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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. That’s so hot that rock can melt like a chocolate bar on a summer day! The hot, melted rock becomes thick, flowing stuff called magma. Magma is lighter than the solid rock around it, and it pushes up through cracks in the Earth. Wherever magma comes out, volcanoes form and grow. Once it’s 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. They’re 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. Single minerals in it are big enough for you to see without a microscope. This rock forms from magma that cools underground. Because the minerals cool slowly, they have time to grow into large crystals. Granite is a sturdy building stone that can be polished to a mirrorlike shine.
Basalt has a fine texture. It forms on the surface of the Earth and cools faster than granite. That means it has smaller crystals inside it. Usually, you can only see the minerals in basalt with 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 fast. Like basalt, obsidian hardens above the ground. However, it cools in minutes or days instead of years or centuries. For many centuries, Native Americans made sharp weapons from obsidian.
The volcano Parícutin created a cinder cone in a Mexican farmer’s cornfield. The steep pile of cinders grew 1,000 feet in just one year. That’s about as high as a 60-story building! ▶
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. This type of volcano is a cone with gently sloping sides and a broad base. Mauna Kea has been dormant for 4,500 years.
