Structure of the Earth: Important Facts
Geology- It is the science that deals with nature and earth history. Example- the study of rocks and stones; how the earth was formed, etc.
Physiography– It is the study of physical patterns and processes of the earth.
Endogenic Forces– Horizontal and vertical movements caused by the forces coming from the origin of the earth.
Exogenic Forces– External processes that occur above the earth’s surface.
Theory of tectonic plates– The crust of the earth has been formed out of seven major and some minor plates. The movement of the plates results in the building up of stresses leading to folding, faulting, and volcanic activity. Given by scientist Alfred Wegener in 1912.
Structure of the Earth:
Most of the information about the interior of the earth is obtained indirectly as samples from deep inside cannot be collected.
Earthquakes, gravitation, magnetic field, and meteors include some of the indirect sources.
Volcanic eruptions, hot springs, rocks, deep drillings, deep mines, etc are direct sources of internal information.
Just like an onion, the earth is made up of several concentric layers with one inside another.
The uppermost layer over the earth’s surface is called the crust. It is the thinnest of all the layers, just like an eggshell. It is about 35 km on the continental masses and only 5 km on the ocean floors. It is lighter than the layer beneath it and generally density of material goes on increasing as we go down. Thus core (NiFe – Nickle+Ferrous) is the heaviest.
Oxygen is the largest constituent of the Crust. It is present in form of oxides of various elements. Silicon is the second-largest substance. The upper layer of the crust is very thin and is made up of sedimentary rocks, while most of the crust is composed of crystalline igneous and metamorphic rocks which are generally acidic in nature.
Half of the crust is made from Feldspar which is a mineral made of Silicon, Oxygen, and other elements.
Crust is of two types:
- Continental Crust
- Oceanic Crust
The main mineral constituents of the ‘continental crust’ are silica and alumina. It is thus called ‘sial‘ (si- silica, and al- alumina). It is lighter than the oceanic crust.
The ‘oceanic crust’ mainly consists of silica and magnesium; it is therefore called ‘sima‘ (si- silica, and ma- magnesium).
Oceanic Crust vs Continental Crust:
Oceanic crust is thinner as compared to the continental crust. The mean thickness of the oceanic crust is 5-10 km whereas that of the continental is around 40 km.
Oceanic crust is made up of heavier rocks having a density of 3 g/cm3. This type of rock found in the oceanic crust is Basalt. The mean density of material in the Continental crust is 2.6 g/cm3.
The second layer is the mantle. It has a higher density than that of the crust. Mantle temperatures range from about 2800°C near the core to about 1800°C near the crust. The mantle is the largest of the Earth’s layers, making up more than 80 percent of the Earth’s total volume. The lower mantle is hotter than the upper mantle, but it is largely rigid, as a result of intense pressure at this depth.
Aesthenosphere – The mantle contains a weaker zone called Asthenosphere. It is from this that the molten rock materials find their way to the surface. The asthenosphere is a plastic layer type which has high temperature and upon which lithosphere floats. The material in the upper mantle portion is called magma. The slow movement of the asthenosphere also disturbs the layer of the lithosphere and leads to ‘folding’ and ‘faulting’ (termed as tectonic activities).
The Third Layer is the core, the innermost layer is the core with a radius of about 3500 km. It is mainly made up of nickel and iron and is called ‘nife’ (ni – nickel and fe – ferrous i.e. iron). The central core has a very high temperature (somewhere between 3000°C and 5000°C) and pressure. Due to this, it remains in solid-state.
The lithosphere is the solid crust or the hard top layer of the earth (covers both Oceans and Land surface). It is made up of rocks and minerals and covered by a thin layer of soil. It is an irregular surface with various landforms such as mountains, plateaus, plains, valleys, etc. Landforms are found over the continents and also on the ocean floors.
The lithosphere floats on the plastic asthenosphere, much like an iceberg floats in water, in an equilibrium state known as isostasy. As erosion reduces the height and weight of a mountain block over time, it becomes lighter and floats higher on the asthenosphere by the process of isostasy. This causes the block to rise, making the mountain higher again and subject to more erosion. The process continues until the mountains become a range of low hills with shallow mountain roots.
Because the asthenosphere is soft and plastic, the rigid lithosphere can move easily over it. The lithospheric shell is divided into large pieces called lithospheric plates. A single plate can be as large as a continent and move independently of the plates that surround it- like a great slab of floating ice on the polar sea. Lithospheric plates can separate from one another at one location, while elsewhere they may collide in crushing impacts that raise great mountains. The major relief features of the Earth- its continents and ocean basins- were created by the continuous movements of these plates in geologic timescales on the surface of the Earth.