Plate tectonics is a set of theories that describes and explains the distribution of earthquakes, volcanos, fold mountains and continental drift. It states that the earth’s core consists of semi-molten magma and that the earth’s surface (or crust) moves around on the magma.
A hotspot is a plume of lava which rises vertically through the mantle. Most are found near plate margins and may be responsible for the original rifting of the crust. However, the world’s most abundant source of lava, the Hawaiian hot spot is not on the plate margin. Hot spots can cause movement, as the outward flow of viscous rock from the centre may create a drag force on the plates and cause them to move.
Collision zones form when two continental plates collide.
Neither plate is forced under the other, instead the two plates crumple into one another and so both are forced up and form fold mountains.
This movement and pressure can often cause earthquakes but no volcanoes will occur on these boundaries.
Example: where the Indian plate collided with the Eurasian plate to form the Himalayas. The Himalayas are still rising today as the two plates continue to collide.
Constructive Plate Boundaries (divergent): this is where plates move apart.
A constructive plate boundary occurs where two plates move away from each other. They are pushed by molten rock, or magma, rising from the mantle crust beneath the Earth’s crust. As soon as the magma reaches the surface it solidifies, forming new crust. This is why this type of plate boundary is referred to as constructive.
An example of a constructive plate boundary is the North Atlantic Ridge.
Conservative Plate Boundaries
-One type of plate boundary is the conservative plate boundary. This is where plates slide horizontally past one another.
Not all of the Earth’s plates are moving apart or crashing into one another. Some plates slide horizontally against each other at conservative plate boundaries. No crust is created or destroyed at these boundaries.
This also means there are no volcanoes at these boundaries. EQs do occur, however, as the plates tend to stick then slip violently rather than sliding smoothly against each other. This occurs at the San Andreas Fault in California.
Destructive plate boundaries
At these margins 2 plates move or CONVERGE together and the destruction of some of the Earth's crust results.
An oceanic plate (denser) is pushed towards a continental plate (less dense) by convection currents deep within the Earth's interior. The oceanic plate is subducted (pushed under) the continental plate at what is called a subduction zone, creating a deep ocean trench. It is the Oceanic crust which sinks down into the mantle because it is denser (heavier).
There are several examples of Destructive plate margins, including along the West coast of the Americas and Japan, where the Philippines sea plate is pushed under the Eurasian plate.