Volcanoes are formed when a vent in the earth reaches all the way down into pockets of molten rock called magma. This magma escapes from the vent and builds up a mound around the vent when it cools off and hardens. This mound might be the size of a small hill, or it could grow to be a giant mountain. There are different types of volcanoes, each formed by different types of eruptions. Of all the different types of volcanoes, the most complex is the composite volcano, also called the stratovolcano.
Composite volcanoes are composite in three ways. They are composites of multiple eruptions. The eruptions that build up a composite volcano may be separated by hundreds of thousands of years. They are also composites of multiple materials. The eruptions that form these volcanoes lay down alternating layers of lava, ash and cinders. Thirdly, while this type of volcano might have only one vent, it also might be a composite of several vents.
Composite volcanoes have steep slopes and tend to be symmetrical in shape. They can be very tall, up to 8,000 feet in height. That's over a mile and a half. They can become so tall that their steep slopes become unstable and collapse underneath their own weight. These volcanoes can become very wide as well, up to approximately five miles across.
How They are Formed
Composite volcanoes are formed by viscous lava, which is relatively thick. Viscous is the opposite of runny. After this lava flows for a while, the eruption changes and starts ejecting ash and cinder that falls near the summit of the mountain, which is why these volcanoes have such steep sides. The eruption then changes back to lava, which cements the ash and cinder.
When composite volcanoes go dormant and stop erupting, they are sometimes worn away by erosion until there is almost nothing left of them. They are also destroyed when further eruptions blast apart the volcano cone. The depressions that are left after erosion and explosions are known as calderas.
Most composite volcanoes occur in chains, rather than singly. The volcanoes that make up these chains can be separated by dozens of miles. These chains can form anywhere on earth, but are most numerous around the rim of the Pacific Ocean, which is known as the "Rim of Fire" because of all the volcanoes that form there.
Famous Composite Volcanoes
Mount Rainier, in Washington state, is a composite volcano. So is Mount Fuji in Japan; the Mayon Volcano, in the Philippines; and Mount Vesuvius in Italy.