USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
Ground Cracks and Radial Cracks
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Mount St. Helens Radial Cracks
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MSH81_USGS_scientists_measure_radial_crack_base_dome_05-12-81.jpg
The U.S. Geological Survey established both periodic and continuous 24-hour
monitoring programs at Mount St. Helens to study and predict eruptions. In this
slide, geologists used a steel tape to measure the distance across a crack on the
crater floor. Widening of cracks was an indication that magma was rising and
deforming the area, leading to an eruption. These cracks were generally
radial to the dome, like spokes of a wheel.
USGS Photograph taken on May 12, 1981, by Lyn Topinka.
[medium size] ...
[large size] ...
[TIF Format, 18 M]
From:
Iwatsubo and Swanson, 1992,
Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount
St. Helens, Washington:
IN: Ewert and Swanson (editors),
Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the
Cascades Volcano Observatory, 1980-1990: U.S. Geological Survey Bulletin 1966,
223p.
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In mid-september 1980, cracks
started forming in the crater floor
(of Mount St. Helens)
radially from the vent, which was plugged by a small dome that formed in August.
... prior to the October 1980 explosive episode, the cracks
widened at an accelerating rate in response to a rising magma body. ...
From:
Brantley and Topinka, 1984, Volcanic Studies at the
U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory,
Vancouver, Washington: Earthquake Information Bulletin, v.16, n.2,
March-April 1984
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New ground cracks
appeared on the crater floor from several days to 2 to 4 weeks
before all the 1980 to 1982 eruptions
(of Mount St. Helens).
The cracks, commonly tens of meters long
and tens of centimeters wide, extended outwards from the dome like spokes from
the hub of a wheel. Incandescent rock was visible in some cracks, and
temperatures of escaping gas were measured as high as 840 degrees C. Measured
with a steel tape, the cracks commonly showed continual widening that
accelerated before eruptions. Such accelerated movement was used to predict
several eruptions in 1981 and 1982.
From:
Iwatsubo, Ewert, and Murray, 1992,
Monitoring Radial Crack Deformation by Displacement Meters:
IN: Ewert and Swanson (editors),
Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the
Cascades Volcano Observatory, 1980-1990: U.S. Geological Survey Bulletin 1966,
223p.
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Beginning in September 1980, cracks that extended radially
from the
Mount St. Helens dome
formed in the crater floor.
As magma ascended into the dome the crater floor deformed,
creating segments or blocks bounded by cracks that either widened or,
less commonly, narrowed
before and during extrusive episodes ...
The rate of widening or narrowing of the cracks
accelerated before extrusion occurred. Initially,
a steel tape was used to measure changes in distance
between reinforcing rod (rebar) stakes driven into the
ground on either side of selected cracks (crack stations).
This method
became a reliable tool for predicting dome-building episodes at Mount St. Helens.
... To monitor crack movement continuously, electronic tape measures (displacement
meters) were installed to monitor displacements between rebar stakes. ...
From:
Brantley and Topinka, 1984, Volcanic Studies at the
U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory,
Vancouver, Washington: Earthquake Information Bulletin, v.16, n.2,
March-April 1984
-
New ground cracks
appeared on the crater floor from several days to 2 to 4 weeks
before all the 1980 to 1982 eruptions. The cracks, commonly tens of meters long
and tens of centimeters wide, extended outwards from the dome like spokes from
the hub of a wheel. Incandescent rock was visible in some cracks, and
temperatures of escaping gas were measured as high as 840 degrees C. Measured
with a steel tape, the cracks commonly showed continual widening that
accelerated before eruptions. Such accelerated movement was used to predict
several eruptions in 1981 and 1982.
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Monitoring Ground Cracks and Thrust Faults
Return to:
[Radial Cracks and Thrust Faults Menu] ...
URL for CVO HomePage is:
<http://vulcan.wr.usgs.gov/home.html>
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<http://vulcan.wr.usgs.gov/Glossary/CracksNThrusts/description_cracks.html>
If you have questions or comments please contact:
<GS-CVO-WEB@usgs.gov>
12/01/06, Lyn Topinka
