A volcano is an opening in the Earth's crust through which hot lava, volcanic ash and gases to escape from the deep below the surface.
Mount Stromboli in Italy
An exploding volcano throws lava, gas and ash into the air. The liquid magma, which is melted rock, starts its journey through the volcano from a magma chamber, or lake, deep inside the earth.
Magma reaches the surface through the vertical main pipe of a volcano. It is forced up by pressure from deep inside the earth. When it comes out of the earth's surface it is called lava.
The lava pours out of the hole, or vent, at the surface of the volcano. A vent can be at the top or sides of a volcano.
Different kinds of volcano
Cinder cones are the simplest kinds of volcano. The cone is made up of layers of hardened lava and ash, built up each time the volcano erupts. The lava falls as cinders around the vent, forming a cone-shaped mountain with a bowl-shaped crater at the top. The volcanoes are usually less than 350m high. Cinder cones are common in many places, including Paricutin in Mexico
Composite volcanoes, or stratovolcanoes, are high, steep-sided symmetrical cones. They are usually higher than 2,500 metres, built up by alternate layers: lava flows, volcanic ash, cinders and blocks. Most have a crater at the tip with a central vent or a cluster of smaller vents.
Mayon volcano, Philippines
Examples of composite volcanoes: Mount Fujiyama in Japan; Mount Cotopaxi in Ecador and Mayon in the Philippines, which is considered to be the most perfect cone.
Sometimes the top of the volcano gets blown off and huge amounts of volcanic ash and and dust flow down the sides. This drains the lava inside the mountain and weakens the walls at the top, which collapse into the magma chamber, forming a caldera. When the volcano next erupts, a new cone is formed in the middle of the caldera. Calderas can be very large. Often they gradually fill with water over a long time, and form a lake called a caldera lake.
Examples: Crater Lake in Oregon, USA and Lake Taal in the Philippines.
The volcanic cone in the centre of Lake Taal
Shield volcanoes are built up by layers of lava flows. When lava is thin and fluid, it flows in all directions out of the main vent or vents at the sides, and spreads widely over a great distance. They are broad, gently sloping mountains that look like a warrior's shield.
An example of shield volcanoes: Mauna Loa in Hawaii, and the world's largest active volcano.
Lava domes are formed when the lava is too thick and gooey to flow any great distance, and it piles up around the vent. As this dome grows, its outer surface cools and hardens. Then it shatters, and the bits flow down the sides. Some lava forms a knob over the vent, while others form short, steep flows that are called coulees.
Lassen Peak, California, USA
Lava domes are common on the sides or in the craters of large composite volcanoes.
Examples of lava domes: Mont Pelee in Martinique, Lassen Peak and Mono dome in California.
See a video about volcanoes: http://video.nationalgeographic.com.au/video/kids/forces-of-nature-kids/volcanoes-101-kids/
Read more about different kinds of volcano here: http://library.thinkquest.org/CR0212082/kindvol.htm
Why are there volcanoes? http://volcano.oregonstate.edu/why-are-there-volcanoes
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