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formation of ash, followed towards the end of the eruption by the ejection of tongues of lava. As a result, the volcano is made up of layers, or strata, of ash and lava overlying one another. That is the reason why such volcanoes
are called stratovolcanoes (made of many layers), or composite volcanoes (composed of different types of rock), or central vent volcanoes (volcanoes whose material comes from a single central pipe or vent).
The less reliable and more violent volcanoes throw out a much wider variety of materials and so have a
more complicated shape. They are often referred to as complex volcanoes. In this kind of volcano the longer the time between eruptions, the more likely it is that the next eruption will be very violent and explosive. Conversely, the sooner one eruption follows another, the less violent it will be. The reason for this is that the longer the volcano stays inactive, the more the magma in the magma chamber below it can separate, and the more silica can rise to the top of the magma chamber. The
higher the silica content of the
magma, the more explosive the
eruption.
When a volcano erupts under water, the shape of the volcano and its products are very different from those on land. In this case the pressure of the water tends to prevent any explosive activity, and most of the submarine volcanoes erupt low-viscosity magma in any case. The result is that the lava flows out quietly and is quickly cooled by the water, making
a series of solidified bubbles,
or pillow shapes. Submarine lava characteristically forms interlocking flows of pillow lava.
Because submarine volcanoes erupt quietly, they
(Below) Volcanic cones may contain a variety of layers (strata) of material, often deposited one atop another. Thus many volcanoes of this kind
are known as stratovolcanoes. If they have a single supply pipe, or vent, they will have a roughly symmetrical shape.
The magma chamber is located at a relatively shallow depth in the crust, and its presence weakens the crust, allowing magma to flow to the surface along many routes. Sometimes small vents open on the flanks of the volcano. When this happens, small cones build, and the main volcano loses its symmetrical shape.
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