In this type of eruption, little of the "old" volcanic rock is carried away in the ash cloud. In fact, in many eruptions of similar size, the bulk of the original volcanic mountain is often little affected. In the case of Mount St. Helens, however, the top and north side of the volcano literally broke free and flowed as a giant landslide or rock avalanche into the valleys to the north. The volume of the mountain lost in this way can be estimated from digital topographic maps. This estimate can then be compared to the volume of new lava ejected (see Magma Volume activity).

Procedure Using NIH Image
1. Open the images "mshprecal.tif" and "mshpstcal.tif." These images have been calibrated in kilometers in the x, y plane of the image and in meters in elevation. The east-west coordinate, the north-south coordinate, and the elevation of each point in the images can be found in the Info box next to X, Y, and Value, respectively. To find the coordinates and elevation of any point in either image, select one of the images, move the cursor to a point of interest, and read the quantities in the Info box. (The numbers in parenthesis are the equivalent pixel and DN values, respectively.)

2. Choose Process/Image Math to get the dialog window. Subtract the MSHpst image from the MSHpre image, multiply by 1 (one) and subtract 0 (zero). The resulting image is still calibrated horizontally in kilometers, but the vertical difference is in density units. Use the measuring tools and density values (one density unit equals 10.64 meters) to find the width and depth of the mass of lost material.

3. Choose Analyze/Options and select Area and Integrated Density (deselect all others) to find the volume of the lost material. Use the rectangular selection tool to outline a box that includes the area of the lost material and a small amount of the surroundings. (Keep the box as small as possible and still include all of the lost material.)

4. Choose Analyze/Measure and Analyze/Show Results to get the volume of lost material. The units are square kilometer-density units. To convert to cubic kilometers, multiply by 0.01064. Convert to cubic miles by dividing by 4.17.

Unfortunately, the digital maps were only changed to show the material lost from the mountain, not the thickness of the avalanche deposits in the valley. You can estimate the average thickness of the deposits by measuring the area of the flat valley bottom of the North Fork Toutle River (center left in the maps) in either of the original images and dividing that total into your measured volume. If you had owned a home near the Toutle River, how deep would you have to dig to find what was left of it?

[ Orting in Pictures ] [ Mountain of Risk ] [ Mud / Waterflows around Mount Rainier ]
[ Maps of Rainier Area ] [ Mount St. Helens Magma Volume Activity ]
[ Mount St. Helens Rock Volume Activity ]
[ References ] [ PBL Model ]

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