Mount Mayon and Fujiyama
“A volcanic cone is the result of the accumulation of ejected material around a vent”, and many different shapes can be identified. It is thought that there is somewhere in the region of 600 active volcanoes (erupted in the last 25,000 years ) currently on land or exposed above the sea on islands, with many more extinct ones, but this is nothing compared to the number of submarine volcanoes under the oceans. There is more than 50,000 under the Pacific alone ( Summerfield, 1990 ). The location of volcanoes is not random, almost all are found at plate boundaries where new crust is either created or destroyed.
The actual form of the volcano differs from one place to another, and they can be classified by two separate factors, the shape of the volcano, or the type of eruption that led to its formation. In this essay, I plan to concentrate on the former of these two things, and look at how the shape of a volcano differs, along with what causes it to do so in each case. The main factors important in volcano form include: the shape of the vent, the number of eruptions from a single vent, the surface environment around the vent, how viscous the magma is, the nature of the eruptive activity, the time between eruptions, and the volume of erupted material.
Shield volcanoes are some of the largest volcanic structures on the planet, the absolute height of Mauna Loa and Mauna Kea on Hawaii illustrates this as they are around 30,000 feet, with a base of almost 400 kilometres across. These volcanoes, which can be subdivided into Hawaiian, Icelandic and Galapagos types, have one thing in common, that they are made up of very fluid, non-viscous basaltic lava. The rapid accumulation of successive flows of this lava leads to a volcano with shallow sides, steeper near the flanks as the lava cools and needs more of a gradient to continue to flow (See figure 2).
Hawaiian shields are the largest of the three types; it is thought that the total volume of Mauna Loa is around 40,000 cubic kilometres ( Williams & McBirney, 1979 ). They form due to eruptions from fissure vents, and the thin fluid basaltic lava leads gradually to the building up of the volcano. Convex on the upper slopes but concave further down, the slope angle can vary from 2-12 degrees. The actual form of the volcano is controlled by rift zones, groups of fissures which converge to form a caldera at the top of the mountain.
These rift zones, up to three kilometres wide, meet at an angle of 130-180 degrees and are marked at the surface by splatter cones, collapse pits and small grabens. 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.