Volcanic activity occurs at two types of plate boundaries: mid-ocean ridges and subduction zones. At mid-ocean ridges, basaltic eruptions produce new sea-floor crust. These underwater eruptions don\’t produce big mountainous volcanoes, which is why they are often overlooked as the most volcanically active features on Earth. Commonly, basalt is erupted at mid-ocean ridges as blob-shaped \”pillows. \” These pillows form when basalt is suddenly quenched as it comes into contact with sea water. If you cut a pillow in half, you\’ll find a glassy rind around the outside, where the lava cooled so fast that it couldn\’t form any crystals. Inside the pillow will be a crystalline matrix of cooled basaltic lava. At subduction zones, volcanoes are created on the overriding plate as melt from the subducting plate rises up through the mantle and crust.
See the map below. Hot spot volcanoes occur somewhat randomly around the globe. Their relationship (or lack of one) to the plate tectonic cycle is still being debated. The map below shows several hot spots, but not all the existing ones. In fact, there are over 100 hot spots that have been active sometime during the last 10 million years or so. Notice on the map below that out of the 25 hot spots shown, about 10 occur on top of a mid-ocean ridge. Whether this is a coincidence or not is a current topic of debate among scientists. Mantle plumes (hot jets of material that well up from deep in the mantle at a speed of centimeters per year) were proposed as the source of hot-spot volcanoes at about the time of the plate tectonics revolution.
Until recently, the prevailing wisdom held that hot spots have a deep source (perhaps as deep as the core-mantle boundary) and that they are nearly stationary with respect to the plates. Geologists, therefore, have used hot spots as an absolute reference frame from which to derive plate motions, and they have studied the geochemical signatures of the lava that has erupted at hot-spot volcanoes as a way to learn something about the composition of the lower mantle. Recent observations of some small young sea mounts east of Japan have initiated a vigorous debate about whether the standard plume model needs to be revised, or maybe even thrown out completely.
This particular chain of seamounts occurs away from a plate boundary and the melt is probably coming from a source deeper than 100 km, but researchers who studied the geochemical signature of the lava concluded that the melt cannot have a very deep source, such as the lower mantle or core-mantle boundary. Their hypothesis is that a crack in the plate allowed some partial melt that was present in the upper mantle to rise to the surface and form the sea-mount volcanoes. The schematic diagram below shows their model, which they call \”petit-spot\” volcanism.
As a shifting tectonic plate pushes into the mantle, which is the hot region between the core of the Earth and the crust, fluids inside the tectonic plate are released by the heat.
Water, carbon dioxide and other fluids rise up into the upper part of the plate and form magma if they melt the part of the crust they come into contact with. This process occurs along plate boundaries. San Andreas Fault states that as magma gets closer to the surface of the Earth\’s crust, hot gases build up and put pressure on the magma. When the magma finally reaches the surface, a volcano is born. Without the shifting of tectonic plates, magma would not form. Shifting tectonic plates do not always produce volcanoes, according to HowStuffWorks. For example, mountains are formed when tectonic plates collide and one cannot slide beneath the other. However, this boundary can eventually develop into a volcanic subduction zone.