Plate Tectonics Model
The Earth’s outer layer, the lithosphere, consists of a number of rigid, moving pieces called plates. These lithospheric plate rest on the asthenosphere, which is partially molten igneous rock. As the molten rock flows, very large and slow-moving convection currents are formed. Within the currents, material expands and rises as it is heated, and contracts and sinks as it cools. The currents in the asthenosphere cause the lithospheric plates to move; some plates are pushed apart, some together, and other slide past each other. The plate boundaries are the epicenters of earthquakes and the sites of most volcanic activity.
The study of the formation and movement of the lithospheric plates is called plate tectonics. This model demonstrates the complex relationships involved in this process.
Divergent oceanic boundaries are places where the lithospheric plates are moving apart. Most diverging boundaries have mid-ocean ridges (2) with deep rift valleys (3) along their entire length. Rising convection currents (indicated by arrows) in the asthenosphere (1) carry largely gas-free molten rock into the rift valleys, where new rocks solidify, forcing the ocean floor to spread. Lava from rift eruptions may spread over many kilometers (12). The rift valleys are broken into segments by faults called fracture zones (17). Movements along the fractures zones have been found to be the source of the earthquakes that occur along the ridge.
The left side of the model illustrates a type of transform boundary, where one plates is sliding past another horizontally, causing little or no subduction/accretion. However, because the edges are rough, they catch on one another, allowing stress to build up. When this stress is released suddenly, earthquakes may occur. In the model, sedimentation (5) has filled in a large part of the area but a basin (6) can still be seen. The upper part of the model (16) can be moved slightly to simulate the motion at a transform boundary.
When one plate collides with another and slides beneath it, a deep-sea trench (7) is formed at the subduction boundary (8). When this trench is formed by the convergence of two ocean plates, it is bordered by a chain of volcanic islands on the overriding ocean plate. When an ocean plate (11) converges with a continental plate and the deep-sea trench is bordered by mountains and volcanoes (9) on the continental plate. The volcanoes occur when magma is forced up through cracks formed in the continental plate (10) as the ocean plate slides under it. The subducted part of the ocean plate is recycled into the asthenosphere.
The left side of the model shows a continental plate converging with an ocean plate. Thick, gaseous magma (13) has formed at the subduction boundary. Explosive subduction boundary eruption, consisting mainly of lava fragments, have formed a steep-sided cinder cone volcano (14). Even though the subducting plate as a whole sinks smoothly into the deep-sea subduction trench, the deepest part of the plate breaks into smaller pieces which become locked in place (15) for long periods of time. Sudden shifting of these pieces generates large earthquakes which can move the land several meters.