Geomorphology 33 J Bret Bennington
Volcanoes are places where molten rock (magma) is extruded as lava. In addition to molten rock, magma also contains some solid crystals, as well as dissolved gases (mostly water and CO2).
Volcanic eruptions can range from explosive to non-explosive. To understand the different types of volcanoes and their eruptions, we need to know something about magmas.
Different types of Magma and Lava
There are a range of different compositions of magma that are produced by different plate tectonic mechanisms and are associated with different types of volcanoes.
Mantle magma - mid-ocean ridges and hot spots
Continental margin magma - subduction of ocean floor
Continental magma - melting of continental rock
Differences in magmas: The primary compositional difference in magma is in the amount of silica that makes-up the melt. Three distinct types of magma are most common. All are dominated by silica, but the silica content can vary from 50% up to about 70%. [slide]
Basaltic magma 50% silica - derived from the dry melting of mantle rock.
Andesitic magma 60% silica. - derived from the melting of mantle rock in the presence of water.
Dacitic / Rhyolitic magma 65-70% silica. - derived from the melting of continental crust.
The principle stuctural difference between these magmas is in their viscosity. The complex silica ion likes to bond to others of its kind to form complex three dimensional networks. The greater the amount of silica in the melt, the more bonding occurs and the stickier or more viscous the magma becomes. Also, as magma cools it becomes more viscous.
Another important difference between magmas is in the amount of dissolved gas that they contain. Gas content typically varies from .2% to 3%. Volcanic gases are mostly water vapor and carbon dioxide, with smaller amounts of more noxious gasses such as hydrogen sulfide.
Gas in magma behaves just as gas in carbonated beverages does. Deep in the crust the magma is under pressure and can thus hold more dissolved gas. As magma rises to the surface, the pressure on the magma decreases and the gas comes out of solution, sometimes explosively.
Viscosity and Gas: The more viscous the magma is, the more difficult it is for gas to escape quickly through the lava. Gas pressure that cannot escape from magma will build-up beneath and within the magma.
Six general types of Volcanic Landform
Flood Basalts - widespread, high volume eruption of fluid basaltic magma from fissures.
Shield Volcano - high volume eruption of fluid basaltic magma from vents.
Stratovolcano / Composite Volcano - steep, high volcano built from alternating eruptions of andesitic magma as pyroclastic flows and lava flows.
Cinder Cone - symmetrical cone produced by eruption of lava fragments from a single vent, built up as loose material accumulating around the vent at about the 30° angle of repose. Associated with both shield and stratovolcanoes.
Caldera - wide volcanic crater produced by the post-eruption collapse of a volcano summit into its own emptied magma chambers. Associated with both shield and stratovolcanoes.
Lava Dome - relatively small, steep dome of material consisting of very viscous dacitic / rhyolitic lava that is erupted and builds up directly over the vent.
Because basaltic magma is relatively low in silica (50%) it tends to erupt non-explosively because the fluid nature of the lava permits the gas to escape easily. Even so, the initial release of pressure on a gas-rich basaltic magma can produce a burst of spetacular fountaining. However, when the fountaining dies down after most of the gas has escaped, the lava simply oozes out of the volcanic opening.
Initially, basaltic magmas flow very easily because of their high temperature. They solidify to form a relatively smooth, ropey lava called Pahoehoe. [slide]
However, after moving for some distance the lava mass begins to cool and clump, slowing down its rate of flow. The lumpy, jagged mass of rock produced now is called AA. [slide]
Because of their fluidity, basaltic lavas can flow for great distances, aided by their tendency to form lava tubes ? roofed channels through which the lava can flow and remain hot. [slide]
If basaltic lavas flow from a point source they will form volcanoes called shield volcanoes. Shield volcanoes are extremely wide (because the lava can flow and spread-out over a wide area). Shield volcanoes are by far the largest volcanoes in the solar system by virtue of the incredible volume of lava they can disgorge.
The largest shield volcanoe on the earth is Mona Loa, the large volcano on the Big Island of Hawaii. If measured from its base on the sea-floor to its summit, Mona Loa is 5.5 miles high, significantly higher than Mt. Everest. Other shield volcanoes include the Tahiti, Samoa, and the Galapagos Islands.
