Volcanology
Volcanology is the study of
volcanoes,
lava, magma and related
geological phenomena. A volcanologist is a person who studies in this field. The term
volcanology is derived from the
Latin word
vulcan, the Roman god of fire.
Volcanologists frequently visit volcanoes, especially active ones, to observe
volcanic eruptions, collect eruptive products including tephra , rock and
lava samples. One major focus of enquiry is the prediction of eruptions; there is currently no accurate way to do this, but predicting eruptions, like predicting earthquakes, could save a lot of lives.
Encyclopedia
Volcanology is the study of
volcanoes,
lava, magma and related
geological phenomena. A
volcanologist is a person who studies in this field. The term
volcanology is derived from the
Latin word
vulcan, the Roman god of fire.
Volcanologists frequently visit volcanoes, especially active ones, to observe
volcanic eruptions, collect eruptive products including tephra , rock and
lava samples. One major focus of enquiry is the prediction of eruptions; there is currently no accurate way to do this, but predicting eruptions, like predicting earthquakes, could save a lot of lives.
Volcano formation
Like most phenomena occurring in the earth's interior, the movements and dynamics of
magma are poorly understood. However, it is known that an eruption may follow movement of magma upwards into the solid layer beneath a volcano and occupying a
magma chamber. Eventually, magma in the chamber is forced upwards and flows out across the planet's surface as
lava, or the rising magma can heat water in the surrounding landform and change the water into steam, creating great pressure. As a result, explosive eruptions can occur. Such explosive eruptions can produce a wide range of volcanic debris, such as
volcanic ash ,
volcanic bombs, which can be large enough to kill people and animals. Eruptions can vary from effusive to extremely explosive.
Many volcanoes are formed at
destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust—a process is called
subduction. As the oceanic crust is subducted, it descends into the mantle where temperatures are generally higher than near the surface of the planet. Increases in temperature and pressure with depth cause water trapped in the descending oceanic crust to escape from minerals in the crust. This process is called dehydration, commonly occurs at depths of about 100 km and can also be a source of very deep earthquakes due to an associated change in volume of the dehydrating rock mass . The water that escapes from the dehydrating oceanic crust migrates into the surrounding mantle which has a different composition than the descending crust. At ambient conditions in the mantle at 100km depth, water will induce partial melting of the mantle. This melt is less dense than the surrounding mantle and will consequently rise though the mantle to the overlying crust. As the magma rises through the crust it may melt and assimilate some of the surrounding crust, it may cool and begin to grow crystals, and it may exsolve gas.
The relative importance of these processes depends on the composition, amount and ascent rate of the magma. If the magma reaches the surface, it will generate a volcanic eruption. The style of the eruption will depend on the composition and gas content of the magma. The type of volcano will depend on the type of magma that usually erupts at that location over a long period of time, and the viscosity of the magma. High concentrations of silica are associated with high viscosity and will form steep sided volcanoes. Volcanic arcs forming near subduction zones, on the edges of continental plates, usually form high-silica melt which create steep sided stratovolcanoes due to the high viscosity of the melt. For example,
Mount St. Helens is found inland from the margin between the oceanic
Juan de Fuca Plate and the continental
North American Plate. Other examples of chains of stratovolcanoes include the
Andes, the
Cascade range, and the
Aleutian Islands.
A volcano is often stereotyped as a mountain sending forth from its summit great clouds of smoke with flames. The truth is that a volcano seldom emits either
smoke or
flame, although various combinations of
hydrogen,
carbon,
oxygen, and
sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust , mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire.
Perhaps the most conspicuous part of a volcano is the
crater, a basin of a roughly circular shape, formed by a
vent from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed
calderas. Some volcanoes consist of a crater alone, with scarcely any
mountain at all; but in the majority of cases the crater is situated on top of a mountain , which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a
conical form.
In some volcanoes, smaller cones or vents may form lower down the principal volcano, along rift zones or fractures. Such features are known as
flank vents,
flank cones or
flank craters.
As a volcano becomes extinct and becomes eroded, solidified lava is often less easily eroded than
volcanic ash and as a result, create interesting landforms. Solidified lava filled fractures called dikes often remain. The main vent may remain behind as a volcanic neck.
Shiprock in
New Mexico,
United States is a fine example of these features.
Tectonic environments of volcanoes
Volcanoes can principally be found in three tectonic environments.
Constructive plate margins
These are by far the most common volcanoes on the Earth. They are also the least frequently seen, because most of their activity takes place beneath the surface of the oceans. Along the whole of the
mid-ocean ridge are irregularly spaced surface eruptions, and more frequent sub-surface intrusions without surface expression. The large majority of these are only known because of earthquakes as part of the eruptions, or occasionally if passing shipping happens to notice unusually high water temperatures, unusual rumbling, or chemical precipitates in the seawater. In a few places, midoceanic ridge activity has led to volcanoes reaching to the surface—
Saint Helena and
Tristan da Cunha are examples—allowing them to be studied in some detail. But most activity takes place at considerable water depths. Iceland is also on a ridge, but has different characteristics than a simple volcano.
It could be argued that the volcanoes of the
Great Rift Valley system of East Africa are modified constructive margin volcanoes. However the modifications caused by the presence of thick continental crust are very substantial, and the magmas produced are often very different from the typically very homogenous MORB that makes up the huge majority of constructive margin volcanoes. But still, some MORB lavas are known to have erupted on land, such as in the
Afar Triangle, which makes up the northern end of the
African Rift Valley. In fact, the
Afar Triangle is a chance to see seafloor spreading on dry land, as many parts of it actually lie below sea level and due to the combination of mountain ranges cutting it off from the
Red Sea and the fiercely hot and arid climate, it has largely dried up with extensive salt flats. Erta Ale is probably the best known volcano in this region, and is well known for its semipermanent lava lake activity.
Destructive plate margins
These are the most visible and among the best-known types of volcanoes on earth, forming above the
subduction zones where plates dive into the mantle. Their magmas are typically
calc-alkaline as a result of their origins in the upper parts of altered ocean plate materials, mixed with sediments, and rise through variable thicknesses of more-or-less continental crust. The denser plate sinks under the lighter one and the friction from the melting plate causes magma to force its way out through a crack in the crust. Unsurprisingly, their compositions are much more varied than at constructive margins.
Hotspots
Hotspots were originally a catch-all for volcanoes that didn't fit into one of the above two categories, but today this refers to a more specific circumstance—where an isolated plume of hot mantle material hits the underside of the crust, either . The mantle plume can lead to a volcanic center that is not obviously connected with a plate margin. The classic example is the
Hawaiian Islands, which is generated by a hotspot underneath the oceanic crust of the Pacific. Yellowstone is cited as another classic example; in this case this involves continental crust because it is far inland. Iceland is sometimes cited as a third classical example, but complicated by the coincidence of a hotspot intersecting an
oceanic ridge constructive margin.
There are debates about the simple "hotspot" concept, since scientists cannot agree on whether the "hot mantle plumes" originate in the upper mantle or in the lower mantle. Meanwhile, field geologists and petrologists see considerable variation in the detailed chemistry of magmas generated by mantle plumes. Additionally, high-resolution
seismology of different hotspots is yielding different pictures of the deep sub-structure of
Hawaii versus
Iceland. There is no detailed consensus about how to interpret these varied results, and it seems plausible that eventually several different sub-types of hotspots may be identified in the future.
See also
- Important publications in volcanology
- History of Volcanology
External link
