Chapter 22: Plate Tectonics and Earth’s Interior
Objectives:
Working through this chapter of the study guide will enable you to:
1. Identify the various layers of the Earth, and contrast them in terms of
their thickness, composition, density, and seismic behavior.
2. Discuss the characteristics of the crust.
3. Discuss the properties and composition of the mantle.
4. Discuss seismological and other evidence that indicates that the outer
core is liquid.
5. Understand what is meant by the term Mohorovicic Discontinuity.
6. Contrast the asthenosphere and lithosphere.
7. Define isostasy and tell how this concept can explain the variations in
elevation between mountains and the seafloor.
8. Distinguish among the four kinds of earthquake waves.
9. Explain how ocean-floor spreading accounts for present-day features of
the ocean floors.
10. Identify the San Andreas Fault and indicate its origin.
11. Explain the basics of the theory of plate tectonics.
12. Discuss the major evidence in support of the theory of plate tectonics.
13. List the three types of plate boundaries and describe the type of motion
involved at each.
14. Explain what Pangea was.
Lecture Outline:
Although the processes are often very subtle, the surface of Earth is
constantly changing. No comprehensive explanation of these changes had been
found until early in the 20th century when the ideas of continental drift and
oceanic seafloor spreading were formulated. These processes were the keys to
the development of an overall theory of plate tectonics that has been able to
account for many widespread and diverse geological phenomena.
One result of crustal movement is the occurrence of earthquakes. These tremors
of our planet's crust happen quite frequently, but fortunately most of these
disturbances are too weak to even be noticed. Occasionally, however, large-scale
earthquakes occur that produce enormous amounts of damage and even loss of
life. These destructive earthquakes are naturally of great interest and concern to
the population of our planet. Most earthquakes are caused by the interaction of
huge lithospheric plates as they move across the surface of Earth.
The motion of plates in the lithosphere produces a dynamic system in which
these massive slabs of rock must interact with each other at plate boundaries. In
some areas, these plates converge, while in others they diverge or simply slide
past one another. Especially at convergent regions, the plate boundaries can
buckle and fracture under the tremendous pressures created as the plates are
forced together. This action can lead to massive uplifting and folding that is the
basis for mountain ranges and other geologic surface features that characterize
Earth's crust. Several types of faulting can occur in these processes. Fault-block
mountains and fold mountains are the direct result of such plate boundary
interactions. The formation of the third type, volcanic mountains, has already
been explained in Chapter 21.
Structure of the Earth
A. The two main categories of earthquake waves are "body waves" (which
move through the earth's interior) and "surface waves" (which travel
along the earth's surface).
B. Types of body waves include:
1. Primary (P) waves, which are longitudinal or pressure waves and
can travel through a liquid.
2. Secondary (S) waves, which are transverse waves and cannot
travel through a liquid.
C. Types of surface waves include:
1. Love waves, which are transverse waves in which the earth's
surface vibrates from side to side.
2. Rayleigh waves, which involve orbital motions that cause the
earth's surface to move up and down.
D. P waves travel faster than S waves, and surface waves are the slowest.
E. The earth is composed of a series of layers:
1. The crust (the outer layer)
2. The mantle (the layer beneath the crust)
3. The core (the earth's innermost zone)
F. The internal structure of the earth has been determined with the help of P
and S earthquake waves.
1. P and S waves are refracted as they travel within the earth.
2. P and S waves do not reach shadow zones on the other side of
the earth from their origins. This is evidence for a central core.
3. S waves cannot travel through the core, suggesting that at least
the outer core is liquid.
4. Faint traces of P waves in the shadow zones suggest that the
earth's inner core is solid.
5. The lower boundary of the crust is known as the Mohorovicic
discontinuity.
Basic Structure of the Interior of the Earth
1. Crust - brittle, rocky outer layer of the Earth (5-40 km thick)
There are two major types of crust:
a. Continental Crust- thicker (30-40 km thick; 60 km in some
mountain ranges), granitic (sialic) composition
b. Oceanic Crust - thinner (about 5 km), basaltic (mafic)
composition
2. Mantle - solid rocky layer, dense, high pressure. A solid that flows.
(2885 km thick)
Composition: peridotite (Mg Fe silicates like olivine), kimberlite (diamonds),
eclogite.
3. Outer Core - molten Fe with some Ni
4. Inner Core - solid Fe with some Ni
This diagram shows the internal structure of the Earth. The circular diagram in
the lower left is drawn to scale, illustrating the extreme thinness of the crust.
