Shield volcanoes, the third type of volcano, are built almost entirely
of fluid lava flows. Flow after flow pours out in all directions from a
central _summit_ vent, or group of vents, building a broad, gently
sloping cone of flat, domical shape, with a profile much like that of a
warrior’s shield. They are built up slowly by the accretion of thousands
of highly fluid lava flows called basalt lava that spread widely over
great distances, and then cool as thin, gently dipping sheets. Lavas
also commonly erupt from vents along fractures (rift zones) that develop
on the flanks of the cone. Some of the largest volcanoes in the world
are shield volcanoes. In northern California and Oregon, many shield
volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000
feet. The Hawaiian Islands are composed of linear chains of these
volcanoes including Kilauea and Mauna Loa on the island of Hawaii—two of
the world’s most active volcanoes. The floor of the ocean is more than
15,000 feet deep at the bases of the islands. As Mauna Loa, the largest
of the shield volcanoes (and also the world’s largest active volcano),
projects 13,677 feet above sea level, its top is over 28,000 feet above
the deep ocean floor.
In some eruptions, basaltic lava pours out quietly from long fissures
instead of central vents and floods the surrounding countryside with
lava flow upon lava flow, forming broad plateaus. Lava plateaus of this
type can be seen in Iceland, southeastern Washington, eastern Oregon,
and southern Idaho. Along the Snake River in Idaho, and the Columbia
River in Washington and Oregon, these lava flows are beautifully exposed
and measure more than a mile in total thickness.
[Illustration: Mauna Loa Volcano, Hawaii, a giant among the active
volcanoes of the world; snow-capped Mauna Kea Volcano in the distance.]
[Illustration: The internal structure of a typical shield volcano.]