Download What Is Inside Earth?

What Is Inside Earth?
¯ Figure 1 Over time, the
Grand Canyon in Arizona was
carved out by the flowing water
of the Colorado River. We can see
that Earth’s surface is constantly
changing. But what is happening
deeper inside Earth?
¯ How do Earth’s three
main layers differ from
one another?
¯ How does heat move
through Earth’s interior?
VOCABULARY
geologist
outer core
crust
inner core
continent
conduction
convection
mantle
core
convection
current
eologists are scientists who study Earth. They want to
know about the materials that Earth is made of and how
Earth has changed during its history. Geologists cannot
drill deep enough to study Earth’s interior. The deepest well ever
drilled is only about 12 kilometers (7.5 miles) deep. But the distance to Earth’s center is more than 6,300 kilometers (almost
4,000 miles). So geologists use other ways to collect information
about what it is like inside Earth.
Scientists get clues about Earth’s interior by studying the lava
of volcanoes, which comes from deep inside Earth. Other evidence about Earth’s interior comes from studying certain rocks
at Earth’s surface. These rocks actually formed deep inside Earth.
Over time, they were pushed up closer to the surface and exposed
when the rock above them was worn away. Geologists also study
earthquakes. The energy waves produced by earthquakes move
differently through different materials. By measuring the speed of
earthquake waves and the paths they take, geologists have learned
much about the different materials that make up Earth.
Geologists have learned that Earth is made up of three main
layers: the crust, the mantle, and the core. Earth’s outermost layer
is the crust. The crust covers Earth’s entire surface and varies in
thickness from 6 kilometers (about 4 miles) to 90 kilometers
(about 56 miles). The crust is thinnest under the oceans and
thickest under the continents, or large land masses. The crust is
composed of rock with a thin layer of soil over it. Crust on the
ocean floor is made mostly of the rock basalt. Continental crust is
mostly granite.
G
Under Earth’s crust is the mantle, which is
made of rock that contains iron and magnesium. The mantle is Earth’s thickest layer and it
Inakes up most of Earth’s mass. The upper part
of the mantle is rigid. The lower part of the
mantle flows very slowly.
Earth’s center, called the core, is made
mostly of the metals iron and nickel. The
outer core is liquid and flows slowly. The
inner core is solid.
The density of the materials inside Earth
increases with depth. The core and the lower
mantle are made of heavier materials than the
layers above them. They are also under great
pressure from the weight of the rock above. So
the core and the lower mantle have a greater
density than the upper mantle and crust.
The temperature of the materials inside
Earth also increases with depth. The inner core
is the hottest layer of Earth. Scientists think the
inner core may be as hot as the surface of the
Sun! Heat energy moves from warmer to cooler
areas, so heat moves outward from the core.
The movement of heat energy from one place
to another is called heat transfer. Heat from
Earth’s
Layers
deep inside Earth reaches the surface mainly
through two processes: conduction and
convection.
Conduction is the transfer of heat energy
through solid matter as one particle strikes
another. Heat moves through Earth’s core and
crust by conduction. The same process is at
work when heat from a stove burner is transferred to a pot on the stove.
Convection is the transfer of heat energy
by flowing material such as a liquid or gas. Heat
moves through Earth’s mantle by convection.
As matter in the lower mantle is heated, it
expands, becomes less dense, and is pushed
upward by the cooler, denser material around
it. As the matter rises, it cools and becomes
more dense. As its density increases, the matter
sinks back down. Then it is heated again
and the cycle repeats. This cycling of heated
matter due to density differences is called a
convection current. Some scientists say that
convection occurs throughout the mantle. Others
suggest that this important process takes place
only in the upper mantle. Convection currents
cause dramatic changes on Earth’s surface.
Figure 2 Earth’s rock layers get denser
and hotter with depth. In the mantle,
heated rock moves slowly in circular
paths called convection currents.
crust
6-90 km (4-56 mi)thick
up to 440°C (825°F)
mantle
2,900 km (1,800 mi} thick
1,500°C (2,800°F)
2,200 km (1,367 mi) thick
3,800°C (6,900°F)
1,250 km (777 mi) thick
more than 5,000°C (9,000°F)
How Has Earth’s Surface
Changed Over Time?
Breakup of Pangaea
B. 135 million years ago
¯ Figure 3 This series of maps shows the probable positions
of the continents following the breakup of the supercontinent
Pangaea, more than 200 million years ago.
¯ What evidence supported the
theory of continental drift?
