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Volcano
A volcano is an opening in Earth’s crust that allows molten rock from beneath the crust to reach the surface. This molten rock is called magma when it is beneath the surface and lava when it erupts or flows from a volcano. Along with lava, volcanoes also release gases, ash, and rock. It’s a super hot mix that can be both incredibly destructive and creative.
Volcanoes form at the edges of Earth’s tectonic plates. These huge slabs of Earth’s crust travel atop the partly molten mantle, the layer beneath the crust. If you could see the plates, you might think they look like pieces of a puzzle because the edges fit together. But these puzzle pieces move, usually at the unnoticeable pace of only a few inches every year. Sometimes, though, plates collide with one another or pull apart, and it’s at these active zones where volcanoes form. Volcanoes may also erupt in areas called hot spots where the crust is thin.
Volcanoes erupt in different ways, producing different landforms. Steep, cone-shaped volcanoes form when plates collide. All the pressure and heat of the collision make for a violent eruption. The cone forms when lava and other material eject and build up around the opening. This type of volcano is known as a stratovolcano, and Mt. Rainier is a good example. Sometimes an eruption is so violent that the top of the volcano collapses, leaving a huge pit or caldera. You can see calderas in Yellowstone National Park and Crater Lake. When plates pull apart, lava escapes through the rift. This more gentle flow creates new crust on the seafloor and wide, rounded volcanoes on the surface called shield volcanoes. Hawaii’s Kilauea is a shield volcano. It is also an example of a volcano that formed over a hotspot.
Volcanic eruptions can be deadly. Eruptions of Krakatau and Tambora in Indonesia caused the deaths of more than 100,000 people. Dangerous as they are, volcanoes also build and shape the land, creating mountains and new seafloor and depositing minerals and nutrients that enrich soil.
Types of Volcanos
COMPOSITE VOLCANOS Composite volcanoes, sometimes known as strato volcanoes, are steep sided cones formed from layers of ash and [lava] flows. The eruptions from these volcanoes may be a pyroclastic flow rather than a flow of lava. A pyroclastic flow is a superheated mixture of hot steam, ash, rock and dust. A pyroclastic flow can travel down the side of a volcano at very high speeds with temperatures over 400 degrees celsius. Composite volcanoes can rise over 8000 feet.
When composite volcanoes erupt they are explosive and pose a threat to nearby life and property. Eruptions are explosive due to the thick, highly viscous lava that is produced by composite cone volcanoes. This viscous lava has a lot to do with why they are shaped the way they are. The thick lava cannot travel far down the slope of the volcano before it cools.
Composite volcanoes are usually found at destructive plate margins. Examples of composite volcanoes include Mount Fuji (Japan), Mount St Helens (USA) and Mount Pinatubo (Philippines).
SHIELD VOLCANOS
Shield volcanoes are low with gently sloping sides and are formed from layers of lava. Eruptions are typically non-explosive. Shield volcanoes produce fast flowing fluid [lava] that can flow for many miles. Eruptions tend to be frequent but relatively gentle. Although these eruptions destroy property, death or injury to humans rarely occurs.
Shield volcanoes are usually found at constructive boundaries and sometimes at volcanic hotspots. Examples of shield volcanoes include Mount Kilauea and Maunaloa on Hawaii.
COMPOSITE VOLCANOES
Composite volcanoes are made of felsic to intermediate rock. The viscosity of the lava means that eruptions at these volcanoes are often explosive. The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a composite volcano. Viscosity also causes some eruptions to explode as ash and small rocks. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other. The result is the classic cone shape of composite volcanoes.
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