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Earth’s Interior and Plate Tectonics
You know from experience that Earth’s surface is solid. You walk on it every day. You may have even dug into it and
found that it is often more solid once you dig and reach rock. However, Earth is not solid all the way to the center.
What Is Earth’s Interior Like?
Figure 1 shows Earth’s major compositional layers. We live on thetopmost layer of Earth—the crust. Because the crust is
relatively cool, it is made up of hard, solid rock. The crust beneath the ocean is called oceanic crust and has an average
thickness of 5–8 km (3.1–4.9 mi). Continental crust is less dense and thicker, with an average thickness of about 20–40
km (12–25 mi). The continental crust is deepest beneath high mountains, where it
commonly reaches depths of 70 km or more.
Beneath the crust lies the mantle, a layer of rock that is denser than the crust. Almost
2900 km (1800 mi) thick, the mantle makes up about 70% of Earth’s volume. Because
humans have never drilled all the way to the mantle, we do not know for sure what it is
like. However, geologic events, such as earthquakes and volcanoes, provide evidence
of the mantle’s consistency.
For the most part, the mantle is solid. The outermost part is also rigid, like the crust.
Deeper than a few hundred kilometers, however, it is extremely hot, and said to be
“plastic”—soft a d easily deformed, like a piece of gum. The center of Earth, the core
is believed to be composed mainly of iron and nickel. It has two layers. The inner core,
which is solid metal, is surrounded by the liquid metal outer core.
Earth’s interior gets warmer with depth
If you have ever been in a cave, you may have noticed that the temperature in the cave was cool. That’s because the air
and rocks beneath Earth’s surface are shielded from the warming effects of the sun. However, if you were to travel far
beneath the surface, such as into a deep mine, you would find that the temperature
becomes uncomfortably hot. South African gold mines, for instance, reach depths of up to
3 km (2 mi), and their temperatures approach 50°C (120°F). The high temperatures in
these mines are caused not by the sun but by energy that comes from Earth’s interior.
Geologists believe the mantle is much hotter than the crust, as shown in Figure 2. These
high temperatures cause the rocks in the mantle to behave plastically. This is the reason
for the inner mantle’s deformable, gumlike consistency.
The core is hotter still. On Earth’s surface, the metals contained in the core would boil at
the temperatures shown in Figure 2. Iron boils at 2750°C (4982°F), and nickel boils at
2732°C (4950°F). But in the outer core, these metals remain liquid because the pressure
due to the weight of the mantle and crust is so great that the substances in the outer core
are prevented from changing to their gaseous form. Similarly, pressure in the inner core is
so great that the atoms are forced together as a solid despite the intense heat.
Radioactive elements contribute to Earth’s high internal temperature
Earth’s interior contains radioactive isotopes.
These radioactive isotopes (mainly those of
uranium, thorium, and potassium) are quite rare. Their nuclei break up,
releasing energy as they become more stable. Because Earth is so large, it
contains enough atoms of these elements to produce a huge quantity of
energy. This energy is one of the major factors contributing to Earth’s high
internal temperature.
Plate Tectonics
Around 1915, a German scientist named Alfred Wegener noticed that the
eastern coast of South America and the western coast of Africa appeared to fit
together like pieces of a puzzle. By studying maps, Wegener found that several
other continents’ coastlines also seemed to fit together. He pieced all the continents together to form a supercontinent that
he called Pangaea (pan GEE uh). Figure 3 shows what Pangaea might have looked like approximately 200 million years
ago.
Using fossil evidence, Wegener showed that 200 million years ago the same kinds of animals lived on continents that are
now oceans apart. He argued that the animals could not have evolved on separate continents. Figure 4 shows the fossil
of a Mesosaurus found in Brazil. Identical fossils were found in western Africa, giving scientists strong evidence for a past
connection between the continents.
Evidence for Wegener’s ideas came later
The evidence for continental drift or the theory that Earth’s surface is made up of large
moving plates, was compelling. However, scientists did not have an explanation of how
continents could move. Wegener’s theory was ignored until the mid-1960s, when structures
discovered on the ocean floor gave evidence of a mechanism for the slow movement of
continents, or continental drift.
In the 1960s, evidence was discovered in the middle of the oceans that helped explain the
mechanisms of continental drift. New technology provided images of “bands” of rock on the
ocean floor with alternating magnetic polarities, like the bands illustrated in Figure 6. These
bands differ from one another in the alignment of the magnetic minerals in the rocks they
contain.
Alignment of oceanic rocks supports the theory of moving plates
As molten rock pours out onto the ocean floor, as shown in Figure 5, iron minerals such as
magnetite align themselves parallel to Earth’s magnetic field, just as compass needles do.