Sometimes the structure of the Earth is compared to that of a boiled egg, or of a
nectarine, to emphasize the thinness of the crust. The pie-shaped wedge on the
right enlarges the crust and upper mantle to show more detail.
Summary of the Major Layers of the Earth
and the Names of the Boundaries or Discontinuities
which separate them
Density P-wave velocity
(g/cm³ )
(km/sec)
Layer
Continental crust
Oceanic crust
2.6 - 2.8
6
3.5
7
Mohorovicic discontinuity (Moho)
Mantle
4.5 - 10
8 - 12
Gutenberg discontinuity
Core (average)
12
-
Outer core
(liquid)
-
8 - 10
Inner core
(solid)
13.5
11 - 12
The major layers of the Earth were detected before 1950.
Fine details were delineated in 1960's during nuclear testing.
Magnetic Field
Note that the Earth's magnetic field is generated in the core by processes
which include convection in the liquid outer core and the rotation of the solid
inner core.
(Note that convection refers to hot material rising and cold material sinking.)
Circular convection cells develop in materials that are heated from the bottom.
The Earth's magnetic field behaves as if there were a giant bar magnet buried
within the Earth.
Note that the Earth has a north magnetic pole and a south magnetic pole.
The magentic poles are NOT THE SAME as the north and south poles which
correspond to the rotational axis of the Earth and the site where the longitude
lines converge (latitude = 90o). The north magnetic pole is somewhere in
northern Canada.
The position of the Earth's magnetic poles changes slightly from year to year (we
call this polar wandering). Polar wandering is probably due to fluctuations in
the convection patterns in the liquid outer core.
Heat Flow and Geothermal Gradient
Recall that the Earth is hot inside.
The deeper you go into the Earth, the higher the temperature.
This is called the geothermal gradient.
(Geo means "Earth" and thermal means "heat".)
The geothermal gradient in crust is about 25° C/ km (75° F/mile)
In diamond mines, at 11,788 ft deep, temperature is about 90° F or more.
Temperature in the Interior
We don't really know the temperature in the center of the Earth. We cannot
measure or observe it directly. We do know that temperatures and pressures
increase with depth. We know the rate at which temperature increases with
depth (the geothermal gradient), and we know the thickness of the various
layers of the Earth, so the temperature in the interior can be calculated
mathematically. We can also model in the laboratory the high pressures and
temperature like those in the Earth's interior, and see how mineral properties
and structures change, and compare these results to the changes that occur
within the Earth, as determined by seismic studies. So we can get a pretty good
idea of what the temperatures are like inside the Earth.
The temperature of the mantle is calculated to be about 870 degrees Celsius.
The temperature of the outer core ranges from about 4,400 degrees Celsius to
about 6,100 degrees Celsius.
The temperature of the inner core of the Earth is estimated to be about 7000
degrees Celsius.
Earth's Layered Structure and Plate Tectonics
Lithosphere and Asthenosphere
A. The lithosphere is a shell of hard rock that consists of the crust and the
upper mantle.

Lithosphere (crust and uppermost mantle, 0-100 km deep), cool,
rigid, brittle
B. The asthenosphere is a layer of hot, soft rock lying just below the
lithosphere.

Asthenosphere (upper mantle, 100-700 km deep), hot, weak, solid
that flows
The Earth's lithosphere is divided into a dozen or so rigid plates that move
relative to one another. These plates "float" or ride along on the underlying
asthenosphere, which flows and undergoes convection, causing the plates to
move.
Probing the Earth's Interior with Seismic Waves
P-waves and S-wave velocities (or travel times) depend on the properties of the
rocks through which they travel. (Review "Earthquake" notes if you need to
review what P-waves and S-waves are.) Differences in velocities (or travel times)
correspond to differences in rock properties. Wave velocity depends on the
density and elasticity of the rock. Seismic waves travel faster in denser rock.
The velocity of seismic waves increases with depth. (This is because
pressure and density increase with depth in the Earth).
P-waves can travel through all of the layers of the Earth because they can
travel through both solids and liquids.
On the other hand, S-waves can travel through the Earth's crust and mantle,
but they STOP when they reach the Earth's core. This causes a shadow zone
(or area in which no S-waves are detected by seismographs) on the side of the
Earth away from the earthquake epicenter.
