Table of Contents
- 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.
Watch video for better and quick understanding
Earth’s Layers based on chemical properties
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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 – 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.