Geological Hazards
Volcanoes
Meteorological Hazards
Biological and Technological Hazards
Home
Contact the
webmaster.
What is a volcano?   According to the United States Geological Survey, a volcano is "a vent at the Earth's surface through which magma (molten rock) and associated gases erupt, and also the cone built by effusive and explosive eruptions." Figure V-2 illustrates both parts of this definition - the eruption of ash and gases through a vent, and the cone formed by such eruptions.  
  Physical geographers differentiate among active, dormant, and extinct volcanoes. An active volcano is one that has erupted in recorded history (which, geologically speaking, is but an instant in time). A dormant volcano has not been seen to erupt, but it shows evidence of recent activity. When a volcano shows no sign of life and exhibits evidence of long-term weathering and erosion, it is tentatively identified as extinct. Such a designation is always risky because some volcanoes have come to life after long periods of dormancy.     Where do volcanoes occur?
  Fig. V-3 shows the global distribution of active volcanoes (and recent earthquakes). There are three different areas where volcanoes may occur:   a. Midoceanic spreading ridges (see Fig. V-4). Along some 50,000 kilometres (31,000 mi) of ocean-floor fissures, molten rock penetrates to the surface and begins its divergent movement, creating new lithosphere in the process. It is a dramatic process involving huge quantities of magma, the formation of bizarre submarine topography, the heating and boiling of seawater, and the clustering of unique forms of deep-sea oceanic life along the spreading ridges. About 75% of the world's volcanoes are on the seafloor.  
  b. Subduction zones at convergent plate boundaries. Volcanoes can occur where continental and oceanic plates converge (see Fig. V-5). When a continental plate meets an oceanic plate which is traveling in the opposite direction, the process of subduction carries the heavier oceanic plate downward beneath the thicker but lighter continental plate. In this process, high relief develops along the coastline, the continental crust is heavily deformed, and magma can penetrate through vents and fissures to erupt as lava at the surface. Many of the world's most famous mountains are volcanic peaks standing astride or near plate boundaries: Mount Fuji (Japan), Mount Vesuvius (Italy), Mount Rainier (US Pacific Northwest), Mount Chimborazo (Ecuador), and many others.  
  Volcanoes can also occur at the convergence of two oceanic plates. The contrast between lithospheric plate densities is not present, and the crust is thrown into huge contortions. One of the plates will override the other, resulting in subduction; deep trenches form and volcanoes protrude, often above sea level (see Fig. V-6). Island Arcs, such as the Aleutian and Japanese archipelagoes, are products of the convergence of two oceanic plates.  
  Widespread volcanism does not typically occur at the convergence of two continental plates.   c. Hot Spots. There is some volcanic activity that is associated neither with midoceanic ridges nor with subduction zones. The island of Hawai'i, for example, lies in an archipelago near the middle of the Pacific Plate. This distribution is difficult to explain. Geologists theorize that the Pacific Plate has been moving over a hot spot in the mantle, a "plume" of extraordinarily high heat that remains in a fixed location, perhaps stoked by a high concentration of radioactivity. As the Pacific Plate moved over this hot spot, shield volcanoes formed over it. See Fig. V-7 for an illustration of a volcanic chain formed by the seafloor moving over a geologic hot spot.  
    Hazards Associated with Volcanoes  
    a. Magma and Lava. According to the USGS, magma is "molten or partially molten rock beneath the Earth's surface. Magma typically consists of (1) a liquid portion (often referred to as the melt); (2) a solid portion made of minerals that crystallized directly from the melt; (3) solid rocks incorporated into the magma from along the conduit or reservoir, called xenoliths or inclusions; and (4) dissolved gases." When magma reaches the earth's surface (as it does in a volcanic eruption), it is then called lava.   The viscosity of magma and lava varies with its composition. Basaltic lavas, such as those flowing from the midoceanic spreading ridges, are relatively low in silica and high in iron and magnesium content, and are therefore quite fluid when they erupt. Other lavas are poorer in magnesium and iron but richer in silica - and thus more acidic. These lavas tend to be more viscous, and as a result they flow more slowly. Basaltic lava can flow like motor oil; the less mafic lava moves more like a thick porridge would.   b. Pyroclastics. Magma also contains steam and other gases under pressure, with variation again a function of its mineral content. The acidic, silica-rich magmas tend to contain more gases, and when they erupt as lavas, these gases often escape explosively. Gobs of lava are thrown high into the air, solidifying as they fall back to the mountain's flanks. Such projectiles, not unreasonably, are called volcanic bombs. Smaller fragments may fall through the air as volcanic cinders or volcanic ash.   After the explosive 1980 eruption of Mount St. Helens in Washington State (see Fig. V-8 ), lighter volcanic dust fell over a wide area downwind from the mountain. Fig. V-9 shows pre-and post-eruption views of Spirit Lake at Mt. St. Helens. The dust is clearly visible in the second photo. Geologists use the term pyroclastics for all such solidified fragments erupted explosively from a volcano (pyroclastic is ancient Greek for broken by fire).  
