What does an active volcano look like?
Climbing up Mount St. Helens and looking into the crater at the steaming dome is an incredible experience. The slope is steep and the landscape is like something from another planet. Nothing's alive up there, except maybe a bird. When you're standing on the top, you can see other Cascades volcanoes: Mt. Adams, Rainier, Hood, Jefferson, and sometimes more.
A volcano is a vent through which molten rock and gas escape from a magma chamber. Volcanoes differ in many features, such as height, shape, and slope steepness. Some volcanoes are tall cones, and others are just cracks in the ground (Figure below). As you might expect, the shape of a volcano is related to the composition of its magma. The three types of volcanoes are composite, shield, and cinder cones.
Mt. Fuji is a well-known composite volcano.
Figure above shows Mt. Fuji, a classic example of a composite volcano. Composite volcanoes have broad bases and steep sides. These volcanoes usually have a large crater at the top. The crater was created during the volcano's last eruption.
Composite volcanoes are also called stratovolcanoes. This is because they are formed by alternating layers (strata) of magma and ash (Figure below). The magma that creates composite volcanoes tends to be thick. The steep sides form because the lava cannot flow too far from the vent. The thick magma may also create explosive eruptions. Ash and pyroclasts erupt into the air. Much of this material falls back down near the vent. This creates the steep sides of stratovolcanoes. The composition of magma that erupts at composite volcanoes is usually felsic (rhyolite) or intermediate (andesite).
A cross section of a composite volcano reveals alternating layers of rock and ash. Frequently there is a large crater at the top from the last eruption.
Composite volcanoes are common along convergent plate boundaries. When a tectonic plate subducts, it melts. This creates the thick magma needed for these eruptions. The Pacific Ring of Fire is dotted by composite volcanoes.
Mauna Kea volcano in Hawaii.
Shield volcanoes look like a huge ancient warrior’s shield laid down. Pictured above is Mauna Kea Volcano (Figure above) taken from Mauna Loa Volcano. Both volcanoes are part of the Big Island of Hawaii. A shield volcano has a very wide base. It is much flatter on the top than a composite volcano. The lava that creates shield volcanoes is relatively thin. The thin lava spreads out. This builds a large, flat volcano, layer by layer. The composition of lava at shield volcanoes is mafic. Shield volcanoes are very large. For example, the Mauna Loa Volcano has a diameter of more than 112 kilometers (70 miles). The volcano forms a significant part of the island of Hawaii. The top of nearby Mauna Kea Volcano is more than ten kilometers (6 miles) from its base on the seafloor.
Shield volcanoes often form along divergent plate boundaries. They also form at hotspots, like Hawaii. Shield volcano eruptions are non-explosive.
Cinder cones are the smallest and most common type of volcano. Cinder cones have steep sides like composite volcanoes. But they are much smaller, rarely reaching even 300 meters in height. Cinder cones usually have a crater at the summit. Cinder cones are composed of small fragments of rock, called cinders. The cinders are piled on top of one another. The cinders can be mafic, intermediate, or felsic in composition. These volcanoes usually do not produce streams of lava. Cinder cones often form near larger volcanoes. Most composite and shield volcanoes have nearby cinder cones.
Cinder cones usually build up very rapidly. They only erupt for a short time. Many only produce one eruption. For this reason, cinder cones do not reach the sizes of stratovolcanoes or shield volcanoes (Figure below).
A cinder cone volcano in Lassen National Park.
- Magma composition determines both eruption type and volcano type.
- Composite volcanoes are common at convergent boundaries. Shield volcanoes are produced at divergent plate boundaries and intraplate.
- Cinder cones are made of small fragments of a variety of compositions usually from a single eruption. They are found with composite and shield volcanoes.
- Why do mafic lavas produce shield-shaped volcanoes? Why do felsic lavas produce cone-shaped volcanoes?
- How did composite and shield volcanoes earn their names?
- What features would you use to identify each of the three volcano types?