Shield volcanoes are composed primarily of lava flows. From a central summit vent, basaltic lava flows out onto the surface and spreads out as it cools. As a result, a volcano with a very low slope but wide diameter is formed. The process repeats itself as the volcano grows in size. Eventually the volcano resembles a shield in its domed form.
Large shield volcanoes are abundant in northern California and Oregon, with diameters extending 3 to 4 miles wide and having heights of 1,500 to 2,000 feet. In Hawaii, the Kilauea and Mauna Loa are also examples of very large and active shield volcanoes. Mauna Loa is in fact, the largest active volcano in the world with a height of 13,677 feet above the sea level and a base that reaches more than 14,000 feet into the ocean.