Figure 8.21: Mt. Fuji is a dormant composite volcano that is the highest mountain in Japan.
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Composite volcanoes are also called stratovolcanoes because of the alternating layers, or
strata,of which they are made (Figure8.22). The magma that creates stratovolcanoes
tends to be more viscous, or thick. Viscous lava creates greater pressure which, in turn,
tends to create explosive eruptions. In addition, the viscous lava cannot travel far down
the sides of the volcano before it solidifies. This viscous lava thus creates steep sides on
stratovolcanoes.
When a stratovolcano erupts, it ejects a great deal of pyroclastic material into the air, which
then settles back down on the Earth. After an initial explosion, lava then flows from the
volcano creating a second layer of material. As these layers solidify, they create alternating
levels, or strata, of material. Ash from the volcanic eruption is also present between the lava
layers along the edge of the volcano. Composite volcanoes are common along the Pacific
Ring of Fire and other major tectonic plate boundaries where the presence of water in the
magma chamber creates explosive eruptions.
Shield Volcanoes
Shield volcanoes get their name from their shape—a huge shield laid on its side. Figure
8.23shows the Mauna Loa Volcano. You can see that shield volcanoes do not have the
steep mountainous sides of composite volcanoes. They have a very wide base and are much
flatter on the top than composite volcano. Although they are not steep, they may be very
large. The Mauna Loa Volcano has a diameter of over 112 kilometers (70 miles) and forms a
significant part of the island of Hawaii. The Mauna Kea Volcano, also in Hawaii, is another