FIGURE 5.18 Diagrammatic cross section of a hydrovolcanic eruption in which magma interacts with near-surface groundwater. The interaction fragments the magma and country rocks, vaporizes the water, and produces steam explosions that excavate a crater and eject tephra. From K. H. Wohletz and R. G. McQueen, NRC, 1984, in Explosive Volcanism.

slowly but nearly continuously from a depth of about 65 km and accumulates in a shallow chamber about 3 km beneath Kilauea's summit. When that chamber becomes filled but the delivery of molten rock continues, the rock walls of the reservoir split, and molten rock either erupts to the surface or fills underground fractures in the volcano's flank.

Many questions about the Hawaiian volcanoes remain unanswered, but their general structure and dynamics are understood relatively well by comparison with those of the Cascade volcanoes like Mount St. Helens and Mount Rainier, which are less continually active. This understanding comes from familiarity; the spattering eruptions of Hawaiian volcanoes are part of everyday life. As convergent plate volcanoes, the Cascade volcanoes rise frigid and silent above the jumbled ridges clustered around them—Mount Shasta and Mount Rainier are over 4,000 m high; Mount Hood and Mount Baker reach heights over 3,000 m; Mount St. Helens had grown to only 2,550 m before its eruption in 1980. At these volcanoes, typical of those around the Pacific rim, earthquake swarms and ground deformation episodes are much more intermittent than at Kilauea. Long periods of quiet are suddenly interrupted by intense episodes of shallow intrusion and eruption. When these volcanoes erupt, they can be violently explosive.

Although violent eruptions are easily perceived as a major hazard, so can associated mudflows or lahars. About 4,000 years ago a massive mudflow having a volume of over 3 km3 originated on the edifice of Mount Rainier and partially filled many surrounding valleys and inlets of Puget Sound to hundreds of feet. Such a mudflow can be a major hazard. They might not necessarily be directly associated with a volcanic eruption but can be triggered by a nearby earthquake that might destabilize the glaciers on the volcanic edifice. Mount Rainier currently has over 4 km3 of perennial ice on its peaks; such mudflows are poised to occur again. Because of the potential mudflow and volcanic hazards at Mount Rainier, it has been designated as a ''Decade Volcano" within the context of the International Decade of Natural Disaster Reduction. The intent is to focus hazard-related research activities; these activities are currently being developed.

So far, earthquake and deformation data on the convergent-boundary volcanoes reveal little about their dynamic behavior. A possible reason for this may be that much significant activity at these volcanoes is beyond the penetration capabilities of deformation-monitoring instruments, perhaps 25 to 35 km beneath the surface. When the sources of potential ground-surface deformation are so deep, the changes at the surface are either too small or too widely distributed around the volcano's summit to be measured by conventional surveying techniques.

The advent of satellite-mounted surveying systems, such as the Global Positioning System (GPS), may help to resolve this problem. With an accuracy of ± 2.5 cm over 50 km, satellite surveys may be able to detect such small local or widely distributed changes in the elevations and horizontal positions of benchmarks around potentially active convergent plate margin volcanoes.

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