||Mount St. Helens Magma Volume Activity
Although there is no standard scale for measuring the sizes of volcanic eruptions comparable to the Richter scale for earthquakes, one useful measure is the volume of new magma or lava ejected onto Earth's surface. In an explosive eruption like Mount St. Helens, there are no lava flows, instead the new magma is blasted into tiny fragments of volcanic rock called ash that is blown far and wide. Thus the volume of new magma erupted by Mount St. Helens can be found by measuring the volume of ash surrounding the volcano and scattered across the country side.
Image file "ashplote.tif" is a map of the thickness of ash from the main eruption of Mount St. Helens (May 18, 1980). The ash, blown by the prevailing winds, spread across Washington, Oregon, Montana, and beyond. You can use this map to measure the volume of ash and the volume of new lava produced by Mount St. Helens.
Procedure using NIH Image
2. Set Scale: The dimensions of pixels in the image need to be converted to a common linear measure. To do this in kilometers, use the line selection tool to measure the length of 300 kilometers on the length scale at lower left. Choose Analyze/Set Scale and change units to kilometers and type "300" in the box. (You should get about 0.497 pixels per km.)
3. Choose Analyze/Options and select Area and Mean Density (deselect all others).
4. Choose Options/Density Slice and narrow the red bar to fill one density unit. By sliding the red bar up and down, different areas will be highlighted.
5. Choose Analyze/Show Results and move the Results box below the map image so you can work on the image and see the measurement results.
6. Measure area/volume for each area: Click on the middle of the red bar in LUT and drag it up or down to highlight desired area in red, then choose Analyze/Measure. The area and mean density value will appear in the results box. Repeat for each area. The order is not important, but you might systematically measure from thickest to thinnest or thinnest to thickest to make sure you do not miss any (of the 11 areas). To get the volume for each area multiply the measured area by the mean density, and divide by 2 million to convert to cubic kilometers. To get the total volume add all 11 volumes together. Divide the sum by 2,000,000 to convert to cubic kilometers. If you wish, you may divide the sum by 4.17 to convert to cubic miles.
This volume of ash or new lava can be compared with the new lava ejected in other volcanic eruptions. It should also be compared with the volume lost from the cone of Mount St. Helens by a giant landslide (see Rock Avalanche activity).
Note: The map does not include all of the ash. About one-third of the measurable ash fall is beyond the map limit. You should correct your volume estimate by this factor.
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