How does plate geography affect the earth

A plate is made of crust (continental or oceanic type) and the uppermost layer of the Earth’s mantle. Together, the two components are termed lithosphere. The lithosphere is around 93 miles (150 km) thick under the continents and around 43 miles (70 km) thick under the oceans. The crustal part of a plate can comprise entirely oceanic crust, or part oceanic and part continental crust.

The main plates are classified in this way as follows: oceanic plates: Pacific, Nazca, Philippine, and Cocos; composite plates: South American, North American, African, Indo/Australian, Eurasian, Antarctic, and Arabian.

A plate, by definition, must be bounded by a spreading center, and a combination of convergent and conservative margins. The crust between these boundaries can be either oceanic, or both oceanic and continental, but never completely continental.

The Indo-Australian plate, for example, is bounded by the Mid-Indian Ridge to the south. Australia, a large block of continental crust, sits on the plate, and is bounded on almost all sides by a passive continental margin (also called a mid-plate

Radar altimeters carried aboard satellites are used to map fluctuations in the actual height of the sea’s surface. The sea bulges above an upswelling in the Earth’s mantle, and seems to be depressed above regions where the mantle is moving downward beneath the ocean.

A computer image of the foci of all earthquakes west of the Tonga and Kermadec ocean floor trenches creates a picture of a smoothly descending surface. This is probably the central plane of a tectonic slab on its way down in its convective cycle.

The manned submersible Cyana has taken photographs of heated water billowing out from vents in the ocean floor crust. Known as black smokers, these dark plumes are rich in sulfides dissolved from the crustal rock. Plate motion creates the fissures and vents that suck in sea water and then expel it once it has heated up.

Further west along the Sundra Trench, the nature of the northern boundary of the Indo-Australian plate changes in character. West of Australia, oceanic crust is being subducted in a normal fashion below Java and Sumatra. Still further along the plate boundary is another collision zone: along the Himalayas, India has collided with continental crust of the Eurasian plate. This is an example of a continent – continent collision.

In some regions, deciding exactly where part of a plate boundary occurs is difficult. The South American plate is an example. It is clearly bounded by a spreading center in the east (Mid-Atlantic Ridge) and a subduction zone in the west (Peru – Chile Trench). In the south and north, however, the definition of boundaries is not as clear. In the south is a complex boundary linking the Peru Trench and the Scotia Arc. In the north is the problematic Caribbean plate, which may largely comprise ancient Pacific oceanic crust, trapped when subduction started along the Middle America Trench. Because of complexities such as these, geologists now speak of plates (the major plates) and the microplates (smaller regions which are bounded by spreading centers, trenches or transforms).

The relative motion of a plate (from its constructive boundary toward a destructive boundary, in most cases) is inevitably affected by the motion of adjoining plates. Relative motion for each plate can differ from spreading motion.

J. T. Wilson was the first geologist to realize the significance of transform faults. He realized that they must be parallel to the direction of spreading, and that they are inherited from irregular opening.