Translations of Encyclopedia about Geology

Volcanoes

Volcanoes received their name after the Roman god of fire, Vulcan. Ancient Romans believed that one volcano near Naples was the entrance into the underworld. In the Greek mythology Hephaistos had his forge underneath the volcanoes. When the gods were angry, people were punished by the fire of the erupting volcanoes. To placate the gods, people would throw sacrifice into the lava lakes, sometimes even human sacrifice.

The majority of close to 1500 active volcanoes on the earth are in the regions where the tectonic plate boundaries touch. To be considered active, the volcano must have been active during the last 1000 years. If there was no activity during that time, the volcano is considered extinct. Today, there are approximately 600 active volcanoes on land, mostly in Iceland, Hawaii, Java, the Aleutian Islands, Central America, Italy, and New Zealand. The incidence of volcanic eruptions on the sea floor is much higher.

When an oceanic plate collides with a lighter continental plate, the heavier plate slides underneath the lighter plate. This is called subduction. In this case, the part of the submerged plate melts. This fluid molten rock is called magma. The magma rises and spills onto the plate that is above it. When two plates drift away from each other, they create a rift in the earth's crust. In this case the magma also ascends. When this happens in the ocean, the magma forms new sea floor or volcanic islands. When it happens on land, it forms volcanic crests.

A volcanic eruption is often accompanied by release of liquid and gaseous material from the earth's interior. This material is concentrated from ten to one hundred kilometres underneath the surface. The temperatures at that depth may reach 1200 degrees Celsius. The molten rock collects in the magma chamber in the earth's crust. The concentrated magma is connected by means of a volcanic vent to the summit of the volcano. During a volcanic eruption, boulders, ash, dust, and gases are ejected. These phenomena may occur in the form of an explosion, when these masses are hurled high into the air above the volcano, or the lava just may slowly flow out of the volcano.

We recognise various types of volcanoes, in general by the type of lava. Shield volcanoes have a wide base and flat slopes, which are formed by lava flow. Lava flows far down the mountain until it congeals. Many so-called "hot spots" are shield volcanoes. An example of this type of volcanoes is Kilauea in Hawaii. In contrast, composite volcanoes (also called stratovolcanoes) have moderately steep sides formed by lava and ash. One of these composite volcanoes is Mount Fuji in Japan. Gaseous volcanoes are those that spew almost exclusively gases.

When the volcanic activity diminishes, quite often it leaves behind the so-called post-volcanic phenomena. These include hot springs and gases, and we may observe those in the form of geysers, fumaroles, and mudpots.

A volcanic eruption may cause a lot of damage. The eruption proper is less serious. The mudslides and floods, which in turn have negative impact on the population such as hunger or an onset of epidemics, are of far more serious nature. Another dangerous phenomena which may accompany volcanic eruptions are windstorms, heavy rains, and tornadoes, and may produce considerable damage.

It is seldom possible to predict a volcanic eruption. One positive side of volcanic activity is that it fertilises the soil, uncovers ore deposits, and provides thermal energy from the hot springs, but this occurs only infrequently.

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