirteen major plates and smaller ones are postulated; thought to be
about 100 kilometers (60 miles) thick, and consist of both oceanic and
continental crust.
B. Plate Boundaries
1. Divergent boundary—type of plate association in which two plates are
moving away from each other because of magma welling up from
asthenosphere.
a) Most common in oceanic ridge, but also occurs within a continent, as in
East African Rift Valley.
2. Convergent boundary—type of plate association in which two plates are
colliding.
a) Normal result is one plate being subducted, but showing crumpling at
the edges where they meet (often resulting in massive and spectacular
landforms).
b) Three types:
(1) Oceanic–continental convergence
(2) Oceanic–oceanic convergence
(3) Continental–continental convergence
(a) Oceanic–continental convergence—denser oceanic plate is
subducted, and oceanic trench and coastal mountains are usually
created (e.g., Andes).
(i) Accompanied by earthquakes, and volcanoes develop.
(b) Oceanic–oceanic convergence—creates oceanic trench and
volcanoes on ocean floor, which initiate volcanic island arc (e.g.,
Aleutians and Japan).
(c) Continental–continental convergence—no subduction occurs, so
huge mountain ranges are built up (e.g., Alps and Himalayas).
(i) Volcanoes rare, but shallow-focus earthquakes common.
3. Transform boundary—plate association in which two plates slip past one
another laterally in a typical fault structure.
a) Associated with a great deal of seismic activity (e.g., San Andreas in
California).
C. 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.
D. Modifications to the Original Theory
1. 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.
a) For example, most of Alaska, western Canada, and western U.S. are a
mosaic of several dozen accreted terranes.
2. Hot spot (mantle plume)—a location where molten mantle magma rises to,
or almost to, Earth’s surface.
a) Cause is unknown; creates volcanoes and/or hydrotherma (hot water)
features.
b) Recent research indicates that explanation of hot spots may be more
complex than previous assumed.
(1) Seismic tomography suggests that some mantle plumes may be
shallow and that some may be mobile.
E. The Questions
1. Still many unanswered questions, from why some plates are larger to what is
ultimate cause of plate movement.
2. Most basic question: What is source of enormous amount of energy required?
3. Present knowledge helps to understand gross patterns of most of world’s
major relief features (size, shape, and distribution of continents and ocean
basins, and many of the cordilleras).
a) Cordillera—a chain of mountains, often encompassing many ranges.
IV. Vulcanism
A. Other internal processes are directly associated with tectonic movement; one is
vulcanism.
1. 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.
2. Three types of vulcanism: volcanism, intrusive vulcanism, and plutonic
activity.
a) Volcanism—extrusive vulcanism, in that the magma is expelled onto
Earth’s surface while still molten.
b) Intrusive vulcanism—occurs where magma solidifies in shallow crust
near surface.
c) Plutonic activity—occurs where magma solidifies very deep inside Earth,
far below surface.
B. Volcanism
1. Lava—molten magma that is extruded onto the surface of Earth, where it
cools and solidifies; affects landscape whether gentle or explosive.
2. Pyroclastic material—solid material such as rock fragments, solidified lava
blobs, and dust thrown into the air by volcanic explosions.
a) For example, the Krakatau explosion in 1883 ejected 20 cubic kilometers
(6 cubic miles) of material into air.
3. Active volcanoes—those that have erupted at least once in recorded history.
a) U.S. has 10% of about 550 active volcanoes in world.
b) Pacific Ring of Fire or Andesite Line has some 80% of world’s volcanoes.
4. Volcanic activity is primarily associated with plate boundaries.
a) At divergent boundaries, magma wells up by volcanic eruption and
flooding from fissures.
b) At convergent boundaries, volcanoes form because of turbulent descent
and melting of crust that occurs with subduction.
5. Chemistry of magma largely determines nature of eruption.
a) Critical component appears to be relative amount of silica (Si02).
(1) High silica content can result in explosive eruption.
b) Strength of surface crust and degree of confining pressure can also play
role.
6. Benefit of volcanoes: magma provides essential nutrients for plant growth.
Volcanoes provide fertile environments.
7. Flood basalt —a large-scale outpouring of basaltic lava that may cover an
extensive area of Earth’s surface.
a) 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.
8. Four particular landforms associated with volcanic activity (though there are
almost infinite variety of terrain features): lava flows, volcanic peaks,
calderas, and volcanic necks.
a) lava flow—usually solidifies in a horizontal orientation resembling
stratification of sedimentary rock (often a flattish plain or plateau).
b) volcanic peak—often starts small, can grow into hill or mountain. Most
have crater set at apex of cone.
(1) Shield volcanoes—never steep-sided, though can be very high (e.g.,
Hawaiian Islands).
