|
Mount St. Helens Rock Volume Activity In the major eruption of Mount St. Helens on May 18, 1980, the beautiful symmetric cone of the mountain was heavily damaged. The top 1300 feet of the cone was gone, and the heart and north side of the mountain was gouged out, leaving a gaping hole. Where did the rock go? 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 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 ] [ Home ] [ Teacher Pages ] [ Modules & Activities ] |
HTML code by Chris Kreger
Maintained by ETE Team
Last updated September 30, 2010
Discuss Exploring the Environment!
Some images © 2004 www.clipart.com.
Privacy Statement and Copyright © 1997-2004 by Wheeling Jesuit University/NASA-supported Classroom of the Future. All rights reserved.
Center for Educational Technologies, Circuit Board/Apple graphic logo, and COTF Classroom of the Future logo are registered trademarks of Wheeling Jesuit University.