Activity 4—Volcanic Hazards: Airborne Debris

The ash in an eruption can have surprising side effects. For example, soon after Mt. St. Helens erupted ash into the western United States, announcements on the radio warned people to remove their contact lenses. The explosion had crushed the volcanic rock into such fine (small) ash that it could be trapped beneath the contact lens and scratch a person’s eye.

Differences in chemical composition cause the varying eruptive behavior of lavas. Refer to the Background Information in Activity 2 of this chapter for more information. There is a broad spectrum of magma compositions, ranging from the silica-poor lava that forms most basalt, the most common rock on Earth’s surface (although not the most common on the continents), to the silica- rich magma that forms rhyolite. This latter magma type tends to be highly explosive when it is erupted, because of the relatively high content of volatiles. The key idea for students to grasp is the difference between the end members of volcanic rock composition (those with the most different compositions), rather than focusing on details of four or five categories. If you want an advance understanding of the different categories, you can read ahead in the students’ readings in Activity 5, which detail differences among basalt, dacite, andesite, and rhyolite, but are not required reading for students.

Students might ask you if ash from Mt. St. Helens came from a rhyolitic lava. In fact, it was dacitic, which is slightly less explosive than rhyolitic lava. (Incredible as it may seem, this means that there are volcanoes that are far more explosive than Mt. St. Helens was.)

A volcanic eruption can send a column of material many miles into the atmosphere. The 1991 eruption of Mt. Pinatubo in the Philippines was 10 times more explosive than the 1980 eruption of Mt. St. Helens. It spewed out 10 times the volume of volcanic products. Less than a million years ago
an eruption at what is now Yellowstone, Wyoming erupted more than 1000 times the volume of material released at Mt. St. Helens. On average, volcanic eruptions with a high volcanic explosivity index (VEI) happen far less often than those with a low VEI because their magmas are more viscous and they tend to have their volcanic plumbing stopped up.

The mechanics of explosive volcanic eruptions are still not fully understood. When a body of magma moves upward in the Earth’s crust, the confining pressure due to the weight of overlying rock decreases. If the magma contains no volatiles or only a low concentration of volatiles, as with basaltic magma, this decrease in pressure is not of great consequence, but if the magma contains a relatively high concentration of dissolved volatiles, like water and carbon dioxide, as is common with silica-rich magmas, then the volatiles have a tendency to come out of solution as the pressure is decreased. In some cases this release is gradual, but in other cases it is sudden. In such cases, when the magma is poised just below the surface a crack might propagate randomly through the overlying rocks down to the vicinity of the magma. This allows a sudden release of pressure in the magma and very rapid outgassing of volatiles, causing a large and sometimes truly gigantic explosion. Geoscientists can make tentative predictions of the potential danger of explosions by monitoring such things as Earth movements caused by the moving magma and gradual release of gases through fissures, but it is still not possible to make reliable predictions of the occurrence and timing of volcanic explosions.

The particulate material ejected from a volcano during an explosive eruption varies widely in composition. The most important kinds of particles are as follows: fragments of preexisting rock that lay above the magma before the eruption; crystals of various minerals that had already crystallized in the magma before it was erupted; rounded masses of very fine or glassy igneous rock that formed by the chilling of drops or gobs of magma in flight; and curved shards of broken walls of gas bubbles. The size of the particles also varies widely. Volcanologists use the term tephra for all of the ejected material, regardless of size or composition. Technically, the term ash refers only to the finest part of the tephra. Material of sand and pebble size is called lapilli, and the largest pieces are called blocks or bombs, depending on their shape. Blocks and bombs rain down close to the site of the eruption. (Imagine blocks the size of television sets falling out of the sky.) Great clouds of finer ash are put much higher into the atmosphere. In some photographs of explosive eruptions, you can see blocks and bombs falling out of the rising cloud of ash. Dispersal of the ash depends on the pattern of winds (speed and direction) at all levels in the atmosphere. Students should understand that the fall velocity, or settling velocity, of a solid particle in a fluid increases with the size and mass of the particle, so the finest part of the ash, of sizes down to the micron (0.001 m) range, not only reach the highest levels of the atmosphere but travel the farthest with the wind before falling out. The finest part of the ash can stay in the atmosphere for many months, and during that time it is diffused throughout the entire atmosphere. Somewhat coarser ash falls out in a shorter time over areas that are subcontinental in size, as shown on the map of Mt. St. Helens ash in Part 1 of the Investigate section. Both the thickness and the particle size of the mantle of ash deposited on the land surface decreases downwind of the eruption. The sharp boundary of the ash layer on the map shown in the Investigate section is only a convenience; the thickness of the ash tails off gradually, and there is really no definite boundary to the ash layer.

The EarthComm web site also contains a variety of links to web sites that will help you deepen your understanding of content most relevant to this activity.