Volcano Eruption, Shield Volcano, Pyroclastic Flow

Volcanoes are a natural way that the Earth and other planets have of cooling off. Planets are warm in their mantles. Heat inside planets escapes towards their surfaces. For reasons that are not well understood, heat sometimes melts rocks, which then rise toward the planet's surface. When the hot rocks, called magma, and included gases, break through the crust an eruption occurs. The buildup of ash and lava flows around the eruption hole (or vent) makes a volcano. Some volcanoes erupt for only a short time - a few days to weeks and never erupt again. Large volcanoes, such as stratovolcanoes and shields, erupt thousands of times throughout their lifetimes, which can last hundreds of thousands to a few million years.

The form of a volcano is determined by the composition of the erupting magma and the type of erupted products, such as pyroclastic and effusive lava.

Types of Volcanoes

Shield Volcanoes

Shield volcanoes are made of thousands of thin basalt lava flows. Because the lava has a relatively low viscosity (low resistance to flow) the lava can travel far from the vent, the location where the lava reaches the surface. The resulting volcanic landform has a broad base and very gentle slopes, much like a warrior's inverted shield.

Eruptions at shield volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosive fountaining that forms cinder cones and spatter cones at the vent. In a shield volcano, 90% of the volcano is lava rather than pyroclastic material. Due to high magma supply rates, the lava is hot and changes very little after it is generated. A common product of hotspot volcanism, shield volcanoes can also be found along subduction-related volcanic arcs or all by themselves.

The Big Island of Hawaii is composed of five coalesced volcanoes of successively younger ages, with the older ones apparently extinct. Mauna Loa, one of the main volcanoes on the Big Island, has a higher elevation than any mountain on Earth: 30,000 feet from the floor of the ocean to its highest peak.

Top

Cinder Cones (Scoria Cones)

Cinder cones are mounds of basaltic scoria that are formed by streaming gases, which carry lava into the atmosphere to form lava fountains. The lava blobs commonly solidify during flight through the air before landing on the ground. If gas pressure drops, the final stage of building a cinder cone may be a lava flow that breaks through the base of the cone. Cinder cones can occur alone but commonly occur in groups or fields.

If there is abundant water in the environment, magma interacts with water to build a maar volcano, rather than a cinder cone. Produced by an explosion in an area of low relief, maar volcanoes are generally more or less circular, and often contain a lake, pond, or marsh.

The longer the eruption, the higher the cone. Some are no larger than a few feet, while others rise higher than 21,013 feet, such as the Paricutin volcano, in Mexico, that erupted almost continuously from 1943 to 1952. Accompanying the near-constant pyroclastic activity were lava flows, which emerged from the volcano's base, destroying the village of Paricutin.

Composite Volcanoes ("Stratovolcanoes")

Composite volcanoes are constructed from multiple eruptions, which sometime occur over hundreds of years or hundreds of thousands of years. Andesite magma is the most common, but not the only, type of magma to form composite cones. Due to its higher viscosity, Andesite magma produces more brittle lava than basaltic lava.

Although Andesitic composite cones are composed mainly of fragmental debris, some of the magma intrudes into the cones as dikes or sills. In this way, multiple intrusive events build a structural framework that knits together the voluminous accumulation of volcanic rubble. These cones thus can stand higher than cones composed solely of fragmental material. Composite cones can grow to such heights that their slopes become unstable and susceptible to collapse from the pull of gravity.

Famous examples of composite cones are Mayon Volcano in the Philippines, Mount Fuji in Japan, and Mount Rainier in Washington state, U.S. Some composite volcanoes reach 6,000 - 9,000 feet above their bases. Most composite volcanoes occur in chains and are separated by tens of miles. There are numerous composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".

Top

Domes

Lava domes result from the slow extrusion of highly viscous silica-rich magma. Most domes are rather small, but can exceed 6 cubic miles in volume. Extrusions from the dome may end up as rather slow-moving lava, but many begin explosively, forming 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, which forms thick stubby flows, or domes. As a dome enlarges, its margins slowly creep outward as a lava flow, with steep cliff-like margins and a rubbly surface. If the 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, or grow in craters and along the flanks of composite cones. A dome has been growing slowly within the crater of Mount St. Helens since its eruption in 1980.

From 1991 to 1995, the continued growth of the Mount Unzen dome in Japan spawned the initiation of hundreds of small but highly destructive pyroclastic flows and surges. The eruption of Mount Unzen, taught volcanologists a valuable lesson: that an active dome may collapse and cause the development of pyroclastic flows.

Calderas

Calderas are circular to oblong depressions formed by the collapse along curved fractures associated with the extrusion of pyroclastic materials. Their diameters, which can be as long as 37 miles, are many times larger than those of associated vents. The largest estimated volume of erupted products is over 834 cubic miles, and deposits are known to have covered 9,653 square miles. The frequency of such voluminous eruptions is very low, with volumes of 119 cubic miles having a frequency of about 100,000 years. Such eruptions have occurred at Long Valley, California (Mammoth area) with a caldera diameter of 12.4 miles, in the Colorado Rocky Mountains (San Juan Mountains), southern New Mexico (Los Alamos area), New Mexico (Valles Caldera in the Jemez Mountains), and many other places. Most calderas in western North America have developed on the thick continental crust. Intraoceanic calderas are usually smaller in size and the eruptive volume and less silicic.

The area of caldera collapse is proportional to the volume of erupted material. Depths of subsidence, as indicated by thickness of caldera fill, is 0.62-1.9 miles. Structural boundaries of calderas are commonly single or composite ring fault zones along which the initial collapse took place. In deeply eroded calderas, these structural boundaries may be expressed by a ring dike along arcuate faults during or after collapse.

Top