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Geography - Geomorphology - Earth’s Layers – Earth’s Composition
                                                                                                    October 27, 2017


Earth’s Layers

Earth’s layers are identified by studying various direct and indirect sources [we studied this in previous post: Earth’s Interior – Earthquake Waves – Shadow Zone].

The structure of the earth’s interior is made up of several concentric layers.

Broadly three layers can be identified—crust, mantle and the core.

Earth’s Layers based on chemical properties

a. crust,
b. mantle, and
c. Core.

Earth’s Layers - 3 layers
Earth’s Layers - crust - mantle core


Earth’s Layers – The Crust

Crust is the outer thin layer with a total thickness normally between 30-50 km.

The thickness of the crust varies under the oceanic and continental areas.

Oceanic crust is thinner (5-30 km thick) as compared to the continental crust (50-70 km thick).

The continental crust is thicker in the areas of major mountain systems. It is as much as 70 -100 km thick in the Himalayan region.
It forms 5-1.0 per cent of the earth’s volume.

Mohorovicic (Moho) discontinuity forms the boundary between crust and asthenosphere [asthenosphere is a part of mantle].

The outer covering of the crust is of sedimentary material (granitic rocks) and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature.

The lower layer of the crust consists of basaltic and ultra-basic rocks.
The continents are composed of lighter silicates—silica + aluminium (also called ‘sial’) while the oceans have the heavier silicates—silica + magnesium (also called ‘sima’).

Earth’s Layers - Crust


Earth’s Layers – Mantle

The mantle extends from Moho’s discontinuity (35 km) to a depth of 2,900 km (Moho-Discontinuity to the outer core).

The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km.

The lower mantle extends beyond the asthenosphere. It is in solid state.

The density of mantle varies between 2.9 and 3.3.

The density ranges from 3.3 to 5.7 in the lower part.

It is composed of solid rock and magma.

It forms 83 per cent of the earth’s volume.

The outer layer of the mantle is partly simatic while the inner layer is composed of wholly simatic ultra-basic rocks.

Earth’s Layers – Asthenosphere

The upper portion of the mantle is called asthenosphere.

The word astheno means weak.

It is considered to be extending up to 400 km.

It is the main source of magma that finds its way to the surface during volcanic eruptions. It has a density higher than the crust’s.

Earth’s Layers – Core

Lies between 2900 km and 6400 km below the earth’s surface.

Accounts for 16 per cent of the earth’s volume.

Core has the heaviest mineral materials of highest density.

It is composed of nickel and iron [nife].

The outer core is liquid while the inner core is solid.

A zone of mixed heavy metals + silicates separates the core from outer layers.

Earth’s Layers – Seismic Discontinuities

Mohorovicic Discontinuity (Moho) – separates the crust from the mantle, its average depth being about 35 km.

A soft asthenosphere (highly viscous, mechanically weak and ductile). It’s a part of mantle.

Gutenberg Discontinuity – lies between the mantle and the outer core. Below 2900 km from earth’s surface.

Earth’s Chemical Composition


Chemical Composition of earth

Composition of Earth’s Crust

Chemical Composition of earth's crust