 
  c. Lahars. On snowcapped volcanoes the hot ash sometimes melts the snow and ice, which forms a flood of ash, mud, and water rushing downslope. Such a mudflow can be extremely destructive. Once it solidifies, the volcanic deposit is known as a lahar. Lahars are mainly triggered by eruptions, but occasionally may also result from intensive, warm-season orographic rainfall against the snowpack of a volcano's uppermost slopes.   d. Nuées Ardentes. Perhaps even more dangerous is the outburst of hot gas and fine dust that may accompany or precede an eruption. Gas is pent up in magma itself, but in some composite volcanoes a large reservoir of gas may accumulate in a chamber below the crater. This happens when the crater becomes clogged by solidified lava, which forms a plug in the top of the vent. The gas cannot escape, and building temperatures inside the mountain may exceed 1000 degrees Celsius. Pressures finally become so great that the side of the volcano may be blown open, allowing the gas to escape, or the plug may be blown out of the top of the volcano's pipe, enabling the gas to rush from the crater along with heavier pyroclastics and lava. Such an event produces a nuée ardente (French for glowing cloud), which races downslope at speeds exceeding 100 km/h. Everything in its path is incinerated.   e. Phreatic Eruptions. Water, poured on advancing lava, can help cool and consolidate the hot crust and slow down or divert the movement of the flow. But when water penetrates into the magma chamber below a volcano, it has quite a different effect. Just as pouring water on a grease fire only intensified the blaze, so water entering a superheated magma chamber results in an explosive reaction - so explosive, in fact, that it can blow the entire top off the volcano above. This may be the reason for the gigantic explosions known to have occurred in recorded history, explosions that involved large composite volcanoes standing in water. Such explosions are called phreatic eruptions, and their effects reach far beyond the volcano's immediate area.     Types and Features of Volcanoes a. Composite Volcanoes. Most of the great volcanoes formed over subduction zones are composite volcanoes - they disgorge a succession of lavas and pyroclastics. In cross section such volcanoes look layered, with lavas of various thicknesses and textures interspersed with strata formed by compacted pyroclastics. Fig. V-10 shows a simplified cross section of a composite volcano, showing a sequence of lavas interspersed with compacted pyroclastics. Neither the heavier pyroclastics nor the rather viscous lava travel very far from the crater. Thus the evolving volcano soon takes on its fairly steep-sided, often quite symmetrical appearance. Many composite volcanoes are long-lived and rise to elevations of thousands of meters. Appropriately, they are also called stratovolcanoes. One of Canada's stratovolcanoes is found in the Garibaldi Volcanic Belt of British Columbia and has an elevation of 2345m.  
  Composite volcanoes, with their acidic, gas-filled lavas, are also notoriously dangerous. They often erupt with little or no warning, and molten lava is not the only threat to life in their surroundings. Pyroclastics can be hurled far from the crater; volcanic ash and dust create health hazards that choke human and animal life even farther away.   b. Shield volcanoes. Shield volcanoes are formed from fluid basaltic lavas. These lavas contain sufficient gases to create a sometimes dramatic "fountain" of molten rock and some cinders, but these are tiny compared to the explosive eruptions at the craters of composite volcanoes. The basaltic lava is very hot, however, and flows in sheets over a countryside being gradually built up by successive eruptions. Compared to their horizontal dimensions, which are very large, the tops of such volcanoes are rather unspectacular and seem rounded rather than peaked. This low-profile appearance has given them the name shield volcanoes.   Fig. V-11 shows a simplified cross section of a shield volcano (vertical scale greatly exaggerated), while Fig. V-12 shows a fountain of lava exploding from Kilauea, a much-studied shield volcano on the main island of Hawai'i.  
 
  c. Lava Domes. When acidic lava penetrates to the surface, it may ooze out without pyroclastic activity. This process usually produces a small volcanic mound, called a lava dome. A lava dome often forms inside a crater following an explosive eruption, as happened at Mount St. Helens. But lava domes can also develop as discrete landforms in a volcanic landscape. Although some grow quite large, lava domes, on average, are much smaller than composite volcanoes.   d. Cinder Cones. Some volcanic landforms consist not of lava, but almost entirely of pyroclastics. Normally such cinder cones (which may also include fragments larger as well as smaller than cinders) remain quite small, frequently forming during a brief period of explosive activity. Probably the most extensive area of cinder-cone development lives in East Africa, associated with the rift-valley system of that region. While many may not expect it, Canada has cinder cone volcanoes as well. Chilcotin Creek Cone (along with many other cinder cones) is found in the Itcha Range of British Columbia. Erupting last in the Pliocene Era, Chilcotin does not prove much risk to humans today.   e. Calderas. A volcano's lavas and pyroclastics come from a subterranean magma chamber, a reservoir of active molten rock material that forces its way upward through the volcanic vent. When that magma reservoir ceases to support the volcano, the chamber may empty out and the interior of the mountain may literally become hollow. Left unsupported by the magma, the walls of the volcano may collapse, creating a caldera (see Fig. V-13). Such an event can occur quite suddenly, perhaps when the weakened structure of the volcano is shaken by an earthquake. A caldera also can result from a particularly violent eruption, which destroys the peak and crater of the volcano. In such cases, however, the magma chamber below is at the peak of its energy and will soon begin to rebuild the mountain.  
|