After the rocks cool to about 550°C (1020°F), the alignment of these magnetic regions in the
iron minerals becomes fixed like the stripes shown in Figure 6. The result is a permanent
record of Earth’s magnetic field as it was just before the rock cooled.
So why are there differently oriented magnetic bands of rock? Earth’s magnetic field has
reversed direction many times during its history, with the north magnetic pole becoming the
south magnetic pole and the south magnetic pole becoming the north magnetic pole. This
occurs on average once every 200 000 years. This process is recorded in the rocks as bands.
These magnetic bands are symmetrical on either side of the Mid-Atlantic Ridge. The rocks are
youngest near the center of the ridge. The farther away from the ridge you go, the older the
rocks appear. This suggests that the crust was moving away from the plate boundary.
Earth has plates that
move
over the mantle
The lithospere is approximately 100 km (60 mi) thick and is made up of the crust and the upper portion of the mantle. The
lithosphere is made up of about seven large pieces (and several smaller pieces) called tectonic plates. The word tectonic
refers to the structure of the crust of a planet. The continents are embedded into these plates, which fit together like
pieces of a puzzle and move in relation to one another. The theory describing the movement of plates is called plate
tectonics.
Tectonic plates move at speeds ranging from 1 to 16 cm (0.4 to 6.3 in) per year. Although this speed may seem slow,
tectonic plates have moved a considerable distance because they have been moving for hundreds of millions or billions of
years.
It is unknown exactly why tectonic plates move
Figure 7 shows the edges of Earth’s tectonic plates. The arrows indicate the direction of each plate’s movement. Note
that plate boundaries do not always coincide with continental boundaries. Some plates move toward each other, some
move away from each other, and still others move alongside each other. One hypothesis suggests that plate movement
results from convection currents in the asthenosphere, the hot, plastic portion of the mantle. The plates of the lithosphere
“float” on top of the asthenosphere.
Some scientists believe that the plates are pieces of the lithosphere that are being moved around by convection currents.
The soft rock in the asthenosphere circulates by convection, similar to the way mushy oatmeal circulates as it boils, and
this slow movement of rock might push the plates of the lithosphere along. Other scientists believe that the forces
generated by convection currents are not sufficient to move the plates, and that instead plates are driven by the force of
gravity acting on their own weight.
Plate Boundaries
The theory of plate tectonics helps scientists study and sometimes predict volcanic
eruptions and has provided information on earthquakes. Volcanoes and earthquakes,
such as the one that caused the damage shown in Figure 8, often occur where
tectonic plates come together. At these plate boundaries, many other dramatic
geological features, such as mountains and rift valleys, can occur.
Mid-oceanic ridges result from divergent boundaries
A divergent boundary occurs where two plates move apart, creating a gap between
them. When this happens, hot rock rises from the asthenosphere and cools, forming
new lithospheric rock. The two diverging plates then pull the newly formed lithosphere
away from the gap. The drop in pressure also causes the rising asthenosphere to
partially melt, forming magma which separates to form new oceanic crust.
Mid-oceanic ridges are mountain ranges at divergent boundaries in oceanic crust.
Unlike most mountains on land, which are formed by the bending and folding of
continental crust, midoceanic ridges are mountain ranges created by magma rising to Earth’s surface and cooling. Figure
9B shows how a mid-oceanic ridge forms. As the plates move apart, magma rises from between the diverging plates and
fills the gap. The new oceanic crust forms a large valley, called a rift valley, surrounded by high mountains. The most
studied mid-oceanic ridge is called the Mid- Atlantic Ridge which is shown in Figure 9A. This ridge runs roughly down the
center of the Atlantic Ocean from the Arctic Ocean to an area off the southern tip of South America.
Oceanic plates dive beneath continental plates at convergent boundaries
Knowing that lithosphere is being created, you may wonder why Earth isn’t expanding. The reason is that while new
lithosphere is formed at divergent boundaries, older lithosphere is destroyed at convergent boundaries. The Andes
Mountains, which are shown in Figure 10B, formed along a convergent boundary between an oceanic plate and the
South American continental plate. The oceanic plate, which is denser, dives beneath the continental plate and drags the
oceanic crust along with it. This process is called subduction. As shown in Figure 10A, ocean trenches, mountains, and
volcanoes are formed at subduction zones. Ocean trenches form along the boundary between two oceanic plates or
between an oceanic plate and a
continental plate. These trenches can be
very deep. The deepest is the Mariana
Trench in the Pacific Ocean. Located off
the coast of Asia, the deepest point in the
trench is more than 11 km (6.8 mi)
beneath the ocean surface. The PeruChile Trench is associated with the
formation of the Andes Mountains and is
more than 7 km (4.3 mi) deep.
