Part 7: The Finale, Tectonic Plate Boundaries
The mid-ocean ridges were formed by the movement of tectonic plates. Before data was obtained in the field of plate tectonics, people accepted the Permanence Theory, stating that continents and oceanic plates had not moved. During the twentieth century, scientists including F.B. Taylor, A. Wegener, and Taylor, Dietz, and Hess, began contributing the necessary information and forming more plausible theories. They all agreed that the continents are moving, known as continental drift.
The most widely accepted hypothesis about continental drift is that of convection currents. This idea suggests that the flow of the mantle is induced by the currents which move the plates in the lithosphere. Convection currents rise and spread below divergent plate boundaries, converging and descending. Convection currents are believed to be caused by the cooling of the earth’s core and the cooling of and radioactivity in the mantle.
There are three possible tectonic plate boundaries. These include divergent, convergent, and transform boundaries. New crust is created at divergent boundaries, where plates diverge or pull away from one another. Oceans are created and widened in this manner. An example of this type of plate boundary would be the expansion of Iceland along the Mid-Atlantic Ridge. This is creating crust in Iceland, but this landmass is expected to break apart along the rift that is forming there.
Convergent boundaries are known as compression zones are subduction zones. Land masses are coming together, often forming mountains and volcanoes. There are three types of convergent boundaries:
Oceanic-Continental convergence occurs when an oceanic plate pushes into and subdues under a continental plate. The continental plate is pushed up, forming mountain ranges. The deepest part of the subjecting plate fragments, and after some time these pieces of earth will suddenly move and cause large earthquakes.
Oceanic-Oceanic convergence occurs when one oceanic plate is subjected under another, forming deep oceanic trenches such as the Marianas Trench. Undersea volcanoes are also formed this way.
Continental-Continental convergence does not result in subduction. The two plates hit head-on, and because they are both relatively light they collide and resist downward motion. The crust is forced upward or sideways. The Himalayas were formed during the slow convergence of the Eurasion Plate and the Indian Plate.
Transform boundaries, or transcurrent boundaries, are not formed by separation or collision. Instead, two plates will slide past each other, forming faults. Most transform faults are found on the ocean floor and are usually defined by small earthquakes. Some do occur on land, such as the San Andreas Fault in California. Faults can be classified geometrically, using the angle of the net slip and the apparent movement of the plates. These classifications include a strike-slip fault, characterized by parallel movement; a dip-slip fault, where one plate appears to have moved down relative to the other; and a diagonal-slip, incorporating a downward and sideward slip that appears as a diagonal movement.
The theory of continental drift helps to explain the biogeographic distribution of present-day life found on different continents with similar ancestors. As discussed in Part 2, the history of species distribution can be uncovered using the theory of plate tectonics; geographical regions with the same biota may one day become separated; and disconnected geographical regions may collide, introducing unfamiliar organisms to new habitats.