Lab 1. Volcanic Hazards
Few things excite the imagination as much as a volcanic eruption, one of nature’s magnificent spectacles, which can result in devastating consequences. The very word “volcano” brings to mind thunderous explosions, rivers of fiery orange lava, and images of smoking destruction. The actual capacity for destruction is not overrated -- volcanic eruptions are one of the most devastating of all geologic hazards. There are about 700 potentially dangerous volcanoes in the world, and about 50 eruptions occur each year worldwide. The United States is the third most volcanically active country in the world. Here, 58 volcanoes have erupted over 470 times just since 1700 A.D. Only Japan and Indonesia have had more volcanic eruptions. DISTRIBUTION OF VOLCANOES
Much of the present-day volcanic activity is clearly related to plate tectonic movements and most volcanoes are located at or close to lithospheric plate boundaries (Fig. I-1). About 80% of the volcanoes occur above subduction zones at convergent boundaries, either as volcanic belts on continental margins or as island-arcs comprised of a chain of volcanoes formed in an oceanic setting. Typical examples of continental volcanic belts are the volcanoes of the Cascade Range in the northwestern United States and of the Andes in the western South America. The Japan and the Philippine islands are good examples of island-arc volcanic chains. The socalled Ring of Fire, defined by the volcanic chains that rim the Pacific Ocean, is actually a ring of subduction zones.
About another 15% of the world’s recent volcanoes occur at divergent boundaries such as the Mid-Atlantic Ridge. By far the largest amount of volcanic rocks in the earth occur along the mid-oceanic ridge system, but the volcanism along this spreading center network is predominantly of the quiet, fissure eruption type rather than through volcanic vents.
Finally, a few areas of volcanic activity lie within lithospheric plates and are not related to plate boundaries. These are attributed to hot spots (or mantle plumes) -- areas below the crust that have enough heat to generate magma which eventually reaches the Earth’s surface as volcanic eruptions. The Hawaiian Islands, near the center of the Pacific Plate, and Yellowstone Park, within the North American Plate, are examples of volcanism associated with hot spots.
Table 1. Selected major volcanic eruptions.
1390 B.C. ±
Carter Lake (Mt. Mazama),
Santorini, (Thera), Greece
Taupo, New Zealand
Mount Pelée, Island of
Martinique, West Indies
Lassen Peak, California
Mount St. Helens,
El Chichon, Mexico
Nevado del Ruiz,.Columbia
Post-eruption collapse formed caldera; 42 billion cubic
meters of new material ejected.
Late Minoan civilization devastated; explosion and tsunami.
Pumpeii and Herculaneum buried; 2.6 billion cubic meters
of new material; 3,000 to 16,000 people killed.
16,000 square km area devastated.
Modern Vesuvius eruptive cycle begins, pyroclastic flows.
Largest historic lava flows; 9,350 and most livestock killed. Debris avalanche and tsunami killed 14,500.
Most explosive eruption in history; 25 billion cubic meters
of new material; 92,000 killed; global cooling ("year without summer") due to encircling volcanic ash.
18 billion cubic meters of new material; caldera...