Volcanoes

As a colder and denser oceanic plate (sima) is forced downwards (subduction), it is heated up by the surrounding mantle.  Eventually, the descending plate melts, giving off steam and gases.  The gases rise to the earth’s surface with liquid rock, known as magma, where it is released as a volcanic eruption.

When liquid rock is underground, it is known as magma, but when it is released at the earth’s surface, it is lava.

In places where eruptions happen repeatedly, layers of ash and/or lava build up over time, and both cinder cone volcanoes and composite cone volcanoes are likely to form.

A cinder cone volcano is a steep cone-shaped hill that is made up of layers of ash that have been deposited during successive explosive eruptions. They occur when a mix of gases and magma rises to the surface and blow violently up into the air.  The violent upward explosion blasts the lava into tiny fragments that solidify as ash and cinders.  The fragments then fall to the ground and form a symmetrical cone around the crater that emitted the fragments. Parícutin

Composite cone volcanoes, which are also known as stratovolcanoes, form when a eruption spews out combinations of ash, lava, pumice or tephra at various times, which are deposited in layers to form a cone. less explosive, most common, lava flows, pyroclastic materials, mudflows

Pyroclastic flows are very fast moving mixtures of hot blocks of lava, pumice, ash and gases that have been extruded from a volcano during its eruption.

Shield volcanoes The volcanoes that form over hot spots have quite a different shape from the cone volcanoes that form near plate boundaries.  Eruptions over hot spots are relatively gentle as lava seeps out to the surface through fissures and vents, adding layer upon layer to previous lava flows. Hawaii

Supervolcanoes