Explain the hazards volcanoes pose and which geographic areas are most at risk.

In Hawaiian myth, Pele is the volcano goddess. She is said to reside in the summit caldera of Kilauea on the island of Hawai’i. Pele embodies the many facets of volcanoes, ranging from life-sustaining benevolence to destructive malevolence. In this section, we turn to Pele’s malevolent side and examine the main geohazards that volcanoes present.

Shield volcanoes, such as those found on Hawai’i, present little threat to human life. Shield volcanoes have nonexplosive effusive eruptions that emit more lava than gases. Mafic lava from shield volcanoes usually flows slowly downhill and can be avoided.

Stratovolcanoes, on the other hand, are potentially serious geohazards. Their eruptions are called explosive eruptions. An explosive eruption is violent and yields large amounts of pyroclasts. Stratovolcanoes may produce effusive outpourings of mafic lava like shield volcanoes, but they are also capable of exploding violently with little warning.

Explosive eruptions send rock, ash, and volcanic gases high into the troposphere, or even into the stratosphere. In the troposphere, rain washes the volcanic material out in a few days or weeks. There is no rainfall in the stratosphere, however, so once ash and sulfur gases enter the stratosphere, they can remain suspended there for five years or more. These materials can encircle the globe and cause climate cooling for a few years (see Section 6.2).

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Large explosive eruptions result when gas-rich felsic magma migrates upward through the crust and encounters less pressure, which causes it to expand rapidly. Once the magma and gases begin expanding, the surrounding magma chamber is enlarged, allowing more gas to expand and further enlarge the magma chamber. This process can unfold over the course of minutes to hours and can result in a catastrophic explosive eruption.

Island volcanoes can become particularly explosive when seawater migrates into the magma chamber, as might occur after an earthquake. As water comes into contact with the intense heat, it turns to vapor and expands rapidly and explosively. Figure 14.13 shows an explosive volcanic eruption in which the force of the expanding gases and collapsing magma chamber sent ash billowing high into the atmosphere.

The volcanic explosivity index (VEI) ranks volcanic eruption magnitude based on the amount of material a volcano ejects during an eruption. A VEI 5 eruption emits more than 1 km³ (0.24 mi³) of pyroclastic material into the atmosphere, and a VEI 6 eruption emits more than 10 km³ (2.4 mi³). During the last 10,000 years, there have been about 50 VEI 6 eruptions. The eruption of Tambora, described in Geographic Perspectives, was the only VEI 7 eruption in historic times. Figure 14.14 compares large historical eruptions to the colossal prehistoric eruption of the Yellowstone caldera, 640,000 years ago.

Stratovolcanoes are among the most dangerous geohazards on the planet. Lava flows from these volcanoes are not their biggest threat because their lava usually flows slowly, so people can escape. The two greatest volcanic hazards are lahars and pyroclastic flows. Together, they account for about half of the volcano-related deaths in any given year.

Torrents of Mud: Lahars

A lahar (a Javanese word that means “mudflow” or “debris flow”) is a mudflow that results when a snow-capped stratovolcano erupts. A lahar is a thick slurry of mud, ash, water, and other debris that moves rapidly down the volcano’s flank. Lahars can travel tens of kilometers down the slopes of volcanoes and into the flatlands below, where people may reside. Lahars are not hot. Their danger lies in the fact that they move quickly and can engulf whole villages in minutes. Figure 14.15 shows a lahar that engulfed Plymouth, the former capital city of Montserrat, an island in the Caribbean Sea.

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Blazing Clouds: Pyroclastic Flows

Pyroclastic flows (also called nuées ardentes, meaning “blazing clouds”) are rapidly moving avalanches of gas and ash. Pyroclastic flows are one of the greatest volcanic hazards because they can travel at speeds up to 700 km/h (450 mph) and they can be as hot as 500°C (930°F). At night, these avalanches can glow orange from their intense heat. The largest flows can travel hundreds of kilometers from the volcanic vent. Figure 14.16 shows a pyroclastic flow on Mount Merapi in Indonesia.

Lahars and pyroclastic flows are by far the most significant geohazards volcanoes present, but they are not the only ones. Volcanoes can also produce large earthquakes, dangerous lava flows, and smothering ashfalls. Picture This explores an unusual and unfortunate volcanic event that happened in Italy many centuries ago.

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Because volcanoes can be such a serious geohazard, predicting their eruptions would save many lives. Scientists can sometimes predict an eruption within weeks or months if a volcano gives warning signs. The monitoring of Mount St. Helens, in Washington State, is a good example of the process of monitoring warning signs and successfully anticipating an eruption, as illustrated in Figure 14.17.

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Plate tectonic theory provides the framework to understand why volcanic landforms occur where they do. All volcanoes are found where the mantle is melted into magma, namely, at or near mid-ocean ridges (divergent plate boundaries), subduction zones (convergent plate boundaries), continental rifts, and hot spots (see Sections 12.3 and 12.4).

More than 60% of the Pacific Ocean’s margins, totaling some 40,000 km (25,000 mi), are subduction zones with active and dangerous stratovolcanoes. These volcanoes make up the Pacific Ring of Fire (see Section 12.3). Although shield volcanoes are found in the Pacific Ring of Fire, explosive stratovolcanoes are the most common type of volcano there. Many of them are dormant, but there are also many active and dangerous stratovolcanoes. Figure 14.18 provides an eruption history for some of the more active stratovolcanoes in the Pacific Ring of Fire.

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