Shield volcanoes are associated with hot spots, localized regions of high heat flow in the mantle that are not particularly well understood. The Hawaiian islands have been produced by the pacific plate moving west over a stationary hot spot in the mantle.
Another type of basalt flow is associated with larger regions of mantle upwelling along mid-ocean ridges and continental rifting zones. These are linear regions of lava eruption called fissure eruptions. Because of their linear nature, fissure eruptions produce broad expanses of basalt.
In the case of mid-ocean ridges, this basalt forms new seafloor where lithosphere plates are diverging.
On continents, fissure eruptions associated with hot spots or rifting can also produce large regions of basaltic lava called flood basalts. For example, the Columbia River Plateau in Washington, Oregon, and Idaho formed from lava flows that erupted from a series of fissures between 16 and 13 million years ago to cover an area of about 200,000 km2 with an average of 50 meters of basalt.
The Watchung Hills of northern New Jersey are the eroding remains of basalt lava flows that erupted when North America and Africa rifted apart some 200 million years ago.
Shield volcanoes can also be observed on Venus and Mars, and every time you look at the moon you see in the dark areas the massive flood basalts that filled the mare basins some 3 to 4 billion years ago.
Composite or Stratovolcanoes
Because of the higher silica content of andesitic magma it is more viscous than basaltic magma. Andesitic volcanoes are associated with subduction zones where seafloor is sliding back into the mantle. The ring of fire around the margin of the pacific ocean is an almost continuous circle of andesitic volcanoes sitting above subduction zones.
The andesitic magma forms at plate collision zones from the melting of mantle rock in the over-riding plate. Melting is triggered by the release of water as the subducting plate descends at a subduction zone.
Andesitic magmas do not flow as readily as basalts, so they tend to pile-up to form large volcanoes. Furthermore, their higher viscosity makes them prone to highly explosive eruptions. Andesitic magmas form what are called stratovolcanoes. Stratovolcanoes build to great heights through a combination of explosive eruption followed by (once the gas pressure is released) viscous flows that cover and cement the tephra. This is why these volcanoes are also called composite volcanoes.
Explosive eruptions blast large quantities of volcanic rock out of the volcano. Some of this rock is excavated from the volcano itself, the rest consists of fragments of lava that has cooled in the air. This material accumulates around the volcano as pyroclastic deposits of tephra. Large fragments are called bombs, gravel size tephra is called lapilli, fine-grained tephra is called ash.
After an eruption, a stratovolcano or shield volcano may collapse into the now empty magma chamber beneath its peak, producing a large crater or caldera. The caldera can sometimes fill with groundwater, precipitation, or meltwater to produce a crater lake. Calderas may also become resurgent if magma again begins to fill the chambers beneath the caldera, resulting in uplift of the caldera floor and eventual eruption of lava within the caldera.
Crater Lake in Oregon, Yellowstone Park in Wyoming, and the Long Valley Caldera in California are all examples of calderas.
Cinder Cone volcanoes
Eruption of solid particles of lava can occur if the composition of the magma is very rhyolitic or as volcanic vents become clogged late in an eruption episode. The ejection of solid fragments from a vent results in a symmetrical cone of loose debris that accumulates with a slope equal to the angle of repose of the material, usually about 30°. A similar feature called a spatter cone forms from the ejection of small blobs of liquid lava from a single vent.
Cinder cones and spatter cones are found associated with most other types of volcanoes.
Lava Domes / Plug Domes
Because the rhyolitic magma is so viscous, rhyolitic volcanoes tend to erupt a pasty lava that flows with great difficulty, often accumulating above the vent as a dome-shaped mass. The initial eruption may be explosive, creating a crater of pyroclastic material, within which develops a plug of rhyolite and volcanic glass.
A coulee is a large mass of viscous magma that erupts and flows downslope for short distances, forming a feature that is a hybrid of a lava dome and a lava flow.
Rhyolitic volcanoes often produce layered deposits of volcanic ash called tuff (as do stratovolcanoes as well). Additionally, the silica-rich rhyolitic magmas commonly produce solid volcanic glass (obsidian), and frothy volcanic glass (pumice).