Earth’s Structure Unit

by | Aug 27, 2024 | Physical Geology & Geomorphology, Structural Geology

Structure-of-the-earth

Three primary structural units of the earth

  1. CRUST
  2. MANTLE
  3. CORE

These structures are divided based on seismic waves (P and S wave velocities)

earth structure

1. Major Characteristics of Crust

  1. The crust is the Outermost and thinnest layer
  2. It is relatively cool and consists of hard rocks
  3. The oceanic crust is about 5-10 km thick and has a basaltic composition dominated by silica and magnesium.
  4. The continental crust is about 20-40 km thick, but it can be 70 km thick under mountains. Its composition is granitic (Silica and Aluminum dominantly).

2. Major Characteristics of Mantle

  • The mantle lies directly below the crust.
  • It is almost 2900 kilometers thick, making up 80 percent of Earth’s volume.
  •  The chemical composition may be similar throughout the mantle
  • Temperature and pressure increase with depth, resulting in the strength of mantle rock varying with depth and creating layering within the mantle.
  • The upper part of the mantle consists of two layers: a) Lithosphere and b) Asthenosphere
Lithosphere and Asthenosphere

2.1 Lithosphere

  • The outer part of the Earth, including both the uppermost mantle and the crust, makes up the lithosphere
  • its mechanical behavior is similar to that of the crust.
  • The lithosphere can be as thin as 10 kilometers where tectonic plates separate. It is about 75 kilometers thick beneath the ocean and 125 kilometers under the continents.
  • A tectonic (or lithospheric) plate is a segment of the lithosphere.

2.2 Asthenosphere

  • The asthenosphere extends from the base of the lithosphere to about 350 kilometers.
  • It gradually increases temperature with depth, resulting in small degrees of partial melting, possibly as much as 10% in regions of high heat flow. This partial melt is an essential source of magma and a lubricant to ease the tectonic movements of the lithospheric plate.
  • This change in rock properties occurs over a vertical distance of only a few kilometers.
  • This zone also called as low velocity zone where the velocities of s wave is decrease.
  • So the asthenosphere flows slowly, perhaps at a rate of a few centimeters per year (Lithospheric plates glide slowly over the asthenosphere like sheets of ice drifting across a pond )
  • At the base of the asthenosphere, increasing pressure causes the mantle to become mechanically stronger, and it remains so all the way down to the core.

3. Major Characteristics of Core

  • The core is the innermost of the Earth’s layers.
  • Outer core: 2900 to 5150 km (liquid)
  • Inner core: 5150 to 6370 km ( solid)
  • Overall, it is a sphere with a radius of about 3470 kilometers, is composed largely of iron and nickel, and has a density of about 12 x 103  Kg/m3 
  • At the boundary of the outer core, P-waves are marked by an abrupt reduction in velocity, bent inwards, and producing a shadow zone. The disappearance of S-waves also suggests that it is in a liquid state (molten) because of the high temperature in that region.
  • Near its center, the core’s temperature is about 6000ºC, as hot as the Sun’s surface. The pressure is greater than 1 million times that of the Earth’s atmosphere at sea level.
  • This extreme pressure overwhelms the temperature effect and compresses the inner core to a solid.