According to the theory of plate tectonics, the outermost layer of the earth, called the lithosphere, is divided in pieces, which float over the mantle layer and move across the surface of the earth relative to each other. There are 12 major plates that float, larger part of them being under water, and the other part represented by the continents. Studies have shown that long time ago the solid crust of the earth was one whole part, called Pangaea, and the splitting of Pangaea created the continents
Earth, excluding the atmosphere and the non-solid part, is divided in layers starting from the surface to the deepest part. The outer layer, on which there is life, is called the crust. Below the crust, the mantle is found, which is divided in the upper mantle and the lower mantle. The deepest part is called the core and is also divided in two parts, the inner core and the upper core. The lithosphere includes the crust and the upper mantle. Geologically, the crust consists of light materials like rocks and granite, in comparison to the core, which consists of heavier materials, for example iron and nickel. The Science website has a deeper explanation about the earth, its layering and the geological properties of the layers.
Earth’s crust is divided into pieces, which are called plates. According to the plate tectonics theory, these plates are floating and moving around the surface of the planet. When these plates collide, new crust is produced and some part of the old crust sinks down and melts in the mantle. In such cases of collision, earthquakes occur. The lifecycle of the crust is around 100 million years, in the end of which it melts down again for the new crust to emerge. However, the continents do not fall back into the mantle of the Earth, so they live longer than the crust.
Movement of the plates
The movement of the plates is slow, a few inches per year, but it has had a great effect shaping the outermost layer of our planet. When landmasses of the plates collide, the outer layer rises and forms mountains. An example of such mountains would be the Himalayas, the highest mountain on this planet, which formed when the crust crumpled from a collision of two plates, the one which belonged to India and the Asian plate, about 55 million years ago.
A recent study from MIT showed that the collision actually happened about 10 million years earlier than what was actually believed till now. One of the scientists from MIT says: “India came running full speed at Asia and boom, they collided, but we actually don’t think it was one collision, this changes dramatically the way we think the India/Asia collision works.” Mountains can also be created when a part of the crust submerges, making the overlying plate to stay on top at a high level. These collisions can also happen underwater, where trenches are formed. Such a formation is the Mariana trench, which is the deepest point on Earth.
An example of a collision was reported by NASA in December 2001. Quoting from the article: “In the Persian Gulf, two tectonic plates-rigid pieces of the Earth's crust-are colliding; the Arabian plate is running up on the Eurasian plate. The Persian Gulf and the Gulf of Oman were once the site of a rift, a place where two plates pull apart from each other, and the Indian Ocean filled in the widening gap between the two plates; however, the process then reversed, and about 20 million years ago, the gulf began to close up. The collision of the two continental plates gives Iran its mountainous terrain.”
Through geological discoveries, humankind has been able to dig deeper into its past and learn more about the changes on the planet, changes that have had a significant effect on extant life. The history of humankind is written in books, but the history of the planet is written on rocks. The tectonic plates are the main factor shaping the world today.