The presence of the S-wave shadow zone tells us that there is an area in
the center of the Earth that is molten (liquid).
The size of the shadow zone gives us a good idea of the size and depth of the
Earth's outer liquid core.
The size and depth of the solid inner core was determined from P-waves that
reflected (or bounced) off the inner core following nuclear detonations.
Diagram showing how P and S waves travel through various layers of the Earth.
Note that S waves do not travel through the outer core. Notice the bending or
refraction of both P and S waves.
Continental Drift and Seafloor Spreading
Geologic studies have suggested for many years that the continents tend to
move across Earth's surface. The first indication of this was the surprising jigsawpuzzle-like fit of the Atlantic coasts of Africa and North and South America.
Additional evidence was found by a German meteorologist named Alfred
Wegener in several areas, for example, similarities in biological species and
fossils found on the various continents suggested that they were once physically
connected. Also, the continuity of geologic structures such as mountain ranges,
and glaciation in the Southern Hemisphere further suggested that the continents
were once connected as a single unit in the past. To account for his observations
Wegener proposed that a giant supercontinent once existed that he called
Pangaea. According to his theory, this supercontinent slowly broke apart and
formed the continents that moved across Earth's surface into their present
positions.
Wegener's theory of continental drift was not generally accepted until 1960,
when an American geologist named Harry Hess proposed a mechanism called
seafloor spreading that could account for the movement of the continents. The
basic idea behind seafloor spreading is that convection cells bring new molten
magma to the surface of the ocean floor, where it forms into mid-oceanic ridges
and forces the continents slowly apart. Deep-sea drilling for core samples of the
crust across these ocean ridges eventually provided conclusive proof of this
theory when the samples showed younger, thinner sediment layers near these
mid-oceanic ridges and older, thicker layers at ever increasing distances on
either side. Symmetrical strips of magnetic anomalies on either side of these
ridges were also discovered about this time, which helped strengthen the
argument. These strips show remanent magnetism that was locked into the new
seafloor as it cooled. This new seafloor was then forced slowly apart by new upwelling magma, producing the symmetrical magnetic patterns.
Continental Drift
A. The German meteorologist Alfred Wegener proposed the theory of
continental drift early in this century.
B. The evidence Wegener cited in support of continental drift included:
1. The occurrence of similar fossils on widely separated landmasses
2. Data on prehistoric climates that indicated that continents had
changed position in latitude with time
C. Wegener suggested that the continents were originally part of a single,
huge landmass called Pangaea.
Pangea
Pangea was a supercontinent which existed during the Permian Period about 225
million years ago.
Diagram of five maps of the Earth showing Pangea and the positions of the
continents as they split apart over time. According to the continental drift theory,
the supercontinent Pangaea began to break up about 225-200 million years ago,
eventually fragmenting into the continents as we know them today. Continental
drift was the forerunner of the theory of plate tectonics.
Plate Tectonics
The theory of plate tectonics is an extension of the ideas of seafloor spreading
and continental drift. About 20 surface plates have been identified as making up
the lithospheric surface of Earth. These plates are driven about by convection
cells in the asthenosphere. Some are pulling apart, forming divergent boundaries,
whereas other are pushed together at convergent boundaries. A third type of
relative motion is also possible where plates slide past one another, creating
transform boundaries. Plates are considered to be passive parts of the
lithosphere that are driven across Earth's surface by deep thermal convection
processes in the dynamic asthenosphere.
If objects of various densities are placed in an even denser liquid, the densest
objects will float lowest in the liquid. This means that more of their volume will
be submerged and less will protrude above the liquid's surface. Low-density
objects will float higher, like a cork on the surface of water. The mantle of Earth
is not actually liquid, but the asthenosphere is relatively plastic and so can make
structural adjustments. The pliable nature of the asthenosphere allows segments
of the lithosphere to "float" on this plastic sublayer.
The ability of Earth's crust (technically, the lithosphere) to float on the
asthenosphere is the basis for an important geologic concept known as isostasy.
Isostasy explains variations in surface heights by assuming that since the rocks
that make up continents and ocean basins have different densities, these areas
of Earth's crust will float at different elevations. The average density of
continental rock is less than that of the ocean basins and both float on the same
plastic sublayer, so the continents must float higher because of their lower
average density.
