Notes - Inside Our Earth-Rocks and Minerals
Category : 7th Class
Inside Our Earth-Rocks and Minerals
Have you ever dug a hole in your backyard? How deep could you dig? One metre? Two metres? Or perhaps three metres if you really work hard. The deepest mines on the earth are only about 3.9 km deep. Some geologists have dug the earth as deep as 11 km beneath its surface. However, this is just scratching the surface. The earth is approximately 6,400 km from the surface to the centre. Let us learn and understand about the interior of the earth in this chapter.
Our earth is dynamic. It is always on the move (rotating and revolving) and constantly changing internally and externally. To know how changes take place on the earth's surface, we need to study the structure of the earth. A detailed study of the interior of the earth has been done by geologists.
INTERIOR OF THE EARTH
There are multiple sections or layers of the earth. Their classification is based on their composition, according to which the earth has three layers namely; the Crust, the Mantle and the Core. The Geologists named these three layers as the Lithosphere, Pyrosphere and Barysphere. These layers have different constituent minerals, density and other features.
CRUST
The crust is the outermost and the thinnest layer of the earth. It is usually 35-80 km and sometimes 100 km thick. This part is known as the Lithosphere. The crust is divided into two layers:
(a) The lower layer of the crust is the oceanic crust. It consists of darker, denser basaltic rocks. Rocks of the oceanic crust contain silicon (Si) and magnesium (Mg). Therefore, the oceanic crust is also called Sima. It is more or less a continuous layer and forms both the ocean floors and the base of the continents.
Interior of the earth
(b) The upper layer of the crust is called the continental crust. It consists mainly of granite rocks which are of lighter colour and density. The granite rocks are more acidic. The upper crust is rich in silicon (Si) and aluminium (Al). For this reason, the upper or continental crust is also called Sial. It is a discontinuous layer which forms the continents.
MANTLE
Geography Reveals
The earth's crust is not in one piece. It is broken into pieces like a cracked eggshell. There are about 26 giant slabs-6 major and 20 small plates-known as the crustal or tectonic plates. (See page no. 110 map no. 3.1) These plates are made up of solid rocks. However, they are lighter than the mantle and so they float on the molten magma of the asthenosphere.
The mantle is a layer between the core and the crust. It is about 2900 km thick. Rocks in this layer are comparatively less dense than those in the core. The temperature range is from 100°C to n2200°C. The inner part of the mantle is, however, more dense and rigid. The upper mantle is called the asthenosphere that stretches from about 100 to 250 kilometres below the earth's surface. The asthenosphere is a viscous, partially melted portion in magma form. This magma in the asthenosphere induces convection current which causes the tectonic plates (explained later in the chapter) to move. The lower mantle is the one with various minerals. Temperature in this zone is 3000°C. It is in a semi solid state and the material found here is magma.
CORE
The core, also known as the barysphere, is the innermost layer of the earth. It is made up of very mdense rocks which are rich in nickel (Ni) and iron (Fe)1. It's radius is about 3500 km. The core is divided into two parts-the inner core (which is solid) and the outer core (which is molten). The core has extremely high temperature varying from 3000°C to 6000°C
Geography Reveals
A person who studies about the formation and composition of the earth and rocks is called a Geologist. Thus, Geology is the study of the earth's internal and external structures as well as the formation and composition of rocks and minerals.
ROCKS AND MINERALS
Rooks are naturally formed consolidated material composed of one or more minerals. Rocks, as per their formation and properties, are classified into three types-1. Igneous rocks 2. Sedimentary rocks 3. Metamorphic rocks
IGNEOUS ROCKS
The word "igneous" comes from the Latin word ignis, meaning 'fire'. Igneous rocks are formed when molten magma cools and solidifies. Igneous rocks are called primary rocks as they solidity directly into rocks from magma. There are two types of igneous rocks-(i) Intrusive Igneous rocks, and (ii) Extrusive Igneous rocks.
When lava, the fiery red molten magma, comes out from the interior of the earth and cools down on its outer surface, the resulting solidified material is called Extrusive Igneous rocks. Basalt is an Extrusive Igneous rock. Extrusive Igneous rocks are also called Volcanic rocks as they are formed from lava that comes out of a volcano.
The lava cools quickly on the earth's crust. The rapid cooling creates small crystals. These small crystals contain a low percentage of silica and high percentage of oxides and are dense. These rocks break up into very fertile soil. The Deccan Plateau is made up of basalt rocks.
Sometimes, the molten magma cools down slowly deep inside the earth's crust and forms rocks. Such rocks are called the Intrusive Igneous rocks or Plutonic rocks. The slow rate of cooling produces large crystals. Granite is an example of Intrusive Igneous rocks. Intrusive Igneous rocks are also called Plutonic rocks as their origin is within the earth's crust. Granite consists mainly of three minerals, i.e. quartz, mica and feldspar. Granite also is not very dense.
