Volcano

A volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame, and it is not infrequently so described. The truth is that a volcano seldom emits either smoke or flame. What is mistaken for smoke consists of vast volumes of fine dust, mingled with steam and other vapours — chiefly sulphurous. What appears to be flames is the glare from the erupting materials, glowing because of their high temperature — this glare reflects off the clouds of dust and steam, resembling fire.

Perhaps the most conspicuous part of a volcano is the crater, a basin, roughly of a circular form, within which occurs a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanos that terminate in a principal crater are usually of a conical form.

Volcanic cones are usually smaller features composed of loose ash and cinder, with occasional masses of stone which have been tossed violently into the air by the eruptive forces (and are thus called ejecta). Within the crater of a volcano there may be numerous cones from which vapours are continually issuing, with occasional volleys of ashes and stones. In some volcanoes these cones form lower down the mountain, along rift zones.

• Short-period earthquakes are like normal fault-related earthquakes. They are related to the fracturing of brittle rock as the magma forces its way upward. These short-period earthquakes signify the growth of a magma body near the surface.

• Long-period earthquakes are believed to indicate increased gas pressure in a volcano's "plumbing system." They are similar to the clanging sometimes heard in your home's plumbing system. These oscillations are the equivalent of acoustic vibrations in a chamber, in the context of magma chambers within the volcanic dome.

• Harmonic tremor occurs when there is sustained movement of magma below the surface.

In December 2000 scientists at the National Centre for Prevention of Disasters in Mexico City predicted an eruption witihin two days from Popocatépetl, on the outskirts of Mexico city. Their prediction used reserarch done by M. Chouet, a Swiss vulacanologist, into increasing long-period oscillations as an indicator of an imminent eruption. The government evacuated tens of thousands of people.

Forty eight hours later, bang on time, the volcano erupted spectacularly. It was Popocatépetl's largest eruption for a thousand years and yet no one was hurt.

Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, solfatare and geysers. Low-magnitude earthquakes often precede eruptions.

Surprisingly, there is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active?

Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years.

Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.

Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For example, since calderas have lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Yellowstone caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted for 70,000 years, yet scientists do not consider Yellowstone as extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system, and rapid rates of ground uplift, many scientists consider it to be a very active volcano.

Famous volcanoes include

• Mauna Loa
• Mauna Kea
• Mount Erebus
• Etna
• Hekla
• Krakatoa
• Vesuvius
• Mount Fuji
• Mount St. Helens.

Public domain picture from NASA of Tambora, Indonesia.

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