Volcanic Eruptions

A volcanic eruption is one of the most awesome spectacles in all nature. Usually earthquakes provide a warning a few hours or a few days beforehand – minor shocks probably caused by the movement of gases and liquids underground. An explosion or a series of explosions begins the eruption, sending a great cloud billowing upward from the crater. In the cloud are various gases, dust, fragments of solid material blown from the crater and the upper part of the volcano’s orifice, and larger solid fragments representing molten rock blown to bits and hurled upward by the violence of the explosions.

Gas continues to issue in great quantities, and explosions recur at intervals. The cloud may persist for days or weeks with its lower part glowing red at night. Activity gradually slackens, and presently a tongue of white-hot lava spills over the edge of the crater or pours out of a fissure on the mountain slope. Other flows may follow the first, and explosive activity may continue with diminished intensity. Slowly the volcano becomes quiescent, until only a small steam cloud above the crater suggests its activity.

Not all eruptions by any means follow this particular pattern. Volcanoes are notoriously individualistic, each one having some quirks of behavior not shared by others. In one group of volcanoes the explosive type of activity is dominant, little or no fluid lava appearing during eruptions. Cones of these volcanoes, built entirely of fragmental material ejected in a solid or nearly solid state, are very steep sided; examples are found in the West Indies, in Japan, and in the Philippines. Other volcanoes, like those of Hawaii, have eruptions characterized by quiet lava flows with little explosive activity. Mountains built by these volcanoes are broad and gently sloping, quite different from the usual volcanic structure. The most common kind of volcano is neither wholly of the “explosive” type nor wholly of the “quiet” type, but has eruptions in which both lava flows and gas explosions occur.

The chief factors that determine whether an eruption will tend to be a largely quiet lava flow or tend to be explosive are the viscosity of the magma and the amount of gas it contains. (The greater the viscosity of a liquid, the less freely it flows: honey is more viscous than water.) Magma is a complex mixture of the oxides of various metals with silica and usually has an abundance of gas dissolved in it under pressure. Like most molten silicates it is extremely viscous, and with rare exceptions lava creeps downhill slowly, like thick syrup or tar. The viscosity depends upon chemical composition; magmas with high percentages of silica are the most viscous. The presence of gas also affects viscosity; magmas with little gas are the most viscous. If the magma feeding a volcano happens to be rich in both gas and silica, the eruption will be explosive. A magma with modest gas and silica contents results in a quiet eruption.

The gaseous products of volcanic activity include water vapor, carbon dioxide, nitrogen, hydrogen, and various sulfur compounds. The most prominent constituent is water vapor. Some of it comes from groundwater heated by magma, some comes form the combination of hydrogen in the magma with atmospheric oxygen, and some was formerly incorporated in rocks deep in the crust and is carried upward by the magma to be released at the surface. Much of the water vapor condenses when it escapes to give rise to the torrential rains that often accompany eruptions.