Earth Formation and Plate Tectonics Dynamics

Earth Formation and Composition

It is believed that the Earth was formed about 4.54 billion years ago as a result of the coalescing of a large collection of material due to gravity. During this fusion process, a large amount of heat was generated and slowly, as the Earth cooled, the heavier and denser materials settled in the center while the light materials rose to the top.

The Interior Layers of the Earth

The Earth's interior consists of the following layers:

• Inner core: Radius of 1,290 km
• Outer core: Radius of 2,200 km
• Mantle: Radius of 2,900 km
• Crust: Radius of approximately 5 to 40 km

Mantle Circulation and Energy Sources

Convection currents develop in the viscous mantle because of the high temperature and pressure gradient between the Crust and the Core, similar to the convective flow of water when heated in a beaker. The energy for this convection comes from two main sources:

1. Radioactive decay: The decay of naturally occurring chemical elements such as uranium and thorium releases energy in the form of heat, which slowly travels toward the Earth's surface.
2. Residual heat: This is the gravitational energy left over from the formation of the Earth by the falling together and compression of cosmic debris.

Continental Drift and Plate Tectonics

The Theory of Continental Drift

Continental drift is the gradual movement of the continents across the Earth's surface. According to this theory, the supercontinent Pangaea (a single plate on the Earth's surface) began to break up about 225–200 million years ago and eventually separated into the present continents. This theory is the forerunner to the theory of plate tectonics.

Understanding Plate Tectonics

This theory states that the Earth's outermost layer (the lithosphere) is fragmented into seven large and several small plates. These large plates slide on the hot molten material below due to the convective flows of the mantle. These plates are composed of both continental and oceanic lithosphere. The seven plates are:

1. Eurasian Plate
2. Indo-Australian Plate
3. Pacific Plate (Largest)
4. North American Plate
5. South American Plate (Smallest)
6. African Plate

Plate Interactions and Seismic Energy

In the course of the sliding and movement of plates, the edges of the plates (plate boundaries) get stuck together while the rest of the part moves. Consequently, the energy that would normally cause the plates to slide past one another is stored up. When the force of the moving plates finally overcomes the friction of the jagged edges of the fault and it unsticks, the stored elastic strain energy is released. This energy spreads out in the form of seismic waves that travel through the body and along the surface of the Earth.

The three types of inter-plate interactions are:

• Convergent
• Divergent
• Transform

Earthquakes and Elastic Rebound Theory

Elastic Rebound Theory

This gradual accumulation and release of stress and strain is called the elastic rebound theory. After the shaking or earthquake is over, the process starts once again.

Defining an Earthquake

An earthquake is the vibration and shaking at the surface of the Earth caused by energy being released along a fault plane, at the edge of a tectonic plate, or by volcanic activity. This process involves elastic stress.