It's spectacular and, oh, so deadly.

A volcanic eruption can be one of nature's most spectacular events. Sprays of lava and debris shoot skyward. A roar that would make thunder seem a mere whisper splits the silence. Flaming lava rivers flow downward through ravines and over ledges creating lava cascades of fiery reds and golds.
A violent eruption may send solid and molten rock fragments, ash, and volcanic gases into the air with tremendous force. The force is so great that the erupting column and the huge, billowing cloud that form above it can rise many kilometers into the atmosphere.

Much of the material ejected from a volcano is known as “tephra.” The largest fragments (greater than 64 mm) are called blocks and bombs. Small, fiery blocks and bombs have been known to travel as far away as 20-80 km. Fragments smaller than 2 mm are often simply called “ash." The ash forms a dust-like blanket of very tiny shards of volcanic rock and glass wherever it settles. It clogs machinery, smothers crops, and, when combined with rain, collapses roofs and breaks tree branches. The smallest particles which are less than .01 mm can stay in the atmosphere for months after a volcanic eruption. Sometimes these particles produce fantastic sunsets such as those seen after the eruption of Mt. Pinatubo in 1991.

Volcanoes also spew gaseous substances. Volcanic gases, made of steam mixed with carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and/or fluorine, filter into the atmosphere, contaminating the air and groundwater, poisoning animals, killing plants with the acid rain that follows, and causing breathing problems. Small droplets of sulfuric acid which remain in the atmosphere for months or years may even contribute to global warming.

Dangers that don't burst out of a volcano flow down it. There are four types of deadly volcanic flows that threaten man and nature.

1. Lava flows are streams of molten rock on the Earth's surface. They pour out of the erupting volcano and flow down its sides at speeds ranging from a few kilometers per hour to over eighty kilometers per hour. Molten rock when it is still underground is called "magma." Once it reaches the surface we call it "lava."
2. Pyroclastic flows contain hot ash, rock fragments, and gases that can flow from volcanoes at speeds in excess of one hundred fifty kilometers per hour.
3. Volcanic landslides consist of rocks, snow, or ice on the slopes of the volcano, which are loosened and sent tumbling downhill by seismic or eruptive activity.
4. Lahars (also referred to as "volcanic mudflows" or "debris flows") are a mixture of volcanic ash and debris and water. This deadly mix, with the consistency of wet concrete, flows down valleys and stream channels, rips out trees and houses, and eventually buries everything in its path. The water that mixes with the debris to start the lahar can come from glacial ice on the volcano's slopes or from heavy rains. As long as these deposits remain on the hillsides, lahars triggered by heavy rainfall can occur without warning, for years after a volcano erupts— a deadly, volcanic legacy.

Predicting Eruptions

No one can predict exactly when a volcano might erupt. There are, however, common clues to look for to let people know that the danger is increasing:

1. Land deformation. Near the volcanic vent, as magma pushes up from below, the land on top of and surrounding the vent may start to deform, and grow larger. In many cases this deformation creates a dome of volcanic debris. As the dome grows in size, it becomes more unstable, and ultimately it will collapse and produce pyroclastic flows or lava flows. Increasing dome size indicates a growing danger.
2. Ash Clouds. Large ash clouds are evidence of increasing volcanic activity. On the island of Montserrat scientists have found that during dangerous times, ash clouds occur in a cycle of 4 to 30 hours apart. What causes an ash cloud? In an active volcano, the vent may become plugged by rocky materials. This plug causes the pressure from the rising magma to build until eventually the plug is blasted apart in a flurry of explosive activity. The released pressure creates an ash cloud, shooting volcanic debris high into the air. Once an ash cloud is produced, the vent may become plugged again and the cycle may repeat. If a plugged volcano does not release its pressure, then scientists become concerned that a massive explosion is imminent. As long as the ash clouds keep appearing regularly, then there is less concern.
3. Tremors. Flowing magma, trying to make room for itself, causes tremors on the surface. Tremors are measured both for their magnitude and the length of time between vibrations, or rate. On the island of Montserrat, scientists study two kinds of seismic data: Volcanic Tremors (VTs) and Rockfall (RF). The number of tremors or falling rocks which exceed a predetermined critical value are counted by the seismometer (see illustration). The more of these “events” the greater the level of the seismic activity. The rate of tremors is determined by charting the number of events per hour. An increase in the number of events can be an indication of possible eruption.
   

Review Questions

1. Describe how an erupting volcano can affect the atmosphere. Explain how this could contribute to global warming.
2. From this article, what are described as the dangerous short-term effects from a volcanic eruption? What are the dangerous long-term effects?
3. Explain how magma under the surface can create seismic tremors.