Volcanic Landforms

In May of 1980, Mount St. Helens erupted. The north face of the mountain collapsed. In a few minutes, a huge rock avalanche swept through the valley, lake, next ridge, and next river valley. The mountain's shape changed forever (photos below). The forest was destroyed. Tons of gray ash moved with the wind around the world. The eruption lasted nine hours, but continued to rumble until 1986. Today, it is quietly building lava inside the crater.

Mt. St. Helens, North Face & Volcano Crater
Mt. St. Helens, South Face
Volcanoes. Volcanoes occur in three forms. Hills or mountains built up (1) hot, molten rock called lava, (2) fragmented rocks, or (3) a combination of lava and fragments. The top of a volcano has a round depression. Small ones are called craters. Large ones are called calderas.

Kilauea Caldera (photo left) is a huge volcano on the 'Big Island' of Hawaii. This caldera contains a series of craters. Active steam vents release the heat two miles below the surface. Today people can hike on trails in the caldera.

Kilauea Volcano is the most active volcano in the world. As recently as 2002, open lava still flowed. Sometimes lava travels down the mountain slope through tubes to the ocean. The steam where the lava hits the water can be seen for miles.

Kilauea Caldera, Hawaii

Volcano Profiles. Volcanoes don't all look the same. Shield volcanoes have flat domes. They are formed of lava that was hot and free-flowing. Many Hawaiian volcanoes are the shield type.

Composite volcanoes are a combination of lava and fragmented material. They are more sloped near the top. They alternate between explosive and quiet eruptions. Most of the world's largest volcanoes are composite including volcanoes in the Philippines, Japan, and Washington.

Cinder Cones are formed by an explosion of ash and fragmented material including cinders. This material builds up and explodes. Cinder cones often are clustered together.
Ancient Cinder Cone, NE Arizona

Eruptions powered by explosive gases can build up rough, steep-sided spatter cones. They are made of pasty blobs or spatters of lava. These are usually small.

Some lava is thick, cool, and barely flows. It heaps up into a steep-sided, plug dome. They are similar in size to spatter cones.

Craters of the Moon, Idaho
Lava flows, Hawaii

Composition of Lava. Deep beneath the Earth's crust lies red-hot molten rock, called magma. Magma is a mushy liquid made of a mixture of melted minerals and crystals. During a volcanic eruption, magma is forced through a weak crack in the surface. Once it emerges, it is known as lava. Lava flows (above photos) are the hard rock that results from molten lava.

The rock is called igneous rock. Magma that cools and solidifies underground is called intrusive igneous rock. Lava becomes extrusive igneous rock.

The three major types of extrusive igneous rock are rhyolite, andesite, and basalt.

Another rock made of lava is obsidian. Obsidian (photo left) is natural glass. It forms when lava cools very rapidly. Obsidian forms because it cools so fast that crystals have no time to develop. It is usually shiny and black. Obsidian is very smooth, but very sharp when chipped. This made obsidian an excellent material for Native Americans arrowheads, spearpoints, and other cutting tools.
Obsidian, Oregon
Types of Lava and Lava Flows. People use Hawaiian words for lava flows. When thick, slow lava cools and breaks, the rocks are rough and jumbled (below left photo). This type of lava is called aa. Thinner, faster flows, cool quickly. They look wrinkled or ropy (below right photo). This type of lava is called Pahoehoe. Can you identify the lava in the Hawaii pictures above?
AA lava
Pahoehoe lava
Large areas of lava flows are also called lava fields. Sometimes a channel of lava may run down a hill. These drained, cooled tubes are called lava tubes or lava caves (Photo, below left).
Lava Tube, Ape Cave, WA
Tree Mold, Craters of the Moon, Idaho
Pressure Ridges, Hawaii

Lava sometimes covers living plants. Sometimes the form of ancient plants are preserved in lava. In the center photograph above, the image of the trunk of a pine is in a lava tree mold.

Pressure ridges (Above right photo) can form as hard lava is squeezed and cracks.

Evolution of Volcanic Terrains. Over time, lava fields change. Wind and water wear down the volcanic rock. Streams and ocean waves cut through rock. Heating and cooling by the sun also break up the rock. These processes move very slowly. Some lava flows are millions of years old. Plants grow in rich volcanic soil. Insects, birds, and other animal life return.
Valley of Fires National Recreation Area, New Mexico

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Note: All photographs taken with a digital camera in Hawaii, Idaho, New Mexico, Oregon, and Washington.
Developed by Annette Lamb and Larry Johnson, 05/02.