Shield volcanoes form in areas where low viscosity magma comes to the surface as fast-flowing lava. The lava travels farther than other types of lava, and the steady build-up caused by this type of magma creates the softer sides seen in shield volcanoes.
The term "shield volcano" comes from the fact that its low profile looks like a shield lying on the ground. Shield volcanoes are typically made of basalt but can be formed by felsic lava as well. Shield volcanoes occur over hot spots in the crust, such as with the Hawaiian shield volcanoes, or over rift zones, such as in the Icelandic shield volcanoes. Shield volcanoes experience almost continuous eruptions that occur over a long period. The most famous of this type of volcano is Mauna Loa, located on the Island of Hawaii. The typical shield volcano has a gentle slope of about two to three degrees, which rises to about 10 degrees at the top of the slope. While this is considered typical, there are many regional differences that depend on local conditions and magma composition. The shield volcanoes of Oregon and California tend to be symmetrical, while the shield volcanoes of Hawaii tend to be off center. This occurs because Hawaiian volcanoes typically erupt from a fissure rather than a single point.