[ v o l c a n o e s : t y p e s o f e r u p t i o n s ]
There are actually many different types of volcanic eruptions. Many people have tried to classify volcanoes into categories, but still it is hard to organize them all into groups. There are numerous classification systems, all of which you may come across when studying volcanoes.
The earliest divisions were created by a volcanologist named G. Paulett Scrope, who made the following distinctions:
- a) permanent - more quiet and regular eruptions (similar to Stromboli)
- b) moderate - violent and irregular
- c) paroxysmal - similar to paroxysmal but less violent
In the late nineteenth century, another system was introduced:
- a) explosive - Explosive volcanoes erupt suddenly, with terrifying force. These form when magma has cooled to make a lava plug blocking a crater. The plug traps hot gas and magma under the ground, and the hot gas builds up until the pressure becomes too great. When this happens, hot gas and magma explodes out of the volcano in a shower of dust, ashes, cinders, and volcanic bombs. Volcanic bombs are large chunks of molten rock that fly high up into the air and then fall on the countryside around. Explosive volcanoes tend to produce steep cones of volcanic ash.
- b) intermediate - Intermediate volcanoes can sometimes erupt explosively and emit dark clouds of ash, but they also produce flowing lava. This type of volcano builds up cones made of alternating layers of ash and lava. Intermediate volcanoes have characteristics of both explosive and quiet eruptions.
- c) quiet - Quiet volcanoes explode with much less suddenness, because lava does not cool and harden. The lava that creates a quiet volcano is still runny and flowing, and do not plug the craters and trap huge quantities of explosive gas. However, pockets of hot gas form that squirt fountains of lava into the air. Because lava flows away before hardening, broad sheets of lava and wide, gently sloping domes are formed around the volcano. When the lava has dried, it forms a rock known as basalt.
As numerous specialists tried to come up with classification systems, a French geologist, Professor A. Lacroix (1908) designed an order recognizing four eruption types. It is used widely today in many geology textbooks:
- a) Hawaiian - Typical of the Hawaiian Island volcanoes, these eruptions come with lots of lava. These are rarely explosive, but fountains of lava can be shot up to 1,000 feet or more by jets of gas. The lava forms flat lava domes, creating some of the largest mountains on earth.
- b) Strombolian - Named after Stromboli, a volcano in the Aeolian Islands off the coast of Sicily, is always active. Known as the “Lighthouse of the Mediterranean,” it has characteristically moderate explosions accompanied by scoria (porous, spongelike lava) and a white vapor cloud. Every so often, gases escape with small explosions, throwing lava clumps and crust.
- c) Plinian/Vulcanian - Named after the Island of Vulcano, north of Sicily, the volcano’s lava is thicker and pastier than the Strombolian type. It forms a solid crust over the crater between infrequent eruptions. Accumulated gases then blow out these obstructions with strong explosions. Broken fragments, fresh lava, and scoria are hurled out of the crater. These eruptions come with a large “cauliflower-shaped” ash cloud. The magama turns into pumice and ash. After the initial explosions, lava flows from the crater and fissures on sides of the cone.
- d) Peléan - Derived from Mount Pelée, a volcano on the island of Martinique in the West Indies, the Peléan type produces the thickest magma and has the greatest explosions. Its distinguishing feature is the nuée ardente, or “glowing cloud,” a highly heated gas charged with glowing ash particles that rushes down mountain slopes with hurricane force. The magma in these eruptions are usually blocked, so that explosions blast out horizontally beneath the plug.
Since this system was created, two additional categories have been added to the Lacroix classification:
- e) Icelandic - This type of eruption emits large amounts of lava from fissures, often miles in length and spread in sheets over the land. The lava is very fluid and can flow for long distances.
- f) Solfataric stage - This term is applied to the final phase of eruption, during which only gases are emitted. This is named after the Solfatara (“sulfur” in Italian) volcano near Naples. Since 1198, it has only emitted gases. Volcanoes can stay in the Solfataric stage for hundreds of years after its last eruption.
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Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/volcanoes/typesoferuptions.shtml