Monitoring
and Assessing Landslide and Slope Failure Hazards in Areas with
Active Volcanoes
Pictured above: A map showing the pathways of lahars from Mt. Rainier in the past 6,000 years. |
All of the USGS hazard assessments of active volcanoes include consideration
of hazards from mudflows or "lahars", debris flows, landslides,
and slope failure. Special attention is currently focused on research
of these phenomena at Mount Rainer National Park, which may be applied
to similar volcanic hazards in other regions.
About 5,600 years ago an enormous landslide (3.8 km3)
removed the volcano's summit and its altered core to form a deep
horse-shoe shaped crater. The event, called the Osceola collapse,
sent altered debris northeast and south into the Puget Sound. The
Round Pass and Electron collapse events 2,700 and 500 years ago,
respectively, sent altered debris west to Orting and Puyallup; the
Puyallup River flows toward the northwest and empties into Puget
Sound near Tacoma. The pathways of these events are illustrated
in red on the map of the Mount Rainier-Puget Sound area at left.
Water circulating through a volcano can become heated and may contain
dissolved chemicals. These "hydrothermal" fluids are capable of
changing or altering the composition of their host rocks. USGS scientists
are studying the distribution of hydrothermal alteration at Mount
Rainier to help understand its hazards. Large volumes of altered
rock located high within a steep volcano are potential sources of
landslides. Such landslides typically transform into lahars that
travel many tens of kilometers downstream from a volcano.
Pictured above: A photograph of Mount
Rainier's Sunset Amphitheater showing locations of altered rocks. |
The experimental
use of high-resolution helicopter-borne magnetic and electromagnetic
surveys over Mount Rainier illustrates a new capability for "sensing"
the locations and sizes of altered zones within a volcano. Sensors
suspended from a helicopter were used to analyze rock beneath
the
surface of Mount Rainier. The techniques used to identify altered
volcanic rocks within a volcano are effective because altered
rocks
have low magnetization and electrical resistivities. In contrast,
fresh volcanic rocks have relatively high magnetic signatures
and
high resistivities, all of which may be detected from the air.
Based on the new data collected, USGS scientists recently reported
that
the core of Mount Rainier has not been significantly by hydrothermal
fluids, contrary to previous assumptions. The results of the new
research suggest that future collapses of hydrothermally altered
rocks will be most probable on the upper west side of the volcano
in and around the Sunset Amphitheater, about 650 m below the summit.
More information is available in the journal Nature, 2001: v.
409,
p. 600-603.
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