What's Inside a Volcano?
In the image at left you see the red magma (hot, liquid rock) flowing upward and out of the mountain through a tube called a vent. When magma comes to the surface it is called lava. This type of volcano is called a composite because of the different layers made up of older eruptions.
A magma flow between two layers is called a sill and a flow through layers is called a dike. During a volcanic explosion hot gases, ashes and cinders can be thrown several thousand feet into the air. After the eruption the top of the mountain falls in on itself and forms a crater. See the Mount St Helens page to see before and after pictures showing the crater. Click on image for a detail view.
A volcano can be made of one or more layers. Wind and water action can erode it just like any other geological feature. When erosion removes the outer layers of the volcano the dike remains as a lava tower like this one in the southwestern US (right).
The Four Types of Volcanoes
Volcanoes are grouped into cinder cones, composite volcanoes, shield volcanoes and lava domes.
Cinder cones are circular or oval cones (left) made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent. Click the image or here for a larger view of these cinder cones in the caldera of Mt. Haleakala on Maui, Hawaii. A caldera is a large volcanic depression, commonly circular or elliptical when seen from above, caused by a volcano collapsing into itself. (Photo by Jim Draggoo.)
Composite volcanoes are steep-sided volcanoes composed of many layers of volcanic rocks, usually made from high-viscosity (thick like honey) lava, ash and rock debris (broken pieces). Mt. Rainier (right), Mount St. Helens and Mt. Shasta in California are examples of this type of volcano. (USGS Photo.)
Shield volcanoes are volcanoes shaped like a bowl or shield in the middle with long gentle slopes made by basaltic lava flows. Basalt lava flows from these volcanoes are called flood basalts. Some very fluid flows can travel for hundreds of miles. The volcanoes that formed the basalt of the Columbia Plateau were shield volcanoes as well as Kilauea in Hawaii (left) and Olympus Mons on Mars.
Click image or here for a larger image of the shield volcano.
Lava domes are formed when erupting lava is too thick to flow and makes a steep-sided mound as the lava piles up near the volcanic vent. The eruption of Mount St. Helens in 1980 was caused in part by a lava dome shifting to allow explosive gas and steam to escape from inside the mountain.
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Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Fragments of less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.
- Volcanic rock caused by partial melting of the Earth's crust.
- A large volcanic depression, commonly circular or elliptical when seen from above, caused by a volcano collapsing into itself.
- Cinder Cone
- A circular or oval cone made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent.
- Composite Volcano
- A steep-sided volcano composed of many layers of volcanic rocks, usually made from high-viscosity (thick like honey) lava, ash and rock debris (broken pieces).
- A steep-sided mound that forms when viscous (thick like honey) lava piles up near a volcanic vent (opening at the surface).
- A vent that releases volcanic gases and steam.
- A mixture of water and rock debris that forms on the slopes of a volcano. Also known as a mudflow or debris flow. The term comes from Indonesia.
- A light-colored volcanic rock containing lots of bubbles from trapped gases. This rock can sometimes float on water.
- Pyroclastic Flow
- A hot, fast moving and high-density (thick like a dust storm) mixture of ash, pumice, rock fragments and gas formed during explosive eruptions.
- Shield Volcano
- A volcano shaped like a bowl in the middle with long gentle slopes made by basaltic lava flows.
- An opening at the surface where magma, gas and steam erupt.
- A vent at the surface where magma, gas and steam erupt. Also, the landform constructed by volcanic material.
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