Volcanoes Forms

Mayon Volcano, a beautifully symmetrical but dangerous composite volcano on Luzon Island, Philippines.

    A volcano is a mound, hill or mountain constructed by solid fragments, lava flows, and or dome-like extrusions deposited around a vent from which the material is extruded. The vent is a conduit that extends from the earth's upper mantle or lithosphere to the surface. Most of the material is deposited close to the vent, but some is carried high into the atmosphere to be spread by winds hundreds or thousands of kilometers from the source.

Types of Volcanoes

Shield Volcanoes

View northward toward Mauna Loa volcano from Pohue Bay, south coast of Hawaii. The broad curving horizon line is the summit of Mauna Loa that stands over 14,000 feet above sea level and    30,000 feet above the sea floor. It is the highest mountain on earth.

    Shield volcanoes are large volcanoes with broad summit areas and low-sloping sides (shield shape) because the extruded products are mainly low viscosity basaltic lava flows. A good example of a shield volcano is the Island of Hawaii (the "Big Island"). The Big Island is formed of five coalesced(接合) volcanoes of successively younger ages, the older ones apparently extinct. Mauna Loa, one of the main volcanoes, has a higher elevation than any mountain on earth — 9090 meters (30,000 feet) from the floor of the ocean to its highest peak.

Domes

Mount Unzen and Shimabara City, Kyushu, Japan. Photograph shows Unzen dome and pyroclastic flow and pyroclastic surge pathways down the mountain to the sea. Photograph 20 January 1992 by Asia Air Survey Co., Ltd.

    Lava domes form by the slow extrusion of highly viscous silica-rich magma. Most domes are rather small, but can exceed 25 cubic km in volume. Domal extrusions may end up as rather slow-moving lavas but many begin explosively, forming reamed-out explosion pits blanketed by pyroclastic debris. The explosive activity wanes as the gas content decreases. With lowered gas pressures, the magma extrudes slowly as viscous lava that forms thick stubby flows, or domes that are spinal or dome-shaped. As a dome enlarges, its margins slowly creep outward as a lava flow with steep cliff-like margins and a rubbly surface. If protrusion occurs on a steep slope, dome margins can collapse in a dangerous mass of hot rubble that can form pyroclastic flows. Domes can be solitary volcanoes, form in clusters, grow in craters or along the flanks of composite cones. A dome has been growing slowly within the crater of Mount St.Helens since the eruption of 1980. Domes have also filled the crater(火山口) of Mt. Pelée, Martinique(馬丁尼克島), and many other volcanoes.

Calderas

Photograph encompassing part of Crater Lake caldera, Oregon, U.S.A. Diameter about 8 km. View is toward the east and includes a late-stage volcano in crater named Wizard Island.

Types and Origins of Calderas

Postcollapse Activity

(1) continued volcanism within or near the caldera,

(2) resurgent uplift of the caldera related to renewed rise of the magma. Renewed rise of magma after caldera collapse has caused uplift of many caldera areas. This is called resurgence. It can consist of doming of the central floor of single calderas, or be a broad regional uplift of one or more calderas and close-by areas. Resurgent(復甦的) calderas are more numerous along the continental margin arc of the Americas than in young volcanic arcs elsewhere in the Pacific Basin. Resurgent structures are associated(組合) with large calderas greater than 10 km diameter.

(3) sedimentation within the caldera basin commonly within a lake.

(4) hydrothermal activity and mineralization(礦化作用) resulting from the interaction of meteoric water and hot country rock. Postcollapse volcanism can continue for millions of years.