Internal structure of the Earth and its outer plates
The universe formed with a big bang about 14 billion years ago. The Solar System and the Earth formed about 4.5 billion years ago and life on Earth began about 3.5 billion years ago. During the first billion years, the Earth experienced immense turbulence, with continuous bombardment by cosmic bodies that made this planet look like a fireball.
The Earth is made up of three main layers: crust, mantle, and core (Fig. 4). Beneath the oceans, the crust generally extends to about 5 km. The thickness of the crust beneath the continents is thicker and averages about 30 km. Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2900 km thick. At the center of the Earth lies the core, which is actually made up of two distinct parts, a 2200 km thick liquid outer core and a 1250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins and generates the Earth’s magnetic field.
The outer rigid layer (about 70-100 km thick) of the Earth, comprising the crust and uppermost mantle, is divided into a number of plates. There are about 12 major plates such as North American, South American, African, Indian, Eurasian, etc., covering the entire surface of the Earth (Fig. 5). The
lithospheric plates are bounded by one of the three main types of geological features: (1) mid-oceanic ridges, (2) subduction zones, and (3) transform faults. Boundaries are narrow deforming zones, which are accompanied by earthquake activity, but the plates’ interiors are rigid. Each plate is in relative motion with respect to others on the surface of the Earth. The relative motion between the plates produces new crust at mid-oceanic ridges, consumes crust at subduction zones, and conserves crust along the transform faults (Fig. 6). Apart from the normal process of construction and destruction at plate boundaries, plates also undergo break-ups and unifications.
In the past, continental landmasses have undergone break-ups (Fig. 7), have collided with other continental masses, new oceans have formed, and some have perished. About 225 million years ago, India was a large island situated off the Australian coast, and a vast ocean called ‘The Sea of Tethys’ separated India from the Asian continent. The Indian subcontinent, moving northwards, collided with Asia about 40 million years ago, and set in motion a chain of events that led to the formation of the Tibetan Plateau and the Himalayan Mountain Range (Fig. 8). This is a spectacular demonstration of a head-on crash between two giant landmasses that began many millions of years ago and continues to date. As a result of this collision, mountain ranges such as the Himalayas and peaks such as Mt. Everest were formed. Mt. Everest has risen to a height of nearly 9 km. The Himalayas continue to rise more than 1 cm a year.