Geoscientists observations

Volcanoes are commonly found along mid-ocean ridges. Basalt magma rises up from the asthenosphere and new oceanic crust is formed.Volcanoes are also found along subduction zones where they form volcanic arcs. Magma is produced above the subducted plate, it is less dense than the surrounding rock so it rises towards the surface. "geoscientists first thought that magma was produced as rock near the top of the down going plate and was heated by friction, but they are now convinced that the melting is for a different reason." Melting at subduction zones goes as follows:

Hot spots originate at the boundary line between the mantle and the outer core. they consist of narrow plumes of extremely hot mantle material. The plumes rise up through the mantle, and then melt the rock at the base of the lithosphere. This creates magma, and this magma then rises to the surface, creating hot spots.

There are some hot spots under continents. A hot spot under the continent is what causes the hot springs of Yellowstone National Park. There is one theory that suggests that the bulge made by a hot spot may cause the rifting of a continent. There is evidence suggesting a hot spot is responsible for the Rift Valley in Africa. Evidence also suggests that the New Material Fault may be an aborted rift zone created by several hot spots.

Most great mountain ranges are located near convergent plate boundaries. Mountain ranges are formed in two ways. Magma is generated above the subducted zone. Most of the magma remains below, cooling to form large underground masses of igneous rock known as batholiths. This combination of batholiths and volcanoes adds much more rock above the subduction zone, and makes the land elevation much higher. Many subduction zones go through compression when two plates are pushed together. This causes large masses of rocks to be pushed and stacked on top of each other. This forms high mountains.

Earth's deepest valleys reside in the ocean. Oceanic plates are bent downward as they enter the subduction zone, as they are being subducted under another plate. here a valley is formed above the zone of the bending known as a trench. These trenches are wide, with side slopes that are not steep. On world maps trenches are shown with the darkest blue shading, making them easy to find. There are many trenches in the western Pacific, where ocean-ocean subduction occurs. And there is a trench along the western coast of South America, due to the Nazca Plate being subducted underneath the continent.

Oceanic sediment is deposited on top of an oceanic plate during it's travels from a mid-ocean ridge to a subduction zone. This material then becomes rock as it is heated and put under pressure. This causes it to then become a solid part of the other plate. When material is added like this, it causes the continent to become larger at its edge. Continents can also become larger when igneous rock and batholiths are added to it above the subduction zone. this is known as continental accretion.

Earthquakes can be caused due to plates not sliding smoothly past one another. Sometimes they hold together for an extended amount of time, and then break apart suddenly. Earthquakes are a common occurrence due to the movement along the transform faults connecting segments of the ridge crest. The transform faults on or close to land are the ones that will most likely cause the most dangerous earthquakes. The Earthquakes that at continent-continent collision zones are the most common on Earth making them the most hazardous. Earthquakes that happen in subduction zones at very shallow or very deep depths along the subducted plate.