PLATE TECTONICS
According to the theory of plate tectonics, the upper portion of the Earth’s lithosphere, which contains the heavier oceanic and the lighter continental crusts, consists of a series of rigid plates that are in constant motion. This theory provides a cohesive model to explain the integrated actions of continental drift, seafloor spreading, and mountain formation.
The Earth’s plates are estimated to have an average depth of approximately 60 miles (or 100 kilometers), but they are believed to vary considerably in size. Some are estimated to be continental or even hemispheric in size, while other are believed to be much smaller. Though the actual boundaries and sizes and shapes of the plates are not known for sure, it has been postulated that there are six major plates and somewhere around the same number of smaller ones. Most of the plates consist of both sial (continental) and sima (oceanic) crust. They are in constant movement, though they move at an extremely slow pace, and these movements cause frequent interactions between plates.
At this time, scientists have identified three different types of boundaries between plates. At a divergent boundary, plates are moving away from each other. This type of boundary occurs at an oceanic ridge, where new material is being added to the seafloor from deeper within the Earth. Shallow earthquakes and underwater volcanoes are associated with this type of plate activity. At a convergent boundary, plates are moving toward each other and collide, causing vast folding and crumpling along the edges of the plates. In addition to the folding and crumpling, one of the plates slowly folds under the other. Though this subduction is slow, it can nonetheless be quite catastrophic as the crustal material of the submerging plate gradually melts into the fiery hot depths below. The area where subduction occurs is usually an area where the crust is relatively unstable and is characterized by numerous deep earthquakes
and a significant amount of volcanic activity. The boundaries between convergent plates are generally found around the edges of ocean basins and are sometimes associated with deep ocean trenches. A third type of boundary is a transcurrent boundary, which involves two plates sliding past each other laterally, without the folding and crumpling that occurs at a convergent boundary. This third type of boundary is thought to be far less common than the other two types of boundaries.
The concept of plate tectonics provides an understanding of the massive rearrangement of the Earth’s crust that has apparently taken place. It is now generally accepted that the single supercontinent known as Pangaea indeed existed, that Pangaea subsequently broke apart into two giant pieces, Gondwanaland in the south and Laurasia in the north, and that the continents attached to the various crustal plates separated and drifted in various directions. As the plates drifted, they may have diverged, which was associated with the spread of the seafloor, or they may have converged, which resulted in collision, subduction, and mountain building.
(1) The majority of the Earth’s major mountain ranges are found in zones where plates converge. (2) The Himalayas, which are the world’s highest mountains, along with the central Asian mountains of varying heights associated with them, were formed by the crumpling and folding of two massive plates that collided at a convergent boundary. (3) The landmass that is today known as India was originally part of Gondwanaland, the giant supercontinent in the Southern Hemisphere, but it broke off from Gondwanaland approximately 200 million years ago and drifted north to collide with part of Laurasia, the giant supercontinent in the Northern Hemisphere, to create the world’s tallest mountains. (4)