Differences in density account for the elevation variations between continental
masses and ocean basins, but they do not explain why there are mountains and
lowlands on the continents themselves. The key to understanding this difference
is in the thickness of the continental crust. In regions where the continental crust
is thin, it floats low on the asthenosphere. Plains or lowlands are found in such
regions. If the crust is thicker, more of the continental mass will float below the
average level of the molten sublayer, but because of the relative thickness more
will also protrude above this level. Uplands and even towering mountains can
thus exist in areas where the crust is exceptionally thick. Again the concept of
isostasy comes into play, and the total surface structure of Earth is in equilibrium.
In this case, we can say that the entire crustal system of Earth is in isostatic
balance.
Plate boundaries are instrumental in producing changes in Earth's crust. Where
plates pull apart, new crust is continually forming, as in the mid-oceanic ridges
discussed earlier. If plates are pushed together, three types of activity can occur.
1. If two oceanic plates are driven together, one plate is forced under the
other, causing a subduction zone. In this region the lower plate is forced
downward into the hot mantle where it melts, releasing gases and lowdensity magma that work their way to the surface causing volcanoes. This
process explains most of the volcanic activity around the Pacific basin Ring
of Fire.
2. Where an oceanic plate is pushed into a continental plate, the denser
oceanic plate is forced under the lighter continental plate, forming
trenches along the coasts of the continents. Large igneous intrusions and
volcanic activity are also produced that can build huge mountain ranges
such as the Andes in South America.
3. The collision of two continental plates leads to crumpling and buckling of
the leading edges of both plates, which produces fold mountains. An
example is the formation of the Alps.
Discoveries leading to the plate tectonic theory came in part from research
associated with the International Geophysical Year (or IGY) (July 1957-December
1958), when scientists from 67 nations around the world worked together to
study various geophysical phenomena in fields such as cosmic rays,
geomagnetism, glaciology, gravity, longitude and latitude determination,
meteorology, oceanography, rocketry, and seismology. (One of the goals of the
research was to launch the first artificial satellite into orbit around the Earth.)
One of the most significant achievements of the IGY (in terms of geology) was
the mapping of large parts of the ocean floor and delineating the mid-ocean
ridge system with its central valley.
Image of the surface of the Earth, derived from a database of land and sea-floor
elevations. World view Mercator projection.
The theory of plate tectonics has revolutionized the understanding of geology.
Many seemingly unrelated geologic facts have been found to be interrelated as a
result of plate tectonic theory. These include:
1. The association of deep ocean trenches with volcanic mountain chains,
2. The geographic distribution of certain types of fossils
3. The distribution of certain types of sedimentary rocks which can be used
as paleoclimate (ancient climate) indicators,
4. The age of the oceanic crust
Plate tectonics is a unifying theory which helps us understand the underlying
causes of the major topographic features of the Earth, as well as the reasons
why some areas of the world are frequently devastated by earthquakes and
volcanic eruptions.
The lithosphere consists of the Earth's crust and part of the uppermost mantle.
The Earth's surface or lithosphere is divided into about 7 large plates and 20
smaller ones.
Asthenosphere is a partially molten part of the mantle, below the lithosphere.
Two types of crust are present in the lithospheric plates:
1. Thin (5 - 7 km), dense (3.0 g/cm3), basaltic oceanic crust (dark, finegrained igneous rock)
2. Thick (35 - 40 km, ranging to 60-70 km in some mountain ranges), low
density (2.7 g/cm3), granitic continental crust (light-colored, coarsegrained igneous rock)
Map of the Earth's tectonic plates from the US Geological Survey.
The tectonic plates move very slowly relative to one another. The rates and
directions of plate movements vary. The rate of movement has been determined
to be approximately 5 - 10 cm per year (2 - 6 inches per year), depending on
location.
All plates are moving. As plate movement occurs, the plates sometimes stick
together and then slip. This sudden slippage causes vibrations known as
earthquakes.
Tectonic plate boundaries tend to be sites of relatively intense geologic activity.
Earthquakes and volcanic eruptions occur predominantly along plate boundaries.
The interiors of plates tend to be less geologically active than the boundaries.
Evidence in support of the Theory of Plate Tectonics:
1. Shape of the coastlines - Africa and South America would fit together like
jigsaw puzzle pieces.
