Volcanic Landforms

Explosive Eruptions

A second category of volcanoes are those characterized by explosive eruptions. Explosive eruptions are common to volcanoes with very viscous lava and high amounts of gas under pressure.  The viscosity, or stickiness, of the lava relates to the silica content. Magma high in silica is more viscous than lavas low in silica. Explosive eruptions are common to volcanoes along the "Ring of Fire", a string of volcanoes extending from the northwest coast of the United States, up through the Aleutian Islands, and into Japan . As the magma rises through the central vent, it gets stuck and gases build to high pressures until an eruption of great force occurs. 

Figure 16.11 Mt. St. Helens, a composite volcano
Source: USGS

Vast amount of ash and pyroclastic material can be ejected from these kinds of volcanoes. Accompanying many of these eruptions are vast outpourings of noxious gases and fine particulate matter called "Nuees Ardentees" or "glowing clouds or avalanches". These clouds can rush down the flanks of a volcano at speeds reaching 60 mph. Escape from such clouds is virtually impossible (See Volcanic Hazards and Episodes: Mt Pelee.)

Figure 16.12 A nearly perfect cinder cone in the Cinder Cones and Lava Beds Natural Landmark Area in the north-central portion of the Mojave National Preserve.
Courtesy USGS (Source: "Our Dynamic Desert")

Two types of volcanoes characteristically produce explosive eruptions, cinder cones and composite volcanoes. Cinder cones are primarily composed of layers of pyroclastic material built from rock fragments once lodged in the central vent of the volcano. Paricutin is one of the most famous cinder cones erupting from a Mexican farmer's field in 1943.

Figure 16.13  Cinder Cone  Example: Paricutin	Figure 16.14  Composite or Stratovolcano Example: Mt. Rainier & Mt. St. Helens

Composite, or stratovolcanoes also produce explosive eruptions. As shown in Figure 16.14, composite volcanoes form from alternating eruptions dominated by pyroclastics or lava. As a result, composite volcanoes display layers of these alternating flows. Composite volcanoes are among the tallest volcanoes on earth, with Mt. Fuji, Mt. St. Helens, and Mt. Kilimanjaro being examples. Composite volcanoes are often associated with convergent plate boundaries where subduction is occurring. 

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