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Description of Plate Tectonic Theory

From volcanoes in Indonesia to the Himalaya Mountains to the Hawaiian Islands, many parts of the world owe their existence to the dynamics of plate tectonics, according to plate tectonic theory. The theory suggests the top layer of the Earth, called the lithosphere, is divided into several sections called plates that float on top of the asthenosphere, a layer of hot liquid rock called magma. Like ice floes on a lake, these plates drift from 1 to 10 centimeters per year. The interactions of these plates as they separate and converge affects the Earth's geography by creating mountains and volcanoes.

Plates
- According to plate tectonic theory, the Earth has several major plates and dozens of minor plates. The major plates are the South American, North American, Eurasian, Antarctic, African, Indian/Australian and Pacific plates. The smaller plates include the Arabian, Caribbean and Philippine Sea plates. Plates are either ocean plates, composed of ocean seafloor, or lighter, thicker continental plates, which makeup dry land. Continental plates often have an outer ring of ocean plates.
Subduction
- When a heavier, more dense plate pushes into a lighter plate, such as when an ocean plate pushes into a continental plate, the denser plate subducts the lighter plate. This means that the heavier plate moves underneath the lighter plate, which pushes the heavier plate into the magma beneath the lithosphere. This subduction allows magma to escape onto the surface, ultimately creating volcanoes. For example, the Pacific plate is subducting the Eurasian plate close to Japan and Russia, which is responsible for the many volcanoes in Japan and the Kamchatka peninsula of Russia.
Convergence
- When a continental plate converges with another continental plate, neither plate pushes the other beneath it. Instead, like a car crash that smashes the hoods of the cars involved, the plates push up massive amounts of earth to create mountain ranges. When the Indian/Australian plate collided with the Eurasian plate millions of years ago, it created the Himalaya Mountains. Geologists who support plate tectonic theory also believe that other mountain ranges, such as the North American Rockies and the European Alps, formed after two continental plates collided.
Divergence
- As two plates move away from each other, they create a gap that allows magma to flow to the surface. This creates new crust for the plates on either side. Typically this occurs on the seafloor, but it has occurred on land in places such as Iceland, which lies between the North American and Eurasian plates. The magma released from the asthenosphere as a result of divergence creates large underwater mountains that can grow high enough to breach the surface of the water, creating islands. The Hawaiian Islands formed in this way.

Nature