Subduction of ocean crust generates volcanoes
Chains of often-explosive volcanic mountains form on the overriding plate at subduction zones—where oceanic crust
meets continental crust. As the water-bearing rocks and sediments of the oceanic plate are heated by
surrounding mantle, they release water into the overlying mantle. Water is very effective at lowering
the melting point of rock at high presure and so magma is formed and rises into the crust. Over time
the accumulation of large volumes of low-density magnetic
rock forms a chain of high mountains and plateaus.
Volcanic mountains also form at convergent boundaries. Magma rises to the surface and cools,
forming new rock. These volcanoes are formed far inland from their associated oceanic trenches.
Aconcagua (ah kawng KAH gwah), the tallest mountain in the Western Hemisphere, is a volcanic
mountain in the Andes. At a height of 6959 m (22 831 ft), the peak of Aconcagua is more than 13.8
km (8.6 mi) above the bottom of the Peru-Chile Trench.
Colliding tectonic plates create mountains
The Himalayas, shown in Figure 11, are the tallest mountains. They formed during the collision
between the continental tectonic plate containing India and the Eurasian continental plate. They
continue to grow in both width and height as the two plates continue to collide. Mount Everest, the
highest mountain in the world, is part of this range. Mount Everest’s peak is 8850 m (29 034 ft) above
sea level.
Transform fault boundaries can crack Earth
Plate movement can cause breaks in the lithosphere. Once a break occurs, rock in the lithosphere continues to move,
scraping past nearby rock. The crack where rock moves is called a fault. Faults can occur in any area where forces are
great enough to break rock. When rock moves horizontally at faults along plate boundaries, the boundary is called a
transform fault boundary as shown in Figure 12A.
Plate movement at transform fault boundaries is one cause of earthquakes. You may have heard of earthquakes along
the San Andreas fault which is shown in Figure 12B, and which runs from Mexico through California and out to sea north
of San Francisco. Transform fault boundaries occur in many places across Earth, including the ocean floor.
Chapter 2
Earthquakes and Volcanoes
Imagine rubbing two rough-sided rocks back and forth against each other. The movement won’t be smooth. Instead, the
rocks will create a vibration that is transferred to your hands. The same thing happens when rocks slide past one another
at a fault. The resulting vibrations are called earthquakes.
What Are Earthquakes?
Compare the occurrence of earthquakes, shown as red dots in Figure
13, with the plate boundaries, marked by black lines. Each red dot
marks the occurrence of an earthquake sometime between 1985 and
1995. You can see that earthquakes occur mostly at the boundaries of
tectonic plates, where the plates shift with respect to one another.
Earthquakes occur at plate boundaries
As plates move, their edges experience
immense pressure. Eventually, the stress
becomes so great that it breaks rock along a
fault line. Energy is released as seismic waves. As the seismic
waves travel through Earth, they create the shaking that we
experience during an earthquake.
The exact point inside Earth where an earthquake originates is called
the focus. Earthquake waves travel in all directions from the focus, which is often located far below
Earth’s surface. The point on the surface immediately above the focus is called the epicenter as
shown in Figure 14A. Because the epicenter is the point on Earth’s surface that is closest to the
focus, the damage there is usually greatest although damage can occur many miles from the
epicenter, as shown in Figure 14B.
Energy from earthquakes is transferred by waves
The energy released by an earthquake is measured as shock waves. Earthquakes generate three
types of waves. Longitudinal waves originate from an earthquake’s focus. Longitudinal waves
move faster through rock than other waves do and are the first waves to reach recording stations.
For this reason, longitudinal waves are also called primary, or P waves.
A longitudinal wave travels by compressing Earth’s crust in front of it and stretching the crust in back of it. You can
simulate longitudinal waves by compressing a portion of a spring and then releasing it, as shown in Figure 15A. Energy
will travel through the coil as a longitudinal wave. The second type of wave is a transverse wave. Transverse waves move
more slowly than longitudinal waves. Thus, these slower waves are called secondary or S waves. The motion of a
transverse wave is similar to that of the wave created when a rope is shaken up and down, as shown in Figure 15B.
Waves move through Earth and along its surface
Both P waves and S waves spread out from the focus in all directions, like light from a
light bulb. In contrast, the third type of wave moves only across Earth’s surface. These
waves, called are the result of Earth’s entire mass shaking like a bell that has been rung.