2. Fossil evidence
a. Glossopteris flora - a type of Late Paleozoic seed ferns (plant
fossils) that were found in Gondwanaland (India, Africa, Australia,
S. America, Antarctica)
b. Mesosaurus, a freshwater aquatic reptile whose fossils were
found in South America and Africa
The locations of certain fossil plants and animals on present-day, widely
separated continents form definite patterns (shown by the bands of
colors), if the continents are put back together. Click image for a larger
version.
c. Same stratigraphic sequence (i.e. same sequence of types of
layered sedimentary rocks)
d. Mountain belts and folded rocks would line up if you could push the
continents back together
3. Paleoclimatic evidence (Paleo = "ancient", climatic = "climate")
If you could push the continents back together, the ancient climatic zones,
as indicated by the rock types, would match up.
Layers of glacial deposits are found at same place in sequence of rocks
Note directions of glacial ice movement as indicated by striations or
grooves in the rock.
Glaciers start to form on continents from the buildup of snow. As they
grow through snow accumulation, they begin to move outwards. They do
not form in the sea and move onto the land.
Directions of glacial striations on the Gondwanaland continents only make
sense if you could push the continents back together to form a single
landmass in the past.
The directions of glacial movement would not make sense in the absence
of plate tectonics.
4. Youth of ocean basins and sea floor.
Before the seafloor rocks were sampled and dated, the hypothesis is that
they would be very old and would be covered by thick accumulations of
sediment. The deep sea drilling efforts have revealed that the basaltic
rocks of the seafloor are actually quite young. Seafloor basalt
dates to less than 200 million years (most is younger than 150
million years).
The seafloor is much younger than we expected. This is because
new seafloor is continually forming at the mid-ocean ridges and spreading
outward away from the ridge in conveyor belt fashion. The Earth is not
expanding. Older seafloor rocks have been subducted, or carried down
into the Earth's mantle in deep sea trenches and melted.
The age of the oceanic crust is shown by colors. The youngest ocean crust
is shown in red along the mid-ocean ridges (0 to 9.6 million years old),
and increasingly older crust is shown in orange, yellow, green, and blue.
The areas of the oldest ocean crust are shown in blue (156.6 to 180.0
million years), and are located farthest from the ridges, such as along the
eastern coast of North America, and the northwestern coast of Africa, and
in part of the western Pacific Ocean. Click on image to seer a
largerversion.
5. Mid-ocean ridges located near ocean centers
a. High heat flow
b. Seismic wave velocity decreases due to high temperatures
c. Valley along center of ridge (graben)
d. Volcanoes along ridge
e. Earthquakes along ridge
6. Magnetic stripes on the sea floor - One of the chief lines of evidence for
seafloor spreading comes from the examination of patterns of ancient
magnetic reversals that are recorded in the basalts of the oceanic crust.
(See below.)
Contributions to plate tectonic theory from paleomagnetism
Igneous rocks, such as seafloor basalts, become magnetized in alignment with
the Earth's current magnetic field as they cool through a specific temperature
known as the Curie point. Magnetization in older rocks has a different orientation
because older rocks cooled and crystallized when the Earth's magnetic poles
were reversed, and/or in a different location. (The magnetic poles are in slow but
continual motion. This is known as polar wandering.) It is possible to determine
the direction and distance to the north magnetic pole from the inclination and
declination of the magnetic field preserved in the rock. Ancient rocks preserve
the past positions of the north magnetic pole.
1. Recently magnetized rocks show alignment of magnetic field consistent
with Earth's current magnetic field
2. Magnetization in older rocks has different orientations (as determined by
magnetometer towed by ship)
Can determine direction to north magnetic pole and distance to north
magnetic pole from inclination and declination of magnetic field in the
rock
.
3. Polar wandering curves
Different polar wandering paths seen in rocks of different continents.
Put continents "back together" and the polar wandering curves are
superimposed (match up)
4. A test of the hypothesis of sea floor spreading (Vine and Matthews, 1963)
Magnetic reversal "stripes" are SYMMETRICAL about the ridge.
Ocean-Floor Spreading
A. In the early 1960s, the American geologists Harry H. Hess and Robert
S. Deitz independently proposed that the ocean floors are spreading.
B. During the process of ocean-floor spreading, molten rock rises up
along a midocean ridge and pushes apart the lithosphere on either
side of the ridge. The molten rock hardens and becomes new crust.
The discovery of normal and reverse magnetic strips on either side of an
ocean ridge is strong evidence for ocean-floor spreading.
5. Magnetic reversal time scale Pattern of reversals in sea floor basalts matches known reversal time scale
as determined from rocks exposed on land.