(2) Composite volcanoes—steep-sided, large, symmetrical cones (e.g., Mt.
Fuji, Japan).
(3) Lava domes—usually small, with irregular shape (e.g., Lassen Peak,
California).
(4) Cinder cones—smallest of volcanic mountains (e.g., Sunset Crater in
Arizona).
c) Caldera—uncommon, but large, steep-sided, roughly circular depression
resulting from the explosion and subsidence of a large volcano (e.g.,
Oregon’s Crater Lake).
d) Volcanic neck—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, lavachoked neck as a remnant (e.g., Shiprock in New Mexico).
C. Volcanic Hazards
1. Wide range of hazards can affect people living near volcanoes:
a) Volcanic gases, lava flows, eruption column and clouds, pyroclastic
flows, volcanic mudflows (lahars).
(1) 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 cause more property damage than loss of life.
(3) Eruption column—violent ejection of pyroclastic material and gases;
can reach elevations of 16 kilometers (10 miles) or more.
(4) Pyroclastic flow—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 mudflow (lahar)—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).
D. Intrusive Vulcanism
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).
a) Batholith—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.
b) Stock—a small body of igneous rock intruded into older rock,
amorphous in shape and indefinite in depth. Many are offshoots of
batholiths.
c) Laccolith—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).
d) Dike—a vertical or nearly vertical sheet of magma that is thrust upward
into preexisting rock; probably most widespread.
e) Sill—a long, thin intrusive body that is formed when magma is forced
between parallel layers of preexisting rock to solidify eventually in a
sheet.
f) Vein—small igneous intrusions, usually with vertical orientation.
V. Diastrophism
A. Diastrophism—a general term that refers to the deformation of Earth’s crust,
and implies that the material is solid and not molten.
1. Two types of diastrophic movements: folding and faulting.
2. Separation of two not always discrete and clear-cut.
VI. Folding
A. Folding—the bending of crustal rocks by compression and/or uplift (with great
pressure being applied for long periods).
1. Can vary from centimeters to tens of kilometers, from simple and
symmetrical forms to complex, asymmetrical features.
2. Figure 14–33 shows the various main types of folds:
a) Monocline—a one-sided slope connecting two horizontal or gently
inclined strata.
b) Anticline—a simple symmetrical upfold.
c) Syncline—a simple downfold.
d) 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.
e) 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.
VII.Faulting
A. Faulting—the breaking apart of crustal rocks with accompanying displacement
(vertical, horizontal, or both).
1. 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]).
2. Earthquakes usually but not exclusively associated with faults.
3. Fault lines often marked by other prominent topographic features.
a) Fault scarp—steep cliff formed by faulting; represent the edge of a
vertically displaced block.
b) Linear erosional valleys and sag ponds.
B. Types of Faults
1. Two dozen types of faults can be generalized into four principal types on
basis of direction and angle of movement:
a) Normal fault, reverse fault, strike-slip fault, and overthrust fault.
(1) 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).
(2) 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).
(3) Strike-slip fault—a fault produced by shearing, with adjacent blocks
being displaced laterally with respect to one another (displacement is
entirely horizontal).
(a) Can result in wide variety of landforms, including
(i) Linear fault trough—a valley marking strike-slip fault, occurs
by repeated movement and fracturing of rock.
(ii) 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.
(iii) Offset stream—offset drainage channel; perhaps most
conspicuous landform produced by strike-slip faulting.
(iv) Shutter ridge—displaces streams flowing across a fault.
(4) Overthrust fault—a fault created by compression forcing the
upthrown block to override the downthrown block at a relatively low
angle; complicated in structure.
C. Prominent Faulted Landforms
1. Fault-block mountain range—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.
2. Horst—an uplifted block of land between two parallel faults.
3. 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.
4. Rift valley—a downfaulted graben structure extended for extraordinary
distances as linear structural valleys enclosed between typically steep fault
scarps.
VIII.The Complexities of Crustal Configuration
A. Internal processes are all interrelated, rather than occurring in isolation.
B. Text examines Glacier National Park to see how processes affected landscape.
1. In addition to sedimentary accumulation, also igneous activity creating a vast
outpouring of flood basalt that created submarine lava flow.
2. Also igneous intrusions, including one large sill and dikes.
3. All underwent contact metamorphism.
4. Sank below ocean, then eventually compressed and uplifted.
5. Extreme lateral pressure from west created a prominent anticline, then
overturned it. Eventual lengthening of western limb and truncating of
eastern limb strained the rock and caused a vast rupture and faulting. Lewis
Overthrust pushed the entire block eastward, and eventually the older rock
was pushed over the younger strata. Created a terrain that is known as
“mountains without roots.”