Other examples of igneous rocks are gabbro, scoria, mica and quartz.
Geography Reveals
The study of rocks is called petrology. Petrology covers the area of mineralogy also. Mineralogy is the specific study of chemical and physical properties of minerals found in rocks. Minerals are of two types:
1. Metallic minerals, which are generally found in Igneous and Metamorphic rocks, e.g., iron, gold and silver.
2. Non-metallic minerals, which are attached with the Sedimentary rocks, e.g., sulphur, salt, and petroleum.
SEDIMENTARY ROCKS
The word 'sediment' is derived from the Latin word sedimentum, meaning 'settling down'. Sedimentary rocks are hardened, compact and fine rocks. These rocks are formed in layers. Sediments carried by the agents of nature such as wind, water and ice are deposited on land or seabeds in layers. Over thousands of years, layers of these rock particles or sediments build up. Each of the layers are hardened and compacted to turn into Sedimentary rocks.
As the Sedimentary rocks have layers or strata, they are also called Stratified rocks. Sandstone, clay, chalk, shale and limestone are the examples of Sedimentary or Stratified rocks.
Geography Reveals
Fossils are the remains of dead plants and animals preserved on the layers of rocks. Thus, they are mostly found in sedimentary rocks and never in igneous.
Petroleum is also found between the two layers of Sedimentary rocks. Petroleum is formed from plant and animal remains whereas coal from plant remains only.
Sandstone is a mechanically formed Sedimentary rock. It is formed when grains of sand are compressed together.
Limestone is another example of Sedimentary rock. It is formed from the remains of shells and skeletons of small marine organisms such as corals. So, limestone has an organic origin.
Rock salt, nitrates, dolomite, and gypsum are also sedimentary rocks. They are formed when the salty water of seas and oceans evaporates. The salts in sea water precipitate to form Sedimentary rocks.
METAMORPHIC ROCKS
The term 'metamorphic' is derived from a Greek word metamorphose meaning 'change of form'.
Metamorphic rocks are those rocks whose properties and appearances have changed within the earth's crust due to high temperature or pressure or both. Metamorphic rocks can be formed either from igneous rocks or from
Sedimentary rocks.
Limestone is a Sedimentary rock. It changes to marble, a Metamorphic rock; due to of great heat and pressure. Granite, an igneous rock, changes to gneiss; a metamorphic rock in the same process. Metamorphic rocks are usually Hard rocks. These rocks do not break easily. Some examples of metamorphic rocks are slate (made from shale and clay), schist, quartzite and marble.
Petra in Jordan where tombs have been cut into sedimentary rocks
THE ROCK CYCLE
Rocks that form the earth's crust keep changing their form. Igneous rocks, when exposed to weather, are broken, eroded and deposited to other places and thus, form Sedimentary rocks. Sedimentary rocks and even igneous rocks change to metamorphic rocks due to excessive heat and pressure. When these metamorphic rocks are destroyed and buried deep beneath the earth's crust, they change into magma and once again, the formation of Igneous and
Sedimentary rocks takes place. The process of change in the forms of rocks is continuous. This endless process of formation, change and re-change, and also the destruction of rocks is known as the Rock Cycle.
Rocks as Building Blocks
The earth's crust is made up of rocks and minerals. Hard bare rocks are clearly visible at some places but at some other places, rocks are covered by layers of soil and vegetation and by some other man-made features. Soil is mostly rocks broken into small particles. Rocks differ from one another in colour, hardness or softness and in texture. Rocks are mixtures of minerals. Most minerals are compounds of several elements such as silica, aluminium, etc. As we know, there are hundreds of different minerals but only about a dozen of them make most of the earth's rocks. These minerals have helped us classify rocks into three main types.
The Importance of Rocks and Minerals
Rocks are important to us and also to all other living things in many ways.
Soil is a very important natural resource. It is formed when rocks break down into very small pieces through physical and chemical processes. These rock pieces finally turn into very fine particles to form soil.
They contain a variety of minerals such as copper, bauxite, iron and many others. These minerals are very important raw materials for manufacturing industries.
Rocks are a storehouse of fossils. Fossils provide very important information to historians and archaeologists. Fossil fuels like coal, petroleum (petrol, diesel and kerosene) and natural gas are obtained from Sedimentary rocks.
Rocks are a source of precious stones and gems such as diamond.
Rocks are used for the construction of roads, bridges and buildings. Limestone is a major raw material for the cement industry.
Geography Reveals
We use some products made from metamorphic rocks. Fill in the missing information in the following table showing examples of Metamorphic rocks, related products and their usage.
|
Metamorphic Rock |
Usage |
Objects |
|
1. Slate |
|
Blackboard |
|
2. Marble |
Building material |
|
|
3. Talc |
Sweat absorbent |
|
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