¯ How does plate tectonics
differ from the original
theory of continental drift?
¯ Why is sea-floor spreading
important?
VOCABULARY
fossil
sonar
Pangaea
mid-ocean
ridge
continental
drift
rift
lithosphere
magma
plate
asthenosphere
trench
subduction
plate tectonics
sea-floor
spreading
Theory of Continental Drift
n 1915 German scientist Alfred Wegener proposed a theory
about Earth’s continents. He noticed that the coastlines of
South America and Africa seem to fit together like puzzle
pieces. Wegener recognized that the types of rocks in South
America match those in Africa. He also found that fossils -the remains, imprints, or traces of living things--in the rocks
on both continents were identical. Based on this evidence,
Wegener concluded that all the continents were once joined
in a single, huge land mass.
Wegener called the giant continent Pangaea, meaning "all
lands." He believed that Pangaea split apart long ago to form
today’s continents. The continents then slid or drifted across the
globe to their present positions (Figure 3). The idea that continents move from one part of Earth to another is called
continental drift.
I
(Figure 4). Plates can be made up of both contb
nental crust and oceanic crust.
Wegener’s theory of continental drift lacked
Underneath the lithosphere is a region of
a convincing explanation of how or why the
upper mantle known as the asthenosphere.
continents moved. Wegener had suggested
This zone of the upper mantle, though solid, is
that the continents moved on their own. He
hot enough to bend and change shape like
thought they "plowed" through the ocean
putty. Convection currents from deep inside
floor. New technology later gave scientists a
Earth can cause the asthenosphere to flow slowtool to learn how the continents really moved.
ly. As the asthenosphere moves, it carries the
Earth’s crust and the very top part of the
plates of the lithosphere. As a result, the sizes,
upper mantle make up a region called the
shapes, and positions of Earth’s continents and
lithosphere. The lithosphere is the brittle
oceans are always changing.
outer layer of Earth. The lithosphere is solid,
Some plates have moved thousands of miles
but it is not an immovable shell. Scientists
over millions of years. Earth’s plates continue to
discovered that the lithosphere is made up of
move today, at rates of a few centimeters a year.
moving sections, or plates. Earth has about
The theory that explains how and why plates
a dozen large plates and several smaller plates
move is called plate tectonics.
Plate Tectonics
~Earth’s Moving Plates
Key
¯ Figure 4 The positions of Earth’s continents are
still changing. Plate movements are causing the
Pacific Ocean to get smaller and the Atlantic Ocean
to get bigger. How do you think Earth will look 65
million years from now?
¯ Figure 5 The recycling of crust that occurs in sea-floor spreading causes oceanic crust to be billions of years
younger than continental crust.
Sea-Floor Spreading
How did scientists arrive at the idea of plate
tectonics? When Wegener proposed continental
drift, little was known about the ocean floor.
Then, in the 1950s, scientists began to map
the ocean floor using sonar technology. In
sonar mapping, ships bounce sound waves
off the ocean bottom. The time between sending the sound and receiving its echo is used to
calculate the depth of the ocean at that point.
The sonar maps showed a long, narrow
chain of mountains rising from the ocean floor.
This underwater mountain range is called the
mid-ocean ridge. A valley called a rift
runs along the crest of the mid-ocean ridge.
Scientists discovered that large amounts of heat
were flowing from the rift. That discovery led
scientists to conclude that the mid-ocean ridge
formed above huge cracks in the oceanic crust.
In the early 1960s, geologist Harry Hess
suggested that new crest forms at the midocean ridge. Scientists found that melted rock,
or magma, from the mantle rises out of cracks
in the rift. The magma hardens and forms new
crust, which piles high to form tire ridges. As
more magma rises, it pushes the older crust
away on both sides.
Hess also suggested that old oceanic crust
moves from the ridge toward underwater
trenches. Trenches are narrow, deep regions
of the ocean floor at the edges of plates. At
trenches, old oceanic crust is carried down into
the mantle as one plate is forced under another.
This sliding of one plate under another is called
subduction. The subducted crust sinks into
the mantle and melts.
This process of oceanic crust being created
at the mid-ocean ridge, moving sideways away
from the ridge, and plunging into the mantle at
trenches is known as sea-floor spreading. By
the late 1960s, discoveries about sea-floor
spreading led scientists to develop and accept
plate tectonic theory.
¯ Figure 6 An underwater mountain range winds
around Earth for about 70,000 kilometers (43,500 miles).