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The roots of volcanoes lie 40 to 120 miles inside the Earth in a layer called the mantle.
Temperatures there are as hot as 4,000°F—so hot that rock can melt like a chocolate bar on a summer day. The hot, melted rock becomes a thick, flowing substance called magma. Magma is lighter than the solid rock surrounding it, and it pushes up through cracks in the Earth. Wherever magma comes out, volcanoes form and grow. Once it is out of the ground, magma is called lava. As lava cools, it hardens into different kinds of rock.
There are three main types of volcanoes:
1. Cinder cones
2. Stratovolcanoes
3. Shield volcanoes
Fiery Rocks
Granite, basalt, and obsidian are igneous (ig-NEE-us) rocks, made from magma mixed with different minerals. The different textures of these three rocks tell you how they were formed.
◀ Granite has a coarse texture, with individual minerals large enough for you to see without a microscope. Granite forms from magma that cools underground. Because the minerals cool slowly, they have time to grow into large crystals. Granite is a durable building stone that can be polished to a mirror finish.
Basalt has a fine texture. It forms on the surface of the Earth and cools more quickly than granite, so its crystals are smaller. Usually, you can see its minerals only by using a microscope. The Olmec of ancient Mexico sculpted this head about 2,000 years ago from one huge block of basalt. ▶
Obsidian has a smooth texture like glass. Crystals do not have time to grow in it because the lava cools so quickly. Like basalt, obsidian hardens above the ground, but in minutes or days instead of years or centuries. For many centuries, Native Americans made sharp weapons from obsidian.
The volcano Parícutin made a cinder cone in a Mexican farmer’s cornfield. It’s a steep pile of cinders that grew 1,000 feet—about as high as a 60-story building—in one year! ▶
Bomb
A lump of lava thrown out of a volcano while still molten
Crust
The upper part of the Earth
Magma Chamber
An underground area where thick, molten rock collects
Central Vent
A channel in the neck of a volcano, where magma erupts
Fissure
A large crack in the Earth through which lava can flow
Crater
The bowl-shaped depression around the mouth of a volcano
Ash and cinders
Tiny airborne pieces of rock and lava
Lava Flow
Molten rock flowing down the sides of a volcano
▲ Hawaii’s Mauna Kea is a large shield volcano that has been dormant for 4,500 years. A shield volcano is a cone with gently sloping sides and a broad base.
