Plate Boundries

All Three Types of Plate Boundries

Mr. Comerford - Plate Boundary Rap

Convergent Plate Boundaries

Convergent Plate Boundaries are places where the lithospheric plates are moving toward each other, causing one of them to be pulled underneath into the mantle. There are three subtypes of Convergent Boundaries, Ocean to Continental, Ocean to Ocean, and Continental to Continental.

Ocean to Continental

These occur when Continental and Oceanic plates collide. When they do, the thinner and more dense plate (Oceanic) is pushed underneath the thicker less dense plate (continental). This is called Subduction. As the Oceanic plate is pushed farther into high temperatures, the material begins to reach its melting point (about 100 miles/160 km), this is when partial melting begins.

The Partial Melting creates magma chambers that appear above the subducting plate. The chambers are less dense than the buoyant mantle materials that are surrounding it. The magma chambers slowly rise through material above them. Melting and fracturing their way up. The size and depth of these can be found by mapping the earthquake activity that surrounds them. If the chamber of magma rises to the surface and is not solid, it will cause a volcanic eruption.

A Convergent Boundary between and oceanic and a continental plate can cause earthquake activity that is shallow by the continent margin but gets deeper beneath the continent. Sometimes the ocean trench will shore immediately off the continent line. Also a line of volcanic activity a few hundred miles inland from the shoreline. And destruction of the oceanic lithosphere.

Ocean to Ocean

This happens when two oceanic plates converge and one of the plates is subducted beneath the other. Usually the older plate will be the one to be pushed under due to its lower density. The subducting plate is heated as it is forced lower into the mantle, and around 100miles/150km the plate begins to melt. Magma chambers are created once again, that have less density than the surrounding rock. It ascends through the rock material that lays over it, the chambers that reach the surface then form a volcanic eruption cone. At the beginning of this type of boundary the volcanic cones will be deep in the ocean but will later grow higher than sea level. This will make an island chain.

The effects of this type of convergent boundary are, a zone of progressively deeper earthquakes. An Oceanic trench, a chain of islands, and the destruction of Oceanic lithosphere.

Continental to Continental

Continental to continental convergences are complex and not understood well, compared to the other types of plate convergences. With continental to continental convergences, a powerful collision occurs. Two thick continental place slam into each other and both have a density that is much lower than the mantle's and prevents subduction.

Pieces of the crust or continent margin can be caught in the collision zone in between the continents. This forms highly deformed mélange rock. The compression can cause extensive folding or faulting of the rock with the colliding plates. These deformations can be hundreds of miles into the plates interior.

Effects found at this type of convergent boundary are, intense folding and faulting, broad folded mountain range, shallow earthquakes, and shortening and thickening of the plates in the collision zone.

Divergent Boundaries

Divergent Boundaries are where the plates move apart. They happen above places where the convection currents are rising. There are two types of Divergent Boundaries, Ridge and Rift (ocean and land).

Ridge (Oceanic)

When divergent boundaries happen under the oceanic lithosphere, the convection currents lift the lithospheric plates and create mid ocean ridges. Forces stretch the lithosphere and create a deep fissure. When it opens, pressure is released on the heated mantle material. The magma responds by melting and flowing into the fissure. It then solidifies and the process is repeated.

Oceanic Divergent Boundaries can cause a submarine mountain range, such as The Mid-Atlantic Ridge.

Rift (Continental)

When the plates beneath the continents pull apart, creating a Divergent boundary, the force is not enough to make a clean break. The thick continental plates become arched from the convection currents lift, being pulled by thin extensional forces, and fracture into rift shapes. As the plates pull apart, faults are created on both sides and the center pieces begin to slide down. All of this causes earthquakes to occur because of the movement.

Early into this rifting process, streams and rivers can flow into the rift creating long stationary lakes. When the rift grows deeper it allows ocean water to flow in creating underground seas, and potentially oceans.

The Effects of this type of boundary are, rift valleys, earthquakes, and volcanic activity.

Transform Boundaries

Transform boundaries are where two plates slide past one another. The fracture zone that is created forms a transform plate boundary, also called a transform fault. Transform faults are places where there are recurring earthquakes. The earthquakes are normally shallow, because there is no subduction happening. Volcanic activity is also not very common, because there is no subduction, nor are there any convection currents.