Icelandic shields are similar, but on a much smaller scale as they are rarely more than 1000 metres high, often under 100 metres. They have a tendency to be symmetrical, as they form from a central pipe or fissure, the lava then spreading out evenly over the landscape. It is now thought that many Icelandic shields formed from a fissure originally and only developed into a shield when the eruptive activity became localised ( Williams & McBirney, 1979 ). These shields have a similar slope angle to the Hawaiian shields.
The third type is to be found on the Galapagos Islands off the coast of South America, and they are different from the above two types in both form and structure. The average slope angle is 25 degrees, and the eruptive fissures are concentric. It is not totally understood why the slopes are so steep, but one theory is lava flows mantling together pyroclastic cones which were formed at an earlier stage of activity ( MacDonald, 1972 ).
The vast majority of continental volcanoes are composite cones ( or stratovolcanoes ), although this latter term is somewhat misleading as pyroclastic cones and shields are also stratified. Their major characteristic is that they are made up of both lava and pyroclastic material, which tend to be erupted alternately, resulting in a volcano which is made up of alternating layers of ash and lava (See figure 1). When the ground plan is looked at, they look circular, this being due to the fact that volcanic products are being produced from a single central vent. The slopes tend to be at an angle of 10-35 degrees with a slight concavity, becoming more and more concave in later stages of eruptive activity. This shape is influenced by the magma composition, which is of intermediate viscosity ( e.g. andesites ), and the manner of growth. The steeper sides near the summit are as a result of short viscous lava flows at the top of the volcano, along with larger deposits of tephra than further down the mountainside. Atop these volcanoes, there is a crater, which is created by the continual blasting out of material during eruptions, and enlarged by the collapse of the vent walls after the magma eruption. As these volcanoes near extinction, not only do they become more concave, but they start to develop parasitic cones at lower levels, quite a common phenomenon.
There are many great examples of composite cones, like Mount Mayon and Fujiyama, which has a base diameter of 30 kilometres and is said to rise 12,000 feet above the surrounding lowlands. The reason that they can reach heights so tall is that the pyroclastic material is strengthened by the ribs of lava interbedded within it, and the lava can also act as a protective 'cap' against erosion. This is why stratovolcanoes are much bigger than pyroclastic volcanoes.
Pyroclastic volcanoes, or cinder cones, as a rule tend to form by explosive eruptions of viscous magma from a monogenetic structure, that is they erupt only once, and they are often found in groups. As they consist almost wholly of tephra ( pyroclastic material ), they rarely reach heights above 1,500 feet due to continual slumping and sliding of the loose material. An example of a cinder cone is Paricutin, which erupted in 1943 but had only reached a height of 410 metres at the end of its short life span nine years later. The cones are well stratified and basically symmetrical, unless the vent was elongated, with smooth slopes of 25-40 degrees, gradually declining towards the foot of the volcano due to the decreasing levels of fallout material reaching these areas. At the top of these structures, you will find a small bowl/saucer shaped crater. As time goes by, the loose material comprising the volcano will become cemented together by groundwater.
Acid cones, domes, or tholoids, are the volcano type associated with great magma viscosity. This viscosity leads to the formation of endogenous lava domes, and finally plug domes and spines, the formation of which is accompanied by very violent explosion phenomena. There are numerous types of domes, many of them typifying the later stages of activity in mature volcanoes. Their simplest form is literally no more than a steep-sided 'cake' of lava, such as the Puy de Sarcoui (See figure 4), which has a height of 150 metres but a diameter of only 400 metres across. The chain of Puy's in Auvergne is probably the best example of acid cones. As the lava has difficulty flowing, it piles up leading to a steep-sided hill over and around the vent, and they are capable of growing both internally and externally. One type of dome is an endogenous dome, while others include intrusive domes and upheaved plugs. The most famous acid cone is Mount Pelee on the Caribbean island of Martinique, which erupted in 1903. In the days before the eruption, the volcano was growing at a rate of up to 78 feet per day, and after the extremely explosive eruption, a spine of viscous lava was left standing atop the volcano before it collapsed. Another place where domes are found is on composite cones, or even inside their craters, as in the case of El Misti.
If when a volcano erupts there is a void created underground in what was formerly the magma chamber, the above rocks will be unsupported and may collapse. This leads to a large, steep-sided, circular basin called a caldera (See figure 3). It is thought that this explosion-collapse theory is the explanation for most of the calderas on Earth. Calderas, diameter greater than one kilometre across, can form within hours of the eruption. Not all calderas form from one solitary outbreak, but in fact a whole series of eruptions. Throughout the world there are numerous well known examples of calderas. Krakatoa erupted in 1883 producing a submerged caldera six kilometres in diameter. Crater Lake in Oregon is 8 kilometres across, while Valle Grande, west of Los Alamos, has a diameter of 20 kilometres and a depth of over one kilometre
Fissure volcanism is the dominant process along mid-ocean ridges, but is also a characteristic of continental areas under stress ( Duff, 1993 ). Although no volcano as such is created, it does lead to a large lava plateau, such as the Deccan in India. It is estimated that over the course of geological time, sheets of fissure lava has covered 2,500,000 square kilometres of continental land area ( Rittmann, 1962 ). Fissure vents are nearly always monogenetic. The separation of the Eurasian and the North American plates has led to many of these eruptions. The Giants' Causeway in Northern Ireland is the result of one such incident, while in Laki, Iceland in 1783, an eruption of this type killed 9336 people after decimating the land and causing the onset of a famine. The fracture was 32 kilometres long, and the lava flowed out 64 kilometres in one direction, covering a total area of 588 square kilometres with a volume of 12 cubic kilometres.
Apart from the volcano types already mentioned, there are many other smaller kinds, such as tuff cones, pumice cones, gas maare, mud volcanoes, fumaroles, solfataras and geysers, such as Old Faithful. As said in the introduction to this essay, submarine volcanoes are numerous throughout the oceans, but surprisingly not one deep water eruption has ever been witnessed ( Decker & Decker, 1989 ). Shallow submarine eruptions are much better understood, and the creation of Surtsey by one such eruption was documented from its beginnings in 1963. Nobody knows for certain how many submarine volcanoes there are, or how many erupt each year.
As many factors have an influence on volcanoes, it is safe to say that very few, if any, volcanoes are the same in profile, particularly when local factors are taken into account. Of all the factors mentioned in this essay, two are of prime importance. Firstly, the shape of the vent, which can range from a tube-like hole to a gaping fissure, determines how the magma spreads out onto the land. The second, as Decker & Decker ( 1989 ) stated, is:
"the viscosity of the magma, as it erupts. Very viscous lavas form a steep sided plug over the vent called a lava dome. Solid fragments thrown from a vent form a pile of debris around or downwind from the crater called a cinder cone. Very fluid lavas flow long distances on gentle slopes, forming lava plateaux or low sloping volcanic piles called shield volcanoes. Since the composition of a lava is generally related to its viscosity - basalt is more fluid and rhyolite more viscous - the shape of a volcano is often an important clue to its composition."
It is the combination of factors that gives a particular volcano a particular shape and form.
Bibliography
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