FIGURE 2.7 Relative volumes of some well-known volcanic eruptions. Figure from R. B. Smithand L. W. Braile (1984), in Explosive Volcanism, National Academy Press.

The largest explosive eruptions have left craters 10 to 50 km in diameter. Fortunately, such eruptions predate human experience, but their occurrence in the future is a certainty. For example, large caldera-forming eruptions occurred in the Yellowstone volcanic area of Wyoming about 2.2, 1.2, and 0.6-million-years ago (Figure 2.7), leading to the not unreasonable expectation of another major eruption within the next few hundred thousand years. The latest Yellowstone eruption created sizable deposits of ash as far away as Kansas. The first caldera explosion at Yellowstone, 2.2-million-years ago, has an identifiable volume of ash of about 2,500 km3. It is estimated that the total volume erupted was about twice this value and represented only 10 percent of the magma chamber. Therefore, a magma chamber of between 20,000 and 40,000 km3 was involved.

Direct observations of volcanic eruptions generate estimates and measurements of ascending columns of steam and ash, duration of eruptions, and rates of dome growth, of magma production within the dome, and of lava flow. Space-based observations are capable of locating eruptions that otherwise would not be detected. Satellite-borne sensing devices also provide a clear image of interaction between volcanic emanations and the atmosphere by tracing the global dispersal of gas and dust following an eruption. A new appreciation of volcanic activity's effect on climate has been realized. Clear evidence from historic and contemporary eruptions is now available to show that significant global decreases in temperature accompany the injection of large volumes of volcanic aerosols into the upper atmosphere.

A better understanding of the interactions of volcanic emissions and the atmosphere and hydrosphere is critical. The Krakatoa eruption is only one of the historic events that have dramatically modified the Earth's climate for several years by introducing large volumes of dust and gas into the atmosphere. Recent speculation suggests that large submarine eruptions along ocean ridges may alter ocean temperatures, establishing the El Niño condition with its subsequent implications for climatic variations. Further understanding will come from global monitoring of volcanism, most likely through satellite-based remote sensing and a better theoretical grasp of the relationships dictating climatic response to heat and mass transfer from the Earth's interior to the hydrosphere and atmosphere.

The large-scale effects of volcanism on the atmosphere, climate, and ecosphere have been recognized as a component of the earth system and of cooperative activities such as the International Geosphere-Biosphere Program (IGBP) and the International Decade for Natural Disaster Reduction (IDNDR). Precursor activity of Lascar Volcano, Chile, was first recognized from thermal measurements taken by Landsat, while several stratospheric volcanic plumes containing sulfur dioxide have been discovered and measured by means of the total ozone mapping spectrometer (TOMS) instrument onboard the Nimbus 7 spacecraft. In addition to the use of satellites to assess volcanic hazards, such

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