[PDF Notes] Useful notes on Plate Tectonic Theory

Plate tectonic theory offers explanations for continental drift, ocean-floor spreading, seismicity, mountain building, etc., developed during the sixties of the twentieth century.

Plate tectonics is the comprehensive model for Wegener’s continental drift theory, seafloor spreading and other related aspects of this twentieth century revolution in earth sciences.

Earth’s present crust is divided into at least 14 plates of which about half are major and the remaining half minor in terms of size. The plate tectonic theory seems to explain most types of mountain building.

Plate boundaries:

The boundaries where plates meet clearly are dynamic places. Different types of plate boundaries are as under:

1.Divergent plate boundaries are characteristic of seafloor spreading centers. Here upwelling material from the mantle forms new seafloor, and crustal plates are spread apart. These are zones of tension.

Although most divergent boundaries occur at mid-oceanic ridges, there are a few within continents themselves. An example is the Great Rift Valley of East Africa, where continental crust is being pulled apart.

2.Convergent plate boundaries are characteristic of collision zones, where areas of continental and/or oceanic crust collide. These are zones of compression.

Examples include the sub-duction zone off the west coast of South and Central America, and the area along the Japan and Aleutian trenches.

Along the western edge of South America, the Nazca plate collides with and is sub-ducted beneath the South American plate creating the Andes Mountain chains and related volcanoes. The collision of India and Asia mentioned earlier is another example of a convergent boundary.

3.Transform boundaries occur where plates slide laterally past one another at right angles to a seafloor spreading center, neither diverging nor converging, and usually with no volcanic eruptions. These are the right angle fractures stretching across the mid-oceanic ridge system worldwide.

Across the entire ocean floor these boundaries are the location of transform faults, always parallel to the direction in which the plate is moving. It may be pointed out hat the mid-oceanic ridges are not the simple straight lines.

When a mid-oceanic ridge begins, it opens at points of weakness in the crust. As new material rises to the surface, building the mid-oceanic ridges and spreading the plates, these areas slide past each other in horizontal faulting motions.

The famous San Andreas Fault system in California, where continental crust has overridden a transform system, is related to this type of motion.

For further details as to how mountains are formed by plate collisions, readers are advised to move back to the earlier section of this chapter, where under the caption ‘Types of Orogenies showing Convergent Plate Collision patterns associated with orogenies’ the matter has been discussed at length.

There is no denying the fact that the subject of mountain building is very thought-provoking, controversial and alive.

Late Prof. Enayat Ahmad has aptly remarked that “it is because of geomorphic, litho-logical, structural and tectonic peculiarities of the mountain zones in comparison with non-orogenicareas that it is one of the most active topics of geography, geology and geophysics”.

Since mountains are phenomena of the remote past, it is quite natural that there should be controversy and uncertainty regarding the orogenic process.

Moreover, such forces, processes and conditions which have been involved in the creation of mountains largely belong to the interior of the earth. Since our knowledge about such things is negligible and limited, any discussion about orogeny becomes hypothetical to a great extent.