project

Geoscience and Environment

The Project

Geomorphology

In his remote sensing tutorial Nicholas Short described a new discipline, global mega-geomorphology (Remote Sensing Tutorial, NASA). Global mega-geomorphology is the study of the form of the earth's surface at the scale of regions and continents, an approach that has become feasible as a result of remote sensing from space. Using this approach, Larry Mayer used digital elevation models to compare Mount Saint Helens before and after a 1980 earthquake triggered the collapse of the bulging north flank, which had been inflated by magma (Applications of digital elevation models to macroscale tectonic geomorphology, in Geomorphology and Global Tectonics, edited by M. A. Sommerfield, Wiley, 2000). The debris avalance from the Mount St Helens event covers 60 km² (23 mi²) and has a volume of about 2.5 km³ (0.6 mi³).

The pleistocene event at Mount Shasta that occurred 300,000 to 380,000 years ago was much larger, covering 675 km² with a volume of 45 km³. Several large blocks measuring tens to hundreds of meters on a side were transported. (See ASTER image.) This large volume of debris is almost 18 times that of Mount St Helens, but Crandell found no evidence that volcanic eruption caused the avalanche, (Bulletin 1861).

Scope of the Project

The aim of this study is to outline the potential of DEMs for the study of landforms with brief reference to errors in DEMs.

The Shape of the Volcano

Popular and academic writers alike use the term cone to describe a strato-volcano like Shasta. This project will accept the cone as a first-order approximation to the shape of the volcano.

The base of an ideal volcanic cone would be a circle. So the first step is to define the elevation and perimeter of the circle that forms the base of the cone. Crandell has suggested that the 1220 meter contour defines the western side of the base and has drawn a circle to define it. The first image on the right represents an attempt to define the base of the volcano and its area, estimated as 230 km², giving a nominal radius of 8.6 km. The radius is nominal because the base is approximately circlar, but not a circle. It is clear from the image that there is saddle of high land between Mt Shasta and hills to the northeast. This saddle makes it difficult to define the shape of the volcano on that flank.

Filling Voids in the DEM

The DEM of Mount Shasta has voids that result partly from the lack of RADAR return from water and/or ice and partly because some steep mountain slopes cause radar shadows. These voids can be filled in a way that does not degrade other parts of the image. The steps are shown in the second image on the right.

Revising the Model

Revisions are needed because the shape of the area left by the mask is not circular. The northeastern flank of the base in obscured by higher deposits. Since the northwest quadrant and its contours are circular, a model could be constructed to analyze that quadrant alone as shown in the third image to the right. (The area of the quadrant is 107.0 km² and the nominal radius 11.7 km.) All quadrants except the northeast were prepared in this way. The northeast quadrant was inferred from the other three quadrants. The spike protruding from the top of the northwest quadrant is anomalous and was removed. The fourth image on the right shows the model used for further analysis. The area defined is not a circle but is circular enough for further analysis.

Profile Analysis

Profiles can be defined to analyse the shape of the volcano. A profile allows viewing a virtual slice through the volcano. The fifth image on the right shows five profiles through the center at 30° intervals from 12 o'clock to 6 o'clock clockwise to 5 o'clock to 11 o'clock. A profile of Bolam Creek and an adjacent ridge are also shown.

Cross-Sections Across the Cone

Cross-sectional profiles show that the northeasten to southeastern side of the cone is more concave than the opposite side. This suggests that some factor or combination of factors may be different depending on the orientation. Field studies could be designed to develop and test explanatory hypotheses.

Profiles of Bolam Creek and Ridge

Bolam Creek illustrates the formation of valleys that allow bulking of debris that is later carried down the valley. The line of the longitudinal profile was obtained by running the RUNOFF module of IDRISI. This module works well in the upper parts of the creek, but not so well in the lower reaches. The reason is that in the lower reaches the water usually sinks into the surface and does not cut a valley that the RUNOFF module can follow. Visual inspection of transverse profiles (cross-sections) assist in locating the lower part of the creek.

Discussion
Digital elevation models are useful for analyzing landforms. All DEMs are flawed to some extent and thus require correction. DEMs and geomorphological models based on DEMs can raise questions concerning factors that may have produced variations in the landscape and can assist in designing field studies.
For example, Crandell indicated that the potential source of the debris is roughly equivalent to the concave region indicated by the profiles presented here.
Crandell pointed out that volcanic activity may not have occurred at the same time as the Pleistocene debris flow. Rather, the largest of the debris flows may have been initiated by an earthquake such as those which have initiated massive debris flows elsewhere. As with Mount St. Helens, part of the volcano may have already been inflated by magma some time before the earthquake. The DEM would be a useful tool for estimating volumes of debris lost from the volcano and gained by the area defined by survey. However, estimating these volumes is beyond the scope of this project.

Return to Cascades Menu
The URL of this site is [http://www.geoscience-environment.com]