How volcanoes grow

Given that eruptions come in many different types it is not surprising that the volcanoes they build do not all look the same. The "typical" volcano pictured in most people's minds is a steep, convex cone resembling those we see in Japanese postcards or movies set in the South Pacific. This may be why some visitors to the Big Island of Hawaii are surprised when they gaze at the gentle slopes of Mauna Loa, which looks very much like the top half of a UFO. "It doesn't look like a volcano!" is the comment I once heard while standing on the terrace of the famous Volcano House Hotel, from where the unassuming shape of Mauna Loa can be viewed in all its glory. I knew at once that the disappointed tourist could not be a native of Iceland, where many volcanoes also look like upside-down saucers. I also wondered how native Hawaiians and Icelanders react when they see for the first time the impressive loom of a typical Ring of Fire volcano like Mt. Fuji.

The fact is that there is no "right" or "typical" shape for a volcano. What a volcano looks like is largely determined by its predominant type of activity and, in turn, by the composition of the magma and the volcano's location on the Earth's mosaic of tectonic plates (Table 3.1). It is therefore possible to tell a lot about a volcano just by looking at its shape - some people call this "reading the landscape." More formally, this is known as geomorphology.

How do the different shapes of volcanoes come about? Let's consider those Hawaiian and Icelandic volcanoes, like Mauna Loa and Skjaldbreid, which look so unimpressive to some. They are known as shield volcanoes because they look like shields dropped face-up on the ground. The term came, naturally, from Iceland and its past history full of shield-wielding, bloodthirsty warriors. The major products of Hawaiian and Icelandic eruptions are long, fluid lava flows. Imagine what happens when flow after flow, all coming out from a single source or from a line of vents, are layered on top of one another. Eventually these flows will build a concave, shallow-sided mound, as illustrated in Fig. 3.9. Some shield volcanoes are topped by large, flat-bottom craters produced by collapse. These craters can be rather spectacular when they are filled up by lava lakes.

If lava flows are erupted but they are more silicic and more pasty than basalts, the result will be a lava dome. Domes are formed by the accumulation of thick, pasty flows and they are quite different in shape from shields, as can be seen from the diagram in Fig. 3.1. Domes are much smaller than shields, rarely exceeding 100 m (330 feet) in height. Their flanks are also much steeper, typically 25 to 30 degrees as opposed to the range of 4 to 8 degrees which is typical for shields. Another major difference is that domes are usually formed by a single phase of eruptive activity rather than by the accumulation of many eruptions. In vol-canological terms, one-episode features or volcanoes are called monogenetic. They are, predictably, smaller than the polygenetic volcanoes that are the result of many eruptive episodes.

Another common type of monogenetic volcanic feature is the cone formed by magma fragments from Strombolian-type activity. Because the falling fragments of lava will tend to accumulate around the vent, most cones have steep and fairly uniform slopes. Most cones are small (tens of meters high) and grow on the sides of larger volcanoes. However, repeated bursts of explosive activity can produce larger cones, including some which are volcanoes in their own right, such as Paricutin and Sunset Crater. Although the activity in those volcanoes lasted for a considerable time, they are considered monogenetic and are not likely to erupt again.

The world's most common type of volcanoes is called stratovolcano or composite volcano (Fig. 3.9).

These usually imposing structures are formed by the accumulation of magma fragments from explosive activity interlayered with lava flows. They tend to have symmetrical shapes with graceful upsweeping slopes and are what many people think of as "typical" volcanoes. In fact, there is a sound basis for such thinking because virtually all continental volcanoes are composite, including some of the world's best known: Vesuvius, Mt. Etna, Mt. Rainier, Mt. St. Helens, Mt. Pelée, and Mt. Fuji. Although they look majestic, these volcanoes are puny compared with oceanic giants like Mauna Loa. For example, Mt. Fuji in Japan has the largest volume of all composite volcanoes: 870 km3 (210 cubic miles). It rises to 3,700 m (12,140 feet) and has a base extending across 30 km (19 miles). The volume of Mauna Loa, however, is larger than that of Mt. Fuji, even when only the shield's above-water volume is considered.