Earth’s surface bends and reshapes as it shakes. The resulting rolling motion of Earth’s
surface is a combination of up-and-down motion and back-and-forth motion. In this type
of wave, points on Earth’s surface have a circular motion, like the movement of ocean
waves far from shore. Surface waves, such as the ones shown in Figure 16, cause more
destruction than either P waves or S waves. P waves and S waves shake buildings back
and forth or up and down at relatively high frequencies. But the rolling action of surface
waves, with their longer wavelengths, can cause buildings to collapse.
Measuring Earthquakes
Because energy from earthquakes is transferred by waves, scientists can measure the
waves to learn about earthquakes, and about the interior of the Earth through which the
waves travel. Scientists hope that learning more will give them tools to predict earthquakes and save lives.
Seismologists detect and measure earthquakes
Seismology is the study of earthquakes. Seismologists use sensitive machines called
seismographs to record data about earthquakes, including P waves, S waves, and surface waves.
Seismographs use inertia to measure ground motion during an earthquake. Examine the
seismograph in Figure 17. A stationary pendulum hangs from a support fastened to Earth as a
drum of paper turns beneath the pendulum with a pen at its tip. When Earth does not shake, the
seismograph records an almost straight line. If Earth shakes, the base of the seismograph moves,
but the pendulum is protected from Earth’s movement by the string. The pendulum draws zigzag
lines on the paper that indicate an earthquake has occurred. Records of seismic activity are called
seismograms. Figure 17 shows a typical seismogram.
Three seismograph stations are necessary to locate the epicenter
of an earthquake
There are more than 1000 seismograph stations across the world. At
each station, three seismographs are used to measure different
motions: north to south, east to west, and up and down. P waves, the
first to be recorded by seismographs, make a series of small, zigzag
lines. S waves arrive later, appearing as larger, more ragged lines. Surface waves arrive last
and make the largest lines. The difference in time between the arrival of P waves and the arrival
of S waves enables seismologists to calculate the distance between the seismograph station
and the earthquake’s focus. When the distances from three seismograph stations are
calculated, three circles can be drawn on a map as shown in Figure 18. Each circle has the
geographic locations of a seismograph station as its center, with the radius being the distance to
the epicenter. Therefore, the epicenter can be found by finding where all three circles intersect.
Geologists use seismographs to investigate Earth’s interior
Seismologists have found that S waves do not reach seismographs on the side
of
Earth’s core opposite the focus, as shown in Figure 19. This is evidence that
part of the core is liquid because S waves, which are transverse waves, cannot
travel through a liquid. By comparing seismograms recorded during
earthquakes, seismologists have noticed that the velocity of seismic waves
varies depending on where the waves are measured. Waves change speed
and direction whenever the density of the material they are traveling through
changes. The differences in velocity suggest that Earth’s interior consists of
several layers of different densities. By comparing data, scientists have
constructed the model of Earth’s interior described in Section 1.
The Richter scale is a measure of the magnitude of earthquakes
The Richter scale is a measure of the energy released by an earthquake. The 1964 Alaskan earthquake, with a
magnitude of 8.4, is the largest earthquake of recent times. Each step on the Richter scale represents a 30-fold increase
in the energy released.
So an earthquake of magnitude 8 releases 304, or 810,000, times as much energy as one of magnitude 4! Table 1
summarizes the effects and number of earthquakes with varying magnitudes. Notice that low magnitude earthquakes
occur frequently. The Richter scale cannot predict how severe an earthquake will be. The amount of damage depends on
several factors, such as the distance between populated areas and the epicenter, and the type of construction used in
area buildings. The Armenian earthquake of 1988 and the San Francisco earthquake of 1989 both had a magnitude of 7
on the Richter scale. In Armenia, there was devastating property damage, and more than 25,000 people died. In contrast,
only 70 people died in San Francisco.
Why was there such a big difference between the effects of these two earthquakes? The focus of the Armenian
earthquake was 5 km down, but in San Francisco it was 19 km down. The deeper the focus, the less the effects will be felt
at the surface. Also, the rock in San Francisco is harder than that of Armenia. Softer rock breaks apart and changes
position more easily than rigid rock. The difference in fatalities was mainly due to building construction. Other factors that
determine how destructive an earthquake will be are the time of day that the event occurs, soil composition, and how
saturated the ground is.
Scientists are trying to predict earthquakes
In the past, people would try to predict earthquakes by watching animals for strange behavior.
Today, scientists are trying to measure changes in Earth’s crust that can signal an earthquake.
Scientists might someday be able to warn people of an impending earthquake and save lives by
learning to observe rock for signs of stress and strain. The random nature of earthquake rupture
makes prediction extremely difficult, but finding a reliable system could save tens of thousands of
lives in the future.
