N A TUREÊS What Are Volcanoes What Are Volcanoes Avolcano is a hill or mountain where molten rock spurts to A spectacular fountain of lava shooting from a volcano on the island of Bali in Indonesia the surface from deep underground Aboveground the magma is known as lava A volcano is made up of lava that has cooled and solidified together with cinders and ash Volcanic eruptions are among the most violent and spectacular natural events on earth Accompanied by red-hot rivers of lava towering clouds of ash and thick flows of mud they can devastate the landscape and people’s lives Volcanoes have been erupting since the earth was formed 4 5 billion years ago Today about 25 volcanoes around the world erupt every year on land Some of these active volcanoes erupt almost all the time Others may erupt only once every few hundred years Some volcanoes erupt with giant explosions Others erupt more gently producing fizzing lava fountains Volcanoes and the landscape Much of earth’s surface is made up of rocks that have come from volcanoes Volcanoes create and build mountains and islands but they can also be destructive They cover the landscape with lava ash and mud burning and burying plants and destroying animal habitats Houses villages and towns may also be buried or burned In the past eruptions have killed thousands of people 4
N A TUREÊS VulcanÊs island The word volcano comes from Vulcano the name of an island off the coast of Italy The ancient Romans believed that the god Vulcan lived in a volcano on the island where he made weapons for the other gods such as arrows armor and lightning bolts Fiery eruptions from the island’s volcanoes were believed to be sparks from Vulcan’s forge Molten lava cools to form new rock This school bus was caught in a lava flow in Hawaii and is now trapped in rock forever CASE STUDY Krakatau One of the biggest volcanic eruptions ever recorded happened in 1883 A volcano on the tiny island of Krakatau in Indonesia erupted with a series of enormous explosions The explosions were so loud they could be heard more than 2,200 miles 3,500 kilometers away The island itself was blasted to pieces leaving a massive crater 957 feet 290 meters deep But the effects of the eruption were felt far and wide Hot ash clouds destroyed villages 30 miles 50 kilometers away across the sea and giant waves swamped nearby coasts killing 36,000 people A new volcano called Anak Krakatau child of Krakatau is growing where Krakatau erupted W h a t A r e V o l c a n o e s 5
N A TUREÊS How Do How Do Volcanoes Form Volcanoes Form Iceland is on a boundary between two tectonic plates that are moving away from each other The rocks here are being torn apart making giant cracks in the landscape 6 Volcanoes form in places where molten rock forces its way upward through a crack in the earth’s surface The molten rock or magma comes from many miles underground from a layer of the earth called the mantle The mantle lies beneath the solid upper layer of the earth called the crust On the continents the crust is about 30 miles 50 kilometers thick while under the oceans it is about 3 miles 5 kilometers thick Moving plates The crust is cracked into huge pieces called tectonic plates The plates drift on the hot rock beneath but they move only an inch or so a year Scientists think they are moved by slow swirling currents in the mantle Most volcanoes form where two plates meet The lines between plates are called plate boundaries At some boundaries two plates collide and one plate dives under the other and into the mantle These boundaries are called destructive boundaries or subduction zones The plate that dives down melts making magma that rises again to produce volcanoes At other boundaries two plates move apart These are called constructive boundaries Magma rises up from the mantle to fill the gap between the plates
N A TUREÊS Submerged volcanoes Many more volcanoes are hidden under the oceans than can be seen on land Most form along constructive divergent boundaries on the ocean floor where magma leaks out between tectonic plates that are moving apart These boundaries are also called spreading ridges centers The magma solidifies to make new crust on each side of the boundary Other undersea volcanoes form at hot spots see box and at destructive convergent boundaries HOT-SPOT VOLCANOES Some volcanoes grow in the middle of tectonic plates far from where plates are colliding or spreading apart Scientists think that they form over hot spots in the mantle where magma forces it way up through the crust Hot-spot volcanoes in the middle of oceans form undersea mountains called seamounts The islands of Hawaii in the middle of the Pacific Ocean are the summits of giant seamounts H o w D o V o l c a n o e s The island of Hawaii is the tip of a hot-spot volcano The volcano is fed by magma that leaks through the crust from below F o r m 7