Because most of this book's volcanoes are composite, it is worth discussing them in a little more detail. Their upsweeping profiles are the result of the evolution of the volcanic activity as the volcano gets older. When these volcanoes are young their eruptions tend to come from the central conduit but, as they age, fractures open on the lower flanks from which lava flows come out. Explosive eruptions still take place at the summit and, gradually, the summit cone gets steeper relative to the lower slopes. Erosion also plays a part in accentuating the shapes of composite volcanoes by removing fine material from the upper slopes and

Complex Volcano Stratovolcano

Complex Volcano Stratovolcano

Somma Volcano Caldera

Somma Volcano Caldera

Laua Dome Crater Row Cinder Cone Tuff Ring Maar

Fig. 3.9. The major types of volcanoes. Schematic profiles are vertically exaggerated by a factor of 2 (types shaded in gray) and by a factor of 4 (types in black). Relative sizes are approximate, as dimensions vary within each group. Somma and complex volcanoes are special types of composite volcanoes. Complex (or compound) volcanoes are multiple, genetically related volcanoes that grow in the same location. Somma volcanoes have collapsed calderas as a result of huge explosions, inside which new composite cones form at a later stage. Volcano types shaded in gray are much longer-lived than the other types. Complex volcanoes generally have lifespans of 1 to 10 million years, somma volcanoes and calderas from 100,000 to 1 million years, and stratovolcanoes from 10,000 to 100,000 years. In contrast, lava domes and cinder cones have lifespans from 1 to 100 years, while the other types generally last less than 1 year. (Modified from Simkin and Siebert, 1994.)

depositing it around the base, where it flattens the lower slopes. In general, the older the volcano and the longer it has been inactive, the more pronounced is its upsweeping profile.

Some composite volcanoes, like Hekla in Iceland, look quite different from the classic Mt. Fuji shape. This is because the eruptions from Hekla are fed from long fissures that cut across the volcano, rather than predominantly from a central vent. The number and location of a volcano's vents (which are places from where magma comes out) exert a major influence on the volcano's shape.

Vents can have the form of pipe conduits, coming out from depth to a single opening at the top, or they can have the form of long cracks or fissures, which can transect the volcano. Although some volcanoes have a single (pipe) vent, most have either a fissure system or else numerous separate vents which are often marked by small cones superimposed on the main volcano.

Single, central vents tend to produce volcanoes with symmetrical profiles, while elongated fissures produce overturned-canoe profiles, elongated in the direction of the fissure. A system of fissures or small vents that cut across the volcano in a well-defined direction is called a rift zone. Mauna Loa is a good example of a volcano with well-defined rift zones. Most of the volcano's eruptions start somewhere along these zones. In plan view, these volcanoes are elongated in the direction of the predominant rift zone.

Some volcanoes have lots of lateral vents, either of the fissure or the pipe kind. Eruptions on these volca noes can start just about anywhere and, as a result, their flanks are usually dotted with cones. Mount Etna is a very good example of this type of activity. The "plumbing" underneath Etna is very complex and new vents have sprung up all over the mountain in what seems to be a disorderly fashion although, even here, there are some preferred zones. Etna's pattern of eruption is not very reassuring for the folks who live on the volcano's slopes as, in theory, a new eruption could start up right in the middle of town. In fact, numerous Etnean towns have been threatened by lava flows and some have been destroyed. However, this fact should not discourage anyone from visiting Mt. Etna, as volcanoes very rarely go bang in the night (or day) without precursor signs. It is, therefore, quite safe to stay in one of the charming Etnean towns despite the volcano's Mediterranean attitude towards orderly behavior.

Volcanoes come in many shapes, sizes, and forms of behavior. The diversity makes them fascinating and no two are totally alike. However, danger always lurks underneath any volcano considered active. Before setting out to explore volcanoes, it is important to understand the hazards and to know how to keep yourself safe.

References

MacDonald, G.A. (1972) Volcanoes. Prentice-Hall. Simkin, T. and L. Siebert (1994) Volcanoes of the World, 2nd edn. Geoscience Press.

Tilling, R.I. (1989) Short Course in Volcanology, vol. 1, Volcanic Hazards. American Geophysical Union.

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Responses

  • veli-matti
    Which volcanoes look like upside down saucers?
    6 months ago

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