4. The Earth - Lithosphere

1. Introduction to Lithosphere

The solid crust of rocks forming the surface of the Earth is called the Lithosphere. The word 'Lithos' means rock or stone; therefore, the term translates to rock-sphere. It is the hard outer shell of the Earth where human beings live, and all visible landforms (mountains, plateaus, plains) are a part of it. It is crucial because it provides valuable soil for agriculture and is the source of most of our minerals.

2. Structure of the Earth

The Earth is composed of several concentric layers, much like an egg. Since the deepest borehole ever drilled is only about 12.2 km (the Kola Superdeep Borehole) compared to the 6,371 km distance to the Earth's center, our knowledge of the interior is based on indirect evidence. This includes studying variations in temperature, pressure, density, and the behavior of seismic (earthquake) waves.

A. The Crust (Lithosphere)

This is the topmost or outermost layer of the Earth. It has an average thickness of 50 to 60 km. The crust is thicker under the continents (20-60 km) and thinner under the oceans (5-12 km). It is divided into two distinct layers:

Concept

SIAL (Continental Crust): The uppermost layer made of lighter rocks like granite. It is rich in Silica (Si) and Aluminium (Al). Its average thickness is 25 km, and its average density is 2.7 g/cm³. It is thick over continents but very thin or absent on the ocean floor.

SIMA (Oceanic Crust): The layer lying below the SIAL. It consists of silicates of Magnesium (Ma) and Silica (Si). Its average thickness is about 35 km. It is heavier (basaltic in nature) with an average density of 2.8 to 3.3 g/cm³.

B. The Mesosphere (The Mantle)

Lying below the Lithosphere is the intermediate layer called the Mantle.

Composition: It is believed to consist of solid, ultrabasic rocks rich in iron and magnesium.
Dimensions: It has an average thickness of about 2,900 km.
Density: Varies between 3.13 to 5.4 g/cm³.
State: Despite extremely high temperatures, the tremendous pressure from overlying layers keeps both the Mantle and the lower crust in a solid state. However, when pressure is released through cracks, molten matter erupts.

Important

Gutenberg Discontinuity: This is the distinct boundary interface between the Mantle and the Core. It begins at a depth of 2,900 km. Here, there is an abrupt change in seismic waves (Primary 'P' waves decrease in velocity, while Secondary 'S' waves disappear completely). The density also jumps suddenly from 5.7 g/cm³ to 9.7 g/cm³.

C. The Barysphere (The Core)

The innermost part of the Earth is known as the Core or Barysphere. The total thickness of this layer is 3,485 km.

Composition: It is comprised mainly of heavy rocks rich in iron (Fe) and nickel (Ni). Hence, it is commonly called "NiFe".
The Outer Core: Starts at 2,900 km depth. It is in a liquid/molten state with temperatures around $4500^\circ\text{C}$ and a density of 9.5 to 12.2 g/cm³.
The Inner Core: Has a temperature of about $5200^\circ\text{C}$. Due to the massive overlying pressure that hinders the melting of rocks, the inner core is in a solid state. Its density reaches up to 15 g/cm³.

Fact

Earth's Magnetism: The presence of heavy metals and an abundance of iron rotating within the Earth's core is the primary cause of the Earth's magnetic field.

3. Earth Movements and Internal Processes

High temperatures and pressure in the Earth's interior generate immense heat. Since rocks are bad conductors of heat, the transfer of this heat to the outer surface causes structural movements inside the Earth.

Isostasy

Definition: Isostasy is the state of gravitational equilibrium between the Earth's lithosphere and the asthenosphere (the semi-fluid layer below the lithosphere, 100 to 410 km deep).
Mechanism: Tectonic plates "float" at an elevation that depends directly on their thickness and density.
Function: Isostasy is a process that restores structural balance to the Earth rather than upsetting it. When a certain area reaches this balance, it is said to be in "isostatic equilibrium."

Plate Tectonics

The Plate Tectonics Theory states that the Earth's outer lithosphere is not a continuous shell but is broken into several massive segments called plates.
The extreme heat and pressure below the crust produce a molten layer (magma/asthenosphere) upon which these lithospheric plates float and move.
Significance of Boundaries: All major geological interactions occur at the boundaries of these plates. Activities such as volcanic eruptions, earthquakes, and the building of fold mountains take place along these active margins.