Width of magnetic stripes on sea floor is related to time.
(Wide stripes = long time; narrow stripes = short time)
What forces drive plate tectonics?
In 1962, Harry Hess proposed the hypothesis that midocean ridges represent
narrow zones where ocean crust forms (called spreading centers).
Mantle material moves upward, carrying heat.
Heat causes expansion and rising of sea floor.
Volcanism occurs (also earthquakes)
Convection in the mantle drives the system
Large scale thermal convection in the mantle.
Convection cells. Roughly circular.
Mantle heat probably due to radioactive decay
If the rising part of a convection cell is beneath a continent, it will cause it to
RIFT apart.
Also causes seafloor to rift apart at mid-ocean ridge.
Continents move along with ocean crust, and do not plow through it.
Plate tectonics is driven by the convection in the asthenosphere (part of
the Earth's mantle).
Conceptual drawing of assumed convection cells in the mantle. Below a depth of
about 700 km, the descending slab begins to soften and flow, losing its form.
Sketch showing convection cells commonly seen in boiling water or soup. This
analogy, however, does not take into account the huge differences in the size
and the flow rates of these cells.
Plate Tectonics
A. According to the theory of plate tectonics, the lithosphere is divided into
seven huge plates and a number of smaller ones.
B. New lithosphere is created where plates move apart at midocean ridges.
C. There are three kinds of plate collisions:
1. Ocean-continental plate collision occurs where a plate bearing
dense oceanic crust slides underneath a plate bearing lighter
continental crust in a region of contact called a subduction zone.
2. Oceanic-oceanic plate collision occurs where a plate bearing
oceanic crust slides underneath a second plate bearing oceanic
crust in a subduction zone.
3. Continental-continental plate collision occurs when the plate
edges are pushed together and buckle, forming a mountain range.
D. A transform fault is a type of plate boundary where the edges of two
plates slide past each other.
1. The San Andreas Fault is an example of a transform fault that has
formed between the boundary of the Pacific and North American
plates.
2. The San Francisco earthquakes of 1906 and 1989 were caused by
movement along the San Andreas Fault.
E. Two plausible mechanisms for plate motion have been proposed.
1. Convection in the asthenosphere
2. Subduction of oceanic plate margins
Types of plate boundaries
There are three major types of tectonic plate boundaries. These include:
1. Divergent plate boundaries where plates move apart from one another.
2. Convergent plate boundaries where plates move toward one another.
3. Transform plate boundaries where plates slide past one another.
Artist's cross section illustrating the main types of plate boundaries. Cross section
by José F. Vigil from This Dynamic Planet
1. Divergent - where the plates are moving apart.
Animation of divergent plate motion at a mid-ocean ridge.
Examples: mid-ocean ridges such as the Mid-Atlantic Ridge (the site of
sea-floor spreading), and continental rifts such as the east African Rift
system.
Animation of divergent plate motion.
2. Convergent - where the plates are moving toward one another.
Example of a convergent plate boundary. In this case, ocean-to-continent
convergence is shown. The deep sea trench is the site of a subduction
zone, where oceanic crust is being carried down into the mantle, where it
begins to melt. The magma rises to form volcanoes along the edge of the
continent.
Example of a convergent plate boundary. In this case, ocean-to-ocean
convergence is shown. The deep sea trench is the site of a subduction
zone, where oceanic crust is being carried down into the mantle, where it
begins to melt. The magma rises to form a volcanic island arc.
Example of a convergent plate boundary. In this case, continent-tocontinent convergence is shown. Continental crust collides with and slides
over other continental crust, forming high mountain ranges like the
Himalayas.
Examples: subduction zones which occur at deep sea trenches such as
the Marianas Trench, and sites of continental collision forming
mountain belts, such as the Himalaya Mountains, the Ural Mountains, the
Appalachian Mountains, and the Alps.
Animation of convergent plate motion.
3. Transform - where the plates are sliding past one another, such as one
sliding to the north and the adjacent plate sliding to the south.
Examples: transform faults (easily seen where they cut at right angles
to the mid-ocean ridges); includes the San Andreas fault.
Animation of transform plate motion.
