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dimecres, 24 d'abril de 2024

> Parts of a Volcano

> Types of Volcanic Eruptions

> Central and Fissure Volcanoes

dimarts, 19 d'octubre de 2004

Volcanoes are generally conical geological formations, formed by the accumulation and solidifying of igneous material, called lava, from a fissure in the earth's crust. One of these formations, the volcano called Mount St. Helens, in the state of Washington in the United States, has been making news in the last few weeks.

In effect, at the end of September, scientists observed an increase in the seismic activity of the volcano, accompanied by the expulsion of vapour and ash. This led to the closing of access to the area and the adopting of emergency prevention measures. However, according to the experts, it is unlikely that there will be a great eruption, although they point to the possibility of there being small emissions of lava.

The last big eruption of Mount St. Helens occurred in 1980, causing fifty-seven deaths and expulsing millions of tons of volcanic material, which spread over an extensive area. The city of Portland (Oregon), situated some eighty kilometres from the volcano, was covered in ash.

+ Webs relacionades dins Nosaltres.Com: Recursos per a escoles. Ciències Naturals

Parts of a Volcano

+ Diagram of a volcano.

A volcano is composed of different parts, four of which are the most important: the magma chamber, where the lava, called magma when it is in the earth's crust, is stored; the chimney, a pipe which allows the magma to flow to the exterior; the crater, situated at the top of the chimney in the form of a funnel; and the volcanic cone, which is simply the mound formed by the accumulation of materials coming from the inside. Some volcanoes can have smaller secondary cones, formed by the emission of lava by other conduits or chimneys.

Types of Volcanic Eruptions

+ Mount St. Helens before and after the 1980 eruption.

Turning to different types of volcanic emission, we find four classes of volcanoes: Hawaiian, Vulcanian, Pelean and Strombolian. The first, typical of the archipelago of Hawaii, emit a very fluid lava, which causes mounds with gentle slopes; in contrast, the Vulcanians have very steep slopes, explained by the violence of the eruptions; the Peleans have a very viscose lava which can end up plugging the crater, resulting in violent explosions; and the Strombolians, which take their name from the volcanic island of Stromboli, alternate the emission of lava and powerful explosions.

Central and Fissure Volcanoes

+ Volcanoes can also be classified as fissure and central.

Based on the types of eruption, volcanoes can also be classified as central and fissure. In the central volcanoes, the most typical, the igneous material exits by a specific point, and the chimney, the cone and the crater are well-defined. In contrast, in the fissure volcanoes, the magma emerges through a crack or fracture, and can end up spread over great areas of land or, on solidifying, taking on the form of a crest.

MATERIALS

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QUE HO SABIES?

The word 'volcano' derives from the Latin 'vulcanus', the name by which Vulcan, the god of fire and blacksmith, was known. According to mythology, the god had his forge at Vulcano, one of the Eolian Islands, situated in the Tyrrhenian Sea, off the Sicilian coast.
In Sicily there is the active volcano Etna, the highest in Europe, at more than 3,300 metres.
It is calculated that there are one thousand nine hundred active or formerly active volcanoes in the world. 90 % of these are concentrated around the so-called Ring of Fire, a strip that circles the Pacific Ocean.