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Ch 1 Sec 1: Earth’s Interior
Guide for Reading
■ How have geologists learned about Earth’s inner structure?
■ What are the characteristics of Earth’s crust, mantle, and core?
Earth’s surface is constantly changing (lifting, sinking, bending &
breaking). Earth looks different today from the way it did millions
of years ago. People wonder, “What’s inside Earth?”
The extreme conditions in Earth’s interior prevent exploration far
below the surface. The deepest mine in the world reaches a depth
of 3.8 km- but it only scratches the surface. You would need to
travel more than 1,600 times that distance- over 6,000 km to reach
the center of the earth.
Geologists have used two main types of evidence to learn about
Earth’s interior:
1. Direct evidence from rock samples: Rocks from inside Earth
give geologist’s clues about Earth’s structure. Holes as deep as 12
km are drilled inside earth to get samples. Geologists can then
make inferences about the conditions deep inside Earth where
these rocks formed. In addition, forces inside Earth sometimes
blast rock to the surface from depths of more than 100 km. These
rocks provide more information.
2. Indirect evidence from seismic waves: Using data from
seismic waves produced by earthquakes, geologists have learned
that Earth’s interior is composed up of several layers. Each layer
surrounds the layer beneath it.
The three main layers of Earth vary greatly in:
Size: Crust: 5-70 km
Mantle: 2,867 km
Outer Core: 2,266 km
Inner Core: 1,216 km
Composition: Mountains & continental crust: Granite
Oceanic crust: Basalt
Core: metals- mainly Iron and Nickel.
Also: oxygen, sulfur & silicon.
Temperature- As you start to tunnel beneath the surface, the
surrounding rock is cool. Beneath the surface, the temperature
decreases for about 20 meters, then increases 1oC for every 40
meters of depth for several tens of kilometers. After that the
temperature rises more slowly, but steadily until the center of Earth
is reached. The higher temperatures are the results from the
formation of the planet and radioactive substances inside earth.
Pressure- results from a force pressing on an area. Because of the
weight of the rock above, the pressure inside Earth increases as
you go deeper.
The three main layers of Earth are:
1. the crust- is the layer of rock that forms Earth’s outer skin. The
crust is a layer of solid rock that includes both dry land and the
ocean floor. It is thinner than lower layers.
Oceanic crust consists mostly of rocks such as basalt, dark rock
with a fine texture. The crust under the ocean is thinner- as thin as
5 km thick.
Continental crust, the crust that forms the continents, consists
mainly of rocks such as granite. Granite is a rock that usually is a
light color and has a coarse texture. Continental crust is thickerthe crust making up some mountains can be up to 70 km thick.
2. the mantle- Below a boundary 40 kilometers beneath the
surface is the solid material of the mantle, a layer of hot rock.
Earth’s mantle is made up of rock that is very hot, but solid.
Scientists divide the mantle into layers based on the physical
characteristics of those layers.
a. The uppermost part of the mantle and the crust together
form a rigid layer called the lithosphere. The lithosphere
averages about 100 km thick.
b. Below the lithosphere is a soft layer called the asthenosphere.
This layer is hotter and under more pressure which makes
the rock less rigid, its somewhat soft and can bend like
plastic. Although it is still solid- it can “flow”.
c. Beneath the asthenosphere, the mantle is solid. This solid
material, called the lower mantle (mesosphere), extends all
the way to Earth’s core.
3. the core- is made mostly of the metals iron and nickel.
Scientists have found the core may also contain oxygen, sulfur and
silicon. The inner & outer core is 3,486 km thick. Because of the
great depth, the core is under a lot of pressure.
It consists of two parts:
a. a liquid outer core - The outer core is a layer of molten
metal that surrounds the inner core. Despite enormous
pressure, it is still liquid. Scientists think that movements in
the liquid outer core create Earth’s magnetic field. Because
Earth has a magnetic field, the planet acts like a giant bar
magnet. When you use a compass, the compass needle aligns
itself with the lines of force in Earth’s magnetic field.
b. a solid inner core- The inner core is a dense ball of solid
metal. The extreme pressure squeezes the atoms of iron &
nickel so much they cannot spread out & become liquid.