Tectonic Plates

Tectonic Plates

A tectonic plate is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest.

Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere.

The Major and Minor Plates

Most of the Earth is covered by seven major plates and another eight or so minor plates.

The seven major plates include the African, Antarctic, Eurasian, North American, South American, India-Australian, and the Pacific plates.

Some of the minor plates include

• Cocos plate: Between Central America and Pacific plate
• Nazca plate: Between South America and Pacific plate
• Arabian plate: Mostly the Saudi Arabian landmass
• Philippine plate: Between the Asiatic and Pacific plate
• Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
• Fuji plate: North-east of Australia
• Juan De Fuca plate: South-East of North American Plate

Facts about Plate Tectonics

One famous transform boundary is the San Andreas Fault in California. It is the boundary between the North American Plate and the Pacific Plate. It is the cause of so many earthquakes in California.

The Mariana Trench is the deepest part of the ocean. It is formed by a convergent boundary between the Pacific Plate and the Mariana Plate. The Pacific Plate is being subducted under the Mariana Plate.

Scientists are now able to track the movement of tectonic plates using GPS.

The Himalayan Mountains, including Mount Everest, were formed by the convergent boundary of the Indian Plate and the Eurasian Plate.

The Indian Plate:

The Indian plate is underthrusting, albeit obliquely, and the Burmese (Myanmar) microplate, the rates of sliding movement varying from sector to sector.

This fact implies that the margin of the Indian plate is segmented (Khan, 2004). This variation in the rate of sliding is attributed to the splitting of the Assam block into two by the NW–SE trending Kopili Fault, which is a highly seismic shear zone.

The rate of convergence varies not only from sector to sector, but also possibly from time to time; thus a clear indication occurs of interrupted ongoing tectonic movement in the I–MBR belt.

The GPS measurement analysis shows that the underthrusting rate is 8 mm/year in the Nagaland salient, 6 mm/year in the Manipur recess, and 1.5 mm/year in the Tripura–Mizoram salient (Jade et al., 2007).

Movement of Plates

The heat from radioactive processes within the planet's interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.

The main driving force of plate tectonics is gravity. If a plate with oceanic lithosphere meets another plate, the dense oceanic lithosphere dives beneath the other plate and sinks into the mantle. However, convection also drives plate tectonics.

The major plate tectonic boundaries are

• Divergent: extensional; the plates move apart. Spreading ridges, basin-range.
• Convergent: compressional; plates move toward each other. Includes: Subduction zones and mountain building.
• Transform: shearing; plates slide past each other. Strike-slip motion.