McDowell Mountains Marcus Landslide

Introduction:

The McDowell Mountains in northeastern Phoenix host one of the largest known landslides in Arizona. The entire landslide is within the McDowell Mountain Regional Park preserve (Figure 1) . Discovery of this large feature took place during the construction of a learner-centered education course funded by the Arizona Board of Regents.

The slide is named in honor of former ASU Professor of Geography Melvin Marcus. A world-renowned physical geographer and student favorite, Professor Marcus passed away in 1997 while leading class field trip in the Rocky Mountains.

Figure 1. The Marcus landslide at different scales.
   A) A satellite image placing Phoenix in geographic context with the Marcus landslide (courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center).
   B) A remotely sensed image of North Scottsdale and the McDowell Mountains with respect to the Marcus Landslide (courtesy of ASU).
    C) Location of the Marcus landslide with respect to growth in Scottsdale, Arizona.Note how the development is starting to grow out next to very steep slopes on the eastern side of the McDowell Mountains, even though the landslide itself rests in a land preserve.

Landslides litter steep unstable hillslopes across Arizona. The Grand Canyon in northern Arizona hosts thousands of landslides, with innumerable failure prone slopes and cliffs (Figure 2). In fact, the Surprise Canyon landslide in western Grand Canyon remains the largest discovered landslide in the state.

Figure 2. A Landsat image of the Grand Canyon, courtesy of NASA, merged with the topography. The rock fall image, caught by Niccole Cerveny, illustrates what happens when very steep slopes are continuously undercut by river erosion.

The McDowell Mountains have one important thing in common with Grand Canyon, steep hillslopes. The Marcus landslide failed along plane with a very steep angle of 33 degrees. The presence of the Marcus landslide cautions against unwise development next to steep slopes of Phoenix mountain ranges.

The Marcus Landslide is technically called a rock avalanche. The Marcus landslide is among the biggest rock avalanches called sturztroms with a volume over 7 million cubic meters. The slide mass extends about one kilometer from the mountain front with a width of 0.5 kilometers. The source exhibits classic features of large landslides of breakaway scar and fall zone (Figure 3).

Figure 3. A west-looking view of the Marcus landslide from a height of 200m, constructed from a 10m resolution digital elevation model in MicroDEM. There is no vertical exaggeration. Ripples in the foreground represent DEM artifacts.

This virtual field provides a tour of the Marcus Landslide intended for anyone interested in desert geomorphology (geomorphology is the science of how the earth's landscape is sculpted). This virtual field trip is more than just a set of pretty pictures, although you can certainly just look at the photos. We explore reasons for landslide failure, the probable age of the landslide, and classic features of large landslides that you can see on the Marcus Landslide.

But like any other geography field trip, we there is more than just the topic of focus. So you will also see the flora, fauna, and cultural landscape features normally encountered (e.g. Figure 4).

Figure 4. Gila monster hiding in a little alcove. The material making up the alcove consists of cemented landslide debris on the east end of the slidemass.

How this virtual field trip works:

The field trip structure involves a series of stops with panoramic images of the surrounding terrain. From these panoramic images, clickable hotspots provide a more in depth means of examining particular locations of interest. After exploring all the hotspots for a particular spot, move on to the next stop and repeat the same procedure. A reference image located at the base of each stop provides a viewshed of your current location, to help you maintain your position with respect to eh landslide. The color rounded triangle represents the viewshed, you virtually stand at the pointy end and look our towards the rounded edge (Figure 5). Along the way, more in depth information provides those so inclined a chance to explore in great detail some aspects of the Marcus Landslide, but by no means required to enjoy your trip.

Figure 5. The upper image is an example of the guidance on this virtual field trip.

The funny pie shaped feature with a number is the viewshed you will see at, in this example, stop 4. At this location, you are virtually standing in the pointed part of the orange viewshed and looking out towards the rounded edge. In this example, the photograph is what you would see from site 4, looking up at the bottom of the slide. The combination of aerial view and ground view helps you know where you and helps you navigate the field trip. The upper image was generated using MicroDEM.