Earthquakes and Earth's Interior
It is possible to study Earth's crust and even its interior using the wave energy
produced by earthquakes. This study is called seismology. Most earthquakes are
related to movements of Earth's crust, although some occur as the result of
violent volcanic eruptions. Crustal movements happen along large fracture lines
known as faults. Rocks along fault lines can be stressed because of crustal
shifting, and potential energy can be stored in the elastic rock itself. When this
stored energy exceeds the fracture point of the surrounding rock, the crust can
snap suddenly back into an unstressed state, producing an earthquake.
The center of an earthquake, no matter where it occurs inside Earth, is called the
focus. The point on the surface of Earth directly above the focus is known as the
epicenter. The locations of both the focus and epicenter of an earthquake can be
determined precisely by analyzing seismic waves that travel across Earth's
surface (surface waves) and those that travel through the interior of Earth (body
waves). Using the amplitude of seismic waves, it is possible to estimate the
amount of energy released by an earthquake. The severity of an earthquake can
be measured on either the Richter scale or the Mercalli scale.
Study of seismic waves has led to a better understanding of Earth's internal
construction. Four identifiable concentric shells exist: the solid, metallic inner
core; the liquid, metallic outer core; the dense, rocky mantle; and the thin, rocky,
outer layer called the crust. However, if the interior of Earth is studied in terms
of its behavior rather than its composition, its structure is quite different. In this
classification the first and outermost layer is called the lithosphere, which has a
thickness of about 70 km and includes all of the crust plus the upper part of the
mantle. The lithosphere is rigid and brittle, and so is relatively resistant to
deformation. Faults and earthquakes occur in this layer. The second layer, the
asthenosphere, extends downward from the bottom of the lithosphere to a depth
of about 700 km. The asthenosphere plays an essential role in continental drift
and seafloor spreading because it moves heat from the inner core of Earth
toward the surface by thermal convection. The asthenosphere is plastic and
mobile enough to allow slow but powerful convection cells to form, and these
cells provide the driving forces necessary to produce continental drift and
seafloor spreading.
A. Most earthquakes are due to the sudden movement of rock along
fracture surfaces called faults.
B. Earthquake magnitudes are expressed on the Richter scale.
Reliable earthquake predictions do not exist.
Types of Deformation
A. A fault is a rock fracture along which one side has moved relative to the
other.
B. Fault scarps are cliffs formed by faulting.
C. Folding occurs when rock strata are compressed by slow, continuous
tectonic movements of the crust.
D. Large-scale crustal movements (called tectonic changes) involve the
rising, falling, tilting, or warping of whole continents or large parts of them.
Crustal Deformation and Mountain Building
Processes by which the major features of Earth's crust are formed and changed
because of the relative movement of lithospheric plates results in the
deformation of rocks. These processes may take place suddenly, producing
earthquakes and other easily observed changes, or they may be extremely slow
and subtle. Even though not readily noticed, slow changes often have a greater
long-term impact on the structural features of Earth than do rapid small-scale
ones.
Two slow processes that produce tremendous changes in Earth's crustal features
are folding and faulting. When extreme horizontal pressure is applied, rocks
eventually reach their elastic limit and begin to buckle and fold. This process
produces wavy-looking geologic formations that consist of alternating "arches"
and "troughs" known as anticlines and synclines, respectively. Faults are
fractures in the crust caused by the shifting of surface plates. The cause of such
motion is the slow convection of plastic rock within the asthenosphere. Such
action can be vertical, which produces upthrusts, or horizontal, which leads to
compression or stretching of the crust itself. Faults can be classified as normal,
reverse, or strike-slip faults. Strike-slip faults are basically the same as the
transform faults that were discussed earlier in this chapter.
Mountains differ greatly in size as well as basic structure. Single, isolated
mountains can be formed by volcanic action; mountains can also be produced in
mountain ranges. Mountains can generally be classified by their characteristic
features as volcanic mountains, fault-block mountains, or fold mountains. The
basic theory behind mountain building involves the process of plate tectonics,
which gives us a good explanation for the hot spots, the upthrusts, and the folds
that produce major mountain
Mountain Building
A. Mountains form in a variety of ways.
1. Mountains can form due to volcanic activity.
2. Some mountains are blocks of the earth's crust raised along faults.
3. The great mountain ranges have a long, complex history involving
sedimentation, folding, faulting, igneous activity, repeated uplifts,
and deep erosions.
B. In mountains having a history of sedimentation:
1. The sedimentary layers are usually thicker than similarly aged
sedimentary layers under adjacent plains.
C. The sedimentary rocks have a complex structure resulting from
intensive compressional forces.