VOLCANOES
A volcano is formed when magma finds it's way through the Earth's crust, erupting and causing lava flows. The lava cools, and ash deposits itself over the cooled lava. Over time, the process repeats until a conical shape is formed from the cooled lava and ash deposits. There are 3 types of volcano shapes:
A stratovolcano is the most recognisable, and common of volcanoes. They are tall, and conical with multiple layers of hardened lava, tephra, and ash. These volcanoes don't erupt often, but are often very destructive when they do. The magma has high levels of silica, which means the lava is very viscous. It cools and hardens before spreading too far. Examples include Mount Fujii in Japan, and Mount St. Helens in the USA.
Shield volcanoes are less common then stratovolcanoes. They have shallow-sloping sides, and erupt quite often, with the Piton de la Fournaise erupting once a year on average. the magma has a low silica lvel, so the lava has a low viscosity. the lava can travel great distances before hardening. Examples include Mauna Kea in Hawaii, and Mount Elgon in Kenya.
A caldera volcano is the opposite to the other two in shape. It is cauldron-like, caused by the collapse of land after a volcanic eruption. If the magma chamber loses enough magma, and is unable to hold the weight of the ground above it, then a circular fracture can appear, causing the ground to sink into the space below. Examples include Yellowstone National Park in the USA, and Krakatoa in Indonesia.
Volcanoes can mainly be found along destructive plate margins, more specifically around the Pacific Plate, in an area known as the 'Ring of Fire'. It has 452 volcanoes, which amounts to 75% of active and dormant volcanoes in the world.