HiSET Science: Theory of Plate Tectonics

Plate Tectonics

Main Concepts
Plate tectonics is a geological theory that was developed to explain the process of continental drift. The theoretical separation of the Earth's lithosphere and asthenosphere is based upon the mechanical properties of the materials in the two respective layers and is distinct from the chemical separation of Earth's crust, mantle, and core.
According to the theory of plate tectonics, the Earth's lithosphere is divided into ten major plates: African, Antarctic, Australian, Eurasian, North American, South American, Pacific, Cocos, Nazca, and Indian; it floats atop the asthenosphere. The plates of the lithosphere abut one another at plate boundaries (divergent, convergent, or transform fault), where the formation of topological features of Earth's surface begins.

Theory
This theory of plate tectonics arose from the fusion of continental drift (first proposed in 1915 by Alfred Wegener) and seafloor spreading (first observed by Icelandic fishermen in the 1800s and later refined by Harry Hess and Robert Dietz in the early 1960s) in the late 1960s and early 1970s.
Prior to this time, the generally accepted explanation for continental drift was that the continents were floating on the Earth's oceans. The discovery that mountains have 'roots' (proved by George Airy in the early 1950s) did not categorically disprove the concept of floating continents; scientists were still uncertain as to where those mountainous roots were attached. It was not until the identification and study of the Mid-Atlantic Ridge and magnetic striping in the 1960s that plate tectonics became accepted as a scientific theory. Its conception was a landmark event in the field of Earth sciences—it provided an explanation for the empirical observations of continental drift and seafloor spreading.

Tectonic Plate Motion
The two main sources of tectonic plate motion are gravity and friction. The energy driving tectonic plate motion comes from the dissipation of heat from the mantle in the relatively weak asthenosphere. This energy is converted into gravity or friction to incite the motion of plates. Gravity is subdivided by geologists into ridge-push and slab-pull. In the phenomenon of ridge-push, the motion of plates is instigated by the energy that causes low-density material from the mantle to rise at an oceanic ridge. This leads to the situation of certain plates at higher elevations; gravity causes material to slide downhill.
In slab-pull, plate motion is thought to be caused by cold, heavy plates at oceanic trenches sinking back into the mantle, providing fuel for future convection. Friction is subdivided into mantle drag and trench suction. Mantle drag suggests that plates move due to the friction between the lithosphere and the asthenosphere. Trench suction involves a downward frictional pull on oceanic plates in subduction zones due to convection currents.

Convergent Plate Boundaries
A convergent (destructive) plate boundary occurs when adjacent plats move toward one another. The Earth's diameter remains constant over time. Therefore, the formation of new plate material at diverging plate boundaries necessitates the destruction of plate material elsewhere. This process occurs at convergent (destructive) plate boundaries. One plate slips underneath the other at a subduction zone. The results of converging plates vary, depending on the nature of the lithosphere in said plates. When two oceanic plates converge, they form a deep underwater trench. If each of the converging plates at a destructive boundary carries a continent, the light materials of the continental lithosphere enables both plates to float above the subduction area. They crumple and compress, creating a mid-continent mountain range. When a continental plate converges with an oceanic plate, the denser oceanic lithosphere slides beneath the continental lithosphere. The result of such convergence is an oceanic trench on one side and a mountain range on the other.

Divergent Plate Boundary
A divergent, or constructive, plate boundary exists when two adjacent plates move away from one another. Observation of activity at diverging boundaries provided unquestionable proof of the seafloor-spreading hypothesis. At this type of plate boundary, kinetic energy generated by asthenospheric convection cells cracks the lithosphere and pushes molten magma through the space left by separating tectonic plates. This magma cools and hardens, creating a new piece of the Earth's crust. In the oceanic lithosphere, diverging plate boundaries form a series of rifts known as the oceanic ridge system. The Mid-Atlantic Ridge is a consequence of undersea diverging boundaries. At divergent boundaries on the continental lithosphere, plate movement results in rift valleys, typified by the East African Rift Valley.

Transform Plate Boundary
A transform (conservative) plate boundary exists when two tectonic plates slide past each other laterally and in opposite directions. Due to the rocky composition of lithospheric plates, this motion causes the plates to grind against each other. Friction causes stress to build when the plates stick; this potential energy is finally released when the built-up pressure exceeds the slipping point of the rocks on the two plates.
This sudden release of energy causes earthquakes. This type of plate boundary is also referred to as a strike-slip fault. The San Andreas Fault in
California is the most famous example of such a boundary.

Geologic Faults
A geologic fault is a fracture in the Earth's surface created by movement of the crust. The majority of faults are found along tectonic plate boundaries; however, smaller faults have been identified at locations far from these boundaries. There are three types of geologic faults, which are named for the original direction of movement along the active fault line. The landforms on either side of a fault are called the footwall and the hanging wall, respectively. In a normal fault, the hanging wall moves downward relative to the footwall. A reverse fault is the opposite of a normal fault: The hanging wall moves upward relative to the footwall. The dip of a reverse fault is usually quite steep; when the dip is less than 45 degrees, the fault is called a thrust fault. In the third type of geologic fault, the strike-slip fault, the dip is virtually nonexistent, and the footwall moves vertically left (sinistral) or right (dextral). A transform plate boundary is a specific instance of a strike-slip fault.