Volcanoes
A volcano is any opening in Earth’s crust through which magma has reached Earth’s surface.
These openings are called vents. Volcanoes often form hills or mountains as materials pour or
explode from the vent, as shown in Figure 20. Volcanoes release molten rock, ash, and a variety
of gases that result from melting in the mantle or in the crust.
Volcanoes generally have one central vent, but they can also have several smaller vents. Magma
from inside a volcano can reach Earth’s surface through any of these vents. When magma
reaches the surface, its physical behavior changes, and it is called lava.
Shield volcanoes have mild eruptions
Magma rich in iron and magnesium is very fluid and forms lava that tends to flow great distances.
The eruptions are usually mild and can occur several times. The buildup of this kind of lava
produces a gently sloping mountain, called a shield volcano. Shield volcanoes are some of the
largest volcanoes. Mauna Loa, in Hawaii, is a shield volcano, as shown in Figure 21A. Mauna Loa’s summit is more than
4000 m (13 000 ft) above sea level and more than 9020 m (29 500 ft) above the sea floor.
Composite volcanoes have trapped gas
Composite volcanoes are made up of alternating layers of ash, cinders, and lava. Their magma is rich in silica and
therefore is much more viscous than the magma of a shield volcano. Gases are trapped in the magma, causing eruptions
that alternate between flows and explosive activity that produces cinders and ash. Composite volcanoes are typically
thousands of meters high, with much steeper slopes than shield volcanoes. Japan’s Mount Fuji, shown in Figure 21B, is a
composite volcano. Mount St. Helens, Mount Rainier, Mount Hood, and Mount Shasta, all in the western United States,
are also composite volcanoes.
Cinder cones are the most abundant volcano
Cinder cones are the smallest and most abundant volcanoes. When large amounts of gas are trapped in magma, violent
eruptions occur—vast quantities of hot ash and lava are thrown from the vent. These particles then fall to the ground
around the vent, forming the cone. Cinder cones tend to be active for only a short time and then become dormant. As
shown in Figure 21C, Parícutin (pah REE koo teen), in Mexico, is a cinder cone. Parícutin erupted in 1943. After 2 years,
the volcano’s cone had grown to a height of 450 m (1480 ft). The eruptions finally ended in 1952. Volcanoes form not
only on land but also under the oceans. In shallow water, volcanoes can erupt violently, forming clouds of ash and steam.
An underwater volcano is called a seamount and looks like a composite volcano.
Most volcanoes occur at convergent plate boundaries
Like earthquakes, volcanoes are linked to plate movement. Volcanoes are common all around the edges of the Pacific
Ocean, where oceanic tectonic plates collide with continental plates. In fact, 75% of the active volcanoes on Earth are
located in these areas. As seen in Figure 22, the volcanoes around the Pacific Ocean lie in a zone known as the Ring of
Fire. As a plate sinks at a convergent boundary, it causes melting in the mantle and magma rises to the surface. The
volcanoes that result form the edges of the Ring of Fire. These volcanoes tend to erupt cooler, less-fluid lava and clouds
of ash and gases. The high-viscosity lava makes it difficult for the gases to escape. Gas pressure builds up, causing
explosive eruptions.
Underwater volcanoes occur at divergent plate boundaries
As plates move apart at divergent boundaries, magma rises to fill in the gap. This magma creates the volcanic mountains
that form the ridges around a central rift valley. The volcanic island of Iceland, in the North Atlantic Ocean, is on the MidAtlantic Ridge. The island is continuously expanding from its center; the eastern and western sides of the island are
growing outward in opposite directions. As a result, a great deal of geologic activity, such as volcanoes and hot springs,
occurs on the island.
Volcanoes occur at hot spots
Some volcanoes occur in the middle of plates. They occur because mushroom-shaped
trails of hot rock, called mantle plumes, rise from deep inside the mantle, melt as they
rise, and erupt from volcanoes at hot spots at the surface. When mantle plumes form
below oceanic plates, lava and ash build up on the ocean floor. If the resulting volcanoes
grow large enough, they break through the water’s surface and become islands. As the
oceanic plate continues moving, however, the mantle plume does not move along with it.
The plume continues to rise under the moving oceanic plate, and a new volcano is
formed at a different point. A “trail” in the form of a chain of extinct volcanic islands is left
behind. The Hawaiian Islands lie in a line that roughly corresponds to the motion of the
Pacific plate. The island of Hawaii is the most recently formed in the chain, and contains
the active volcanoes situated over the mantle plume. Volcanic activity produces fertile
soil which helps tropical plants, like those shown in Figure 23, grow.