Our Earth
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| Inside our Earth |
The earth is about 460 crore year old. Its surface has been going changes throughout the years. WE still have very limited information what is inside our Earth.
A detailed study of the interior of the earth is done by Geologists. An elementary knowledge about the Earth’s structure is necessary to understand the nature of changes that take place on the surface of the Earth.
It is impossible to reach the center of the Earth because the temperature inside increases with the increase in depth. The temperature at the center is estimated to be about 50000C. Thus, it is not possible to access direct information about the interior of the Earth.
Structure of the Earth:
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| Structure of the Earth |
On the basis of varying density and other physical and chemical properties of rocks, the Earth can be divided into three concentric layer-crust, mantle and core. This arrangement can be compared to an onion with its layers one inside the other.
The Crust:
The Crust
The outermost shell, called the crust or
lithosphere is the thinnest layer. Its average thickness varies from 5 to 40
km. The average thickness f the crust on an average is about hand, the
continental crust is less denser than the oceanic crust because it is made of
light and rock layer.
The chief mineral constituents of the continental crust are silica and alumina. It is also called sial (si-silica and al-alumina).
The oceanic crust consists mainly of silica and magnesium. It is called (si-silica and ma- magnesium).and nickel. It is also called nife (ni-nickel and fe-iron). The heavy metals and plenty of iron explain the cause of Earth’s magnetism.
The Mantle:
The Mental
The mantle or the mesosphere
is the second layer of the Earth. It is located beneath the crust and extends
up to the core of the Earth. The average thickness of the mantle is 2900 km and
it is made up of rocks that are far denser that the crust. The mantle is
divided into three layers, depending on temperature and pressure, which cause
the racks to be solid or semi-solid. The temperature of the mantle is extremely
high since it contains molten magma which comes to the surface during volcanic
eruptions as lava. Iron is the constituents of the mantle.
The Core:
The Core
The innermost layer of the Earth is called the core or the barysphere. Being composed of mostly metal, it is also
known as the metallic core. It is separated from the mantle by a boundary. The
core is also divided into two parts. The outer core is composed of iron and is
in liquid state. The inner core is composed of iron and nickel. It is also
called nife ( ni-nickel and fe-iron). The heavy metals and plenty of iron explain
the cause of the Earth magnetism.
Rocks and Minerals:
Rocks
A rock can be defined as an aggregate of minerals. Earth is made up of rocks and minerals. The rocks forming the crust of hand position of the Earth crust can be called as rocks. They make up most of the soft materials of the lithosphere. It has no definite composition. Most of the rocks are mixture of various minerals in different proportions. Rocks may be hard or soft
Mineral
A mineral is a naturally occurring substance having a definite chemical composition and physical properties. A rocks having a large proportion of a single mineral so that the mineral concerned can be extracted economically from it, is called an ore of that mineral. Magnetite, an ore of iron is an example of such rocks.
Types of rocks:
Types of rocks
Rocks are usually classified into three main groups, depending upon the way in which they have been formed.
Igneous rocks:
Igneous
rocks
These
rocks are also known as primary rocks because they are believed to be the first
rocks formed on the Earth. All other rocks are ultimately derived from igneous
rocks. They covered the large percentage of the Earth’s crust. Before formation
of the crust, magma was in liquid state. Its upper layer cooled and became
surface of the Earth which gave rise to these rocks. These rocks continue to be
formed even today. Molten material in the interior of the Earth is known as
magma. When it comes out of the cracks in 100 km deep layer through volcano, it
is called lava which on cooling forms igneous rocks on surface on the Earth. When
these rocks are formed on the Earth, they are called extrusive rocks. Examples
of such rocks are basalt and rhyolite. Intrusive rocks are those igneous rocks
beneath the Earth’s surface.
Igneous rocks have the following characteristics:
- These rocks are hard and compact.
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- These rocks do not occur in distinct beds or stratas.
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- These rocks do not have remains of animals or plants.
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- These rocks do not allow water to percolate through them.
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- Other types of rocks are formed from igneous rocks.
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- Valuable minerals such as iron, gold, silver and aluminum are found in these rocks.
Sedimentary rocks:
Sedimentary rocks
sedimentary rocks are formed consolidation of sediments deposited in water bodies like rivers, lakes and seas. The sediments are derived by denudation of the rocks by agents of erosion like rivers, glaciers, winds and waves. These agents of erosion carry the eroded particles of rocks to the water bodies and deposit them there.
Such deposited material is called loose particles of gravel, sand, silt and clay in various proportions. Over a period of time, a significant thickness of sediments accumulates and it gets consolidated or compacted to form hard rocks. These are sedimentary rocks.
Sedimentary rocks are formed either mechanically (sandstone), chemically (salt) or organically (coal). These rocks are the most widespread and cover about 75 percent of the total land area of the Earth.
Sedimentary rocks have the following characteristics:
1. These rocks are softer than igneous rocks.
2. These rocks have horizontal layers, arranged one above the other.
3. These rocks are mostly formed under water and have marks of waves and mud-cracks.
4. These rocks also contain remain animals and plants, called fossils between the layers.
5. The important examples are sandstone, limestone and coal.
Metamorphic rocks:
Metamorphic rocks
As the name suggests, metamorphic rocks are rocks which have changed their state from their state from their earlier form. They are formed by subjecting any rocks type (including previously formed metamorphic rock) to different temperatures and pressures condition than those in which the original rock was formed. These temperatures and pressures condition than those in which the original rocks was formed. These temperatures and pressures are so high that they change the original mineral into other mineral types or formed by various natural processes like recrystallization etc.
Metamorphic rocks have the following characteristics.
1. These rocks after changes remain in their original position.
2. They are very hard and have a very high specific gravity.
3. These rocks do not have empty spaces in them.
Uses of Rocks and Minerals
1. In ancient times, rocks were used to make tools and weapons.
2. Rocks are widely used in the construction of roads, houses and buildings.
3. Minerals like coal and petroleum provide the sources of power.
4. Chemicals found in some sedimentary rocks are used to make fertilizers.
5. Soil is formed from rocks.
Rock Cycle
Rock Cycle
The rocks cycle in the simple term can be described as the process of transformation of one rock form to another. It is a fundamental concept in geology that describes the change among the three main rock types- sedimentary, metamorphic and igneous. These changes do not happen in a day or even year but take hundreds of years.
To understand the process, let us see the example below:
1. Igneous rocks as you studied above are formed by the cooling of molten magma which comes out through volcanoes.
2. The various agents of weathering like the temperature fluctuation, the flow of wind and water, action of plants and animals all cause erosion of the rocks and these igneous rocks keep breaking and cracking.
3. The broken pieces of rocks which may be as small as pebbles and fine sand keeps shifting and get moved by natural transportation in basins and lower regions and gradually form sedimentary rocks.
4. The layers keep forming over the sedimentary rocks and those which get buried deeper, again get subjected to the pressures and the heat of the Earth and change their state to the metamorphic rocks.
5. Similarly the intrusive rocks under the Earth surface are also subjected to heat and pressure and they change their state to the metamorphic rocks.
6. The metamorphic rocks again melt due to heat and pressures inside the Earth and mix into the existing magma inside the Earth and thus the cycle continues.