Internal Structure of Earth: Crust, Mantle & Core, Discontinuities

The internal structure of the Earth is divided into three main layers: the crust, the mantle, and the core. Each of these layers has distinct characteristics and is separated by discontinuities, which are boundaries where seismic wave velocities change abruptly. Here’s a detailed look at each of these layers and the major discontinuities:

1. Crust

a. Continental Crust:

Thickness: 30-70 km
Composition: Primarily granitic rocks (rich in silica and aluminum, also known as sial)
Density: Approximately 2.7 g/cm³
Characteristics: Less dense, thicker, and older than oceanic crust

b. Oceanic Crust:

Thickness: 5-10 km
Composition: Primarily basaltic rocks (rich in silica and magnesium, also known as sima)
Density: Approximately 3.0 g/cm³
Characteristics: Denser, thinner, and younger than continental crust

2. Mantle

a. Upper Mantle:

Extends from the base of the crust to about 660 km depth
Composition: Peridotite (rich in iron and magnesium silicates)
Includes the lithosphere (rigid outer layer comprising the crust and uppermost mantle) and the asthenosphere (semi-rigid part that flows slowly)

b. Lower Mantle:

Extends from 660 km to about 2,900 km depth
Composition: Similar to the upper mantle but with minerals that have changed structure due to high pressure
Characteristics: More rigid and dense compared to the upper mantle

3. Core

a. Outer Core:

Extends from 2,900 km to about 5,150 km depth
Composition: Liquid iron and nickel
Characteristics: Generates Earth’s magnetic field through convection currents

b. Inner Core:

Extends from 5,150 km to the center of the Earth at about 6,371 km
Composition: Solid iron and nickel
Characteristics: Extremely high pressure and temperature, solid due to immense pressure

Discontinuities

1. Mohorovičić Discontinuity (Moho):

Boundary between the crust and the mantle
Characterized by a sudden increase in seismic wave velocities
Depth: About 35 km on average beneath continents and 5-10 km beneath ocean floors

2. Gutenberg Discontinuity:

Boundary between the mantle and the outer core
Characterized by a drastic decrease in seismic wave velocities (S-waves cannot travel through the liquid outer core)
Depth: Approximately 2,900 km

3. Lehmann Discontinuity:

Boundary between the outer core and the inner core
Characterized by an increase in seismic wave velocities (P-waves speed up as they enter the solid inner core)
Depth: Approximately 5,150 km

Summary

The internal structure of the Earth is complex, with each layer having distinct physical and chemical properties. The crust is the outermost layer, varying in thickness and composition between continental and oceanic regions. Beneath the crust lies the mantle, which is divided into the upper and lower mantle, characterized by varying degrees of rigidity and composition. The core is the innermost layer, with a liquid outer core generating the magnetic field and a solid inner core composed mainly of iron and nickel. Discontinuities such as the Moho, Gutenberg, and Lehmann discontinuities mark the boundaries between these layers, signifying abrupt changes in seismic wave velocities.