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 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.Composite volcanoesSome of the Earth's grandest mountains are composite volcanoes, which are sometimes called strato-volcanoes. Typically they are steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs. They rise to be as much as 8,000 feet above their bases. Figure 5: Shishaldin Volcano, A composite cone that towers 9,272 feet above sea level in the Aleutian Islands, AlaskaThe composite volcano is formed when magma rising upward through a conduit, erupts at the Earth's surface to form a volcanic cone. The lava spreads over the surrounding area. As the volcanic activity continues, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepen their valleys. When the volcanic activity finally ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened "volcanic plug" in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or "volcanic neck," a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau (Fig 5).Calderas Figure 7: Aerial view of Crater Lake Caldera,...
