3. Earthquakes and Seismic Waves
Earthquakes are produced when a part
of the Earth’s uppermost layer moves
suddenly. Earthquakes produce shock
waves that travel through the Earth,
called seismic waves. They are actually
waves of energy.
4. Seismic Waves
All earthquakes produce at least two
types of seismic waves at the same
time: P and S waves. These waves are
detected and recorded by an instrument
called a seismograph. As seismic waves
move through the Earth changes occur
in their movement caused by
differences in the structure of the
Earth’s interior.
5. After observing the speeds of P waves
and S waves, scientists have concluded
that the Earth’s center, or core is
actually made up of two layers with
different characteristics.
6. The Earth’s Core
Both layers of the Earth’s core are made
of the elements iron and nickel.
7. The Inner Core
The solid inner layer is called the inner
core. The temperature reaches 5000
degrees C, the temperature at which
iron and nickel usually melt. The
extreme pressure pushes the particles
so tightly together that they remain
solid. The solid iron produces a
magnetic field.
8. The Outer Core
Surrounding the inner core is the
second layer of the Earth, called the
outer core. The outer core begins
about 2900 km below the Earth’s
surface and is 2250 km thick. The
temperatures range from 2200 to 5000
degrees C. The heat makes the iron
and nickel molten.
9. The Earth’s Mantle
The layer of the Earth directly above
the outer layer is the mantle. The
mantle extends to a depth of about
2900 km below the surface. About
88% of the volume of the Earth and
68% of the mass are in the mantle.
10. The Moho
In 1909, the Yugoslav scientist A.
Mohorovicic observe a change is the
speed of seismic waves as they moved
through the Earth. When the waves
reached a depth of 32 to 64 km their
speed increased. This indicated a
difference in density or composition of
the rock. This is the boundary between
the outermost layer and the mantle, the
Moho.
11. Composition of the Mantle
After studying rocks from volcanoes and
the ocean floor, scientists think the
mantle is made of the elements silicon,
oxygen, iron and magnesium. The
density of the mantel increases with
depth. The high temperature and
pressure in the mantle allow the solid
rock to flow slowly, thus changing
shape. This is the property of plasticity.
12. The Earth’s Crust
The Earth’s crust is its thin outermost
layer. The crust is much thinner than
the mantle and the outer and inner
cores. All life on Earth exists on or
within a few hundred meters above the
crust.
13. Composition of the Crust
The crust is made of three types of
solid rocks: igneous, sedimentary and
metamorphic rocks. The thickness of
the Earth’s crust varies. Igneous Rocks
Igneous rocks form when hot, liquid
rock from deep within the Earth cools
and hardens as it reaches the surface.
The word igneous means, “born of fire”.
14. Sedimentary Rocks
Sedimentary rocks form when
sediments- small pieces or rocks, sand
and other materials – are pressed and
cemented together by the weight of
layers that build up over long periods of
time.
15. Metamorphic Rock
Metamorphic rock forms when igneous
and sedimentary rocks are changed by
heat, pressure, or the action of
chemicals.
16. The Oceanic Crust
Crust beneath the oceans, called
oceanic crust, is less than 10 km thick.
Oceanic crust is made mostly of silicon,
oxygen, iron and magnesium.
17. The Continental Crust
The crust beneath the continental,
called the continental crust, has an
average thickness of about 32 km.
Beneath mountains, the crust is much
thick. Continental crust is made mostly
of silicon, oxygen, aluminum, calcium,
sodium and potassium.
18. The Lithosphere
The Earth’s Crust forms the upper part
of the lithosphere. The lithosphere is
the solid topmost part of the Earth. It
is between 50 and 100 km thick and is
broken up into large section called
lithospheric plates. There are at least
seven major plates.
19. The Asthensphere
The layer directly beneath the
lithosphere is called the asthenosphere.
It is 130 to 160 km thick. It is the
upper edge of the mantle. It is made of
hot, molten material that has the
property of plasticity and thus can flow
easily. The plates move on the hot
molten material that forms the
asthenosphere.
