Chapter 3
The Surface and Interior of the Earth
If we look from the
space, the planet Earth appears to be a large round ball that shines bright and
blue. We have already learnt in our
previous class how the earth was formed about 4.6 billion years ago. We also know that sun is a star and the
planets revolving around it had come out of it in the distant past. However, the earth is the only planet where
conditions are favourable for sustenance of life. Since its birth, the earth as well as other
planets are undergoing changes. Let us,
therefore, study the structure of the earth's interior and the internal forces
which have led to the formation of its surface.
EARTH'S SURFACE The surface of the earth constitutes a great assemblage
of physical features. Our earth has a
variety of landforms on its surface.
These are huge mountains and deep valleys, vast stretches of plains do
you know? and tablelands or
plateaus. This variation in terms of
height and the highest point of the earth's surface is Mount Everest depth
differs from high mountains (8,850m above the mean sea level) in the
Himalayas. The to lowlands and is
measured from lowest point is the bottom of the Marina Trench (11,034m a
baseline called the mean sea below the mean sea level) in the Pacific
Ocean. level. THE INTERIOR OF THE EARTH The interior of the
earth has remained a mystery for years.
We know little about it because deep down, the temperature of the earth
is very high. The intensive heat neither
allows us to dig deep non Is there any possibility of living organisms deep
inside the earth, say at 3 to 8 km below the surface. However, for extracting gold from mines, we
have been able to dig as deep as 3.5 km below the earth's surface. Our knowledge of earth's interior is,
therefore, based on indirect observations, like: Do You Know? the behavior of seismic waves or earthquakes. the incident of volcanoes. earthquake seismology is the science of the
study of the (16
The earthquake is the shaking and trembling of the earth's
surface, caused by internal movements.
The shock - waves generated by an earthquake have their origin at a
focus inside the earth. These waves
then, travel towards the earth's surface in different directions and with
different speed. The speed of these
waves depends upon the nature of the material they pass through there are two
main types of seismic waves. Devastation
by an Earthquake in Nepal The P - Wave or Primary wave which can travel through
both solid and liquid mediums and is faster in speed. The S - wave or Secondary wave, on the other
hand, can pass through only a solid medium and that too at a slower speed. The study of the behavior of these waves
tells us about the earth's interior. Do
you know? A devastating earthquake
struck the Bhuj area of Gujarat on 26 January 2001. It killed thousands of
people and caused extensive damage to property Barren island is an active
volcano located in Andaman and Nicobar islands of India. STRUCTURE OF THE EARTH The earth's interior
is made up of three layers - the Crust, the Mantle and the Core, The Crust
CRUST UPLR NANIE MANTLE 22 The thin solid layer covering the outside of the
earth is called the Crust. The thickness
of the crust differs from place to place.
It is thicker under the continents than on the ocean floor. The average thickness of the crust is 60–100
km, Beneath the sedimentary rocks of the solid earth, the rocks are rich in
SIAL (silica + aluminum). SIAL is made
of lighter rock material. OUTER CORE WER
CORE DIAMETER 12:19 Interior of the Earth 17
The Mantle Beneath the crust is a very thick layer of rocks
called the Mantle. The mantle extends up
to 2900 km in depth. The entire mantle
is not homogenous The mantle is sub - divided into two parts namely, Do You
Know? the upper mantle and the lower
mantle. The upper mantle is up to 100 km
in depth. Beyond 2 The zone that
separates the crust and mantle is called Moho and Asthenosphere. this lies the lower mantle. It is made up of silica and magnesium
(SIMA). The Core Thermosphe Mesotho Stro
Tropospher Crust Upper Mantin Mantie The innermost part of the earth is known
as Core. The core has a thickness of
3470 km. It is further divided into
inner core and outer core. The core of
the earth is rich in iron and nickel (NIFE).
The temperature inside the earth converts the solid rock into molten
state. Sometimes, the hot material
called the lava comes out through cracks or joints. When the molten material comes out from
beneath a sea floor, it creates a new sea floor. On the other hand, the lava deposited on the
land surface makes mountains and plateaus.
Outer Com Inner Com Inner Core Sous Rock THE EARTH: A TRASURE OF ROCK
MATERIALS A rock is a naturally occurring solid substance made up of a mixture
of minerals. All rocks are not
hard. They can be as hard as fast
cooling, fine crystals extrusive granite and as soft as clay or chalk. magma chamber They are found in different
shapes, sizes and colors. There are variety
medium cooling medium crystal size of rocks found on the earth's crust. Geologists have classified the Intrusive slow
cooling Igneous rock rocks into three large groups on large crystal size the
basis of their formation. These are
Igneous, Sedimentary and Metamorphic rocks.
Igneous rocks 18
The word 'igneous means' coming out of fire. Thus, the igneous rocks are formed due to
intensive heat and pressure where molten material called magma comes out and
solidifles over the earth's surface as lava.
The cooling and solidification of magma can take place inside the earth
forming Intrusive Igneous rocks, for example, Granite. If it cools over the earth's surface, It
forms Extrusive Igneous rocks, for example, Basalt. The size of the mineral crystal present in a
rock depends on the rate of cooling. In
general, slow cooling inside the earth results in large size crystals. Igneous rocks are also called primary rocks
as they were formed first over the earth's surface Since their formation, the
igneous rocks are subjected to weathering and denudation over the years by the
agents of gradation. The continuous
action of these agents on rocks breaks them into small rock pieces that are
carried by rivers. They are then
deposited on the river beds or sea beds. These deposits are called sediments which form
layers, one above the other. Due to the
pressure of the above layers and the presence of cementing particles, they are
hardened to form sedimentary rocks. This
type of rocks are layered and therefore, are also called stratified rocks. Sandstone, limestone, clay, shale, chalk are
some examples of sedimentary rocks.
Sometimes, the remains of animals and plants get buried and trapped
under the layers, and over a period of time to get converted into coal, oil and
natural gas. That is why, they are
called fossil fuels. The fossil fuels
have a great economic importance all over the world as they are a major source
of energy rocks which are basically igneous or sedimentary in nature, get
changed in character and appearance due to the heat and pressure and are called
metamorphic rocks. For instance, granite
is igneous but changes into metamorphic as gneiss. The limestone is a sedimentary rock but
changes into metamorphic as marble.
Alike, shale may be transformed by great pressure into slate. Igneous TOCK magma Volcanic eruption sediment
metamorphic Hedimentary rock Rock cycle 19
Minerals As stated
earlier, a rock has single or a combination of several minerals in addition to
mass of th earth material. These minerals
are present in small or large quantities at any given site of the eart The most
common minerals occur inside the earth are iron, silver, gold, mica, bauxite
and lead. The common salt that we use in
our food is also a mineral. Of all the
minerals known to us, diamond is the hardest substance whereas graphite, black
and greasy in nature, is the softest.
Minerals are identified on the basis of their color, hardness and
lustre. Bauxite Iron Keywords geologist:
a scientist who studies geology (study of earth). mean sea level: the level of the ocean
surface between the level of mean high tide and low tide. Mineral: a solid inorganic substance of
natural phenomena. seismic waves: a wave
of energy generated by an earthquake or other earth vibrations that travel
withi the earth or along its surface. 20
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