Classification of Volcano: Shape Pt 4 (final)

Acid or Dome Volcanoes

Internal structure of a typical lava dome
Source: http://pubs.usgs.gov/gip/volc/fig18.gif

Acid or dome volcanoes are steep-sided, convex cone because of the viscous (high silica content) lava which quickly cools and solidifies near the crater on exposure to air. The volcano grows largely by expansion from within, as indicated in the internal structure of the volcano by the layers of lava fanning upward and outward from the center. As it grows its outer surface cools and hardens. When part of a dome volcano collapses while it still contains molten rock and gases, it produces pyroclastic flow, one of the most lethal forms of volcanic event. Ultimately, many volcanic domes are destroyed by large explosive eruptions. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees." Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes.

Volcanic dome atop Novarupta vent, Valley of Ten Thousand Smokes, Katmai National Park and Preserve, Alaska
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Katmai/dds40-015_large.jpg

Next entry: Classification of Volcano by eruption style

Classification of Volcano: Shape Pt 3

Tephra Cone

Internal View of Tephra Cone volcano
Source: http://pubs.usgs.gov/gip/volc/fig8.gif

Cinder cones are the simplest type of volcano. They tend to be mainly explosive volcanoes, but they can also issue lavas. They are small volume cones consisting predominantly of tephra that result from eruptions consisting of rhyolitic and andesitic materials. They are actually fall deposits that are built surrounding the eruptive vent. They show an internal layered structure due to the varying intensities of the explosions that deposit different sizes of pyroclastics. They grow rapidly and soon reach their maximum size, not exceeding 250 meters in height and 500 meters in diameter, although some may rise to as high as 650 meters or more. Cinder cones can occur alone or in small to large groups or fields and most of them have a bowl-shaped crater at the summit. The gradual decrease in volume of the fallout materials (ash, lapilli) at greater distances from the vent leads to gentler slopes at the base of the cone. Cinder cones are commonly found in western North America as well as throughout other volcanic terrains of the world.

Pu`u ka Pele on the flanks of Mauna Kea, Hawaii
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/30424305-084_large.JPG


Mt. Veniaminof in Alaska in its final stages of eruption in 1983-1984
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Veniaminof/dds40-057_large.jpg

Classification of Volcano: Shape Pt 2

Composite Volcano or Stratovolcano

Cross-section of a typical Stratovolcano showing composite,
stratified nature with alternating layers of lava and tephra.
Source: http://pubs.usgs.gov/gip/volc/fig10.gif

Some of the Earth’s grandest mountains are composite volcanoes (aka stratovolcanoes). Stratovolcanoes show alternate layering of lava flows, tephra and volcanic ash, which is why they are called composite volcanoes. They are usually steep-sided, symmetrical cones of large dimension built of alternating layers of viscous andesitic lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. The high silica content of the magma results in it being viscous. It also makes gases trapped in it harder to escape, which frequently results in explosive eruptions. The steep slope near the summit is partially due to the thick, short viscous lava flows that are unable to travel far down the slope. The gentler base is due to the accumulation of material from the volcano and also pyroclastic material.

Most of the composite volcanoes have a crater at the summit which is formed by the explosive ejection of material from the central vent. Sometimes the craters have been filled in by lava flows or domes, or with glacial ice and less commonly they are filled with water.

The sequence of events for a violent eruption of Mount St. Helens, a composite volcano, in 1980. In the first photograph taken at 8:32:47, the original cone is intact, with some steam rising from the vent. The last photograph taken at 8:33:18,
shows the original cone blowing up.
Source: http://erg.usgs.gov/isb/pubs/teachers-packets/volcanoes/poster/graphics/posterfig6-7.jpg

Some of the most beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan (shown in the picture below), Mount Mayon in the Phillipines, Mount Cotopaxi in Ecuador , Mount Shasta in California, Mount Hood in Oregon and Mount Rainier in Washington.

Mount Fuji, Japan
Source: http://www.mt-fuji.co.jp/MMF/07/07feb.jpeg

Classification of Volcano: Shape Pt 1

Volcanoes can be classified based on their shape, their eruptive style and the tectonic environment. This entry will begin on classification of volcano through their shapes.

Shield Volcano

The Internal Structure of a Typical Shield Volcano
Source: http://pubs.usgs.gov/gip/volc/fig15.gif

Shield volcanoes form the largest volcanoes on Earth. They have gentle upper slopes and somewhat steeper lower slopes (somehow resembling a warrior’s shield, thus its name). It is built up slowly by the accretion of thousands of flows of low viscosity basalitic magma (which is very fluid) that easily spreads over great distances from the summit vent, then cooling as thin gently sloped sheets. The viscosity of magma is dependent on its temperature and composition. Shield volcanoes erupt magma as hot as 1,200 °C, compared with 850 °C for most continental volcanoes, which are usually composed of acidic lava. Because of the fluidity of the lava, major explosive eruptions do not occur in shield volcanoes. Lava also erupts from the vents along fractures that develop on the flanks of the cone. This gives the shield volcanoes the circular/oval shape with nearly flat summits.

The largest shield volcano, Mauna Loa volcano on the Island of Hawaii
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/shieldvolcano1_large.jpg

Newberry Volcano, Oregon
USGS photo by Lyn Topinka

The Hawaiian Islands are composed of linear chains of the shield volcanoes. The islands are more than 15,000 feet above the ocean floor. For the Mauna Loa, the largest shield volcano, it is about 13,677 feet above sea level and its summit is 28,000 feet above the ocean floor. In Northern California and Oregon, many of the shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet.