Marcus Landslide EarthCache
Hidden : 5/22/2014
Difficulty:
3 out of 5
Terrain:
3 out of 5

Size: Size: other (other)

Join now to view geocache location details. It's free!

Watch

How Geocaching Works
Looking for a different adventure?

Please note Use of geocaching.com services is subject to the terms and conditions in our disclaimer.

Geocache Description:


  • This EarthCache requires hiking the entire Marcus Landslide Trail in the McDowell Sonoran Preserve, including the end of trail loop. The 3.7 mile round trip hike with a 300 foot elevation change begins at Tom's Thumb Trail Head. The well-marked trail, which includes many informational signs, is rated Moderate.
  • A listing of general rules that all visitors to the Preserve are asked to adhere to can be found posted at the trail heads and in the Preserve maps, which are available at the Trail Head info building. One of the most common ways to destroy habitat is by hikers going off trail. With that in mind you are required to please stay on the designated trails at all times.
  • Be sure to wear sunscreen, appropriate shoes and clothing, take plenty of water/food and be aware that you are visiting the desert which is home to many animals including, but not limited to, snakes.
Marcus Landslide

The Marcus Landslide was discovered in 2002 when John Douglass, at the time an ASU geography Ph.D. student, and his friend Brian Gootee, then a graduate student in geology at ASU, participated in a school field trip in the McDowell Mountains. Curious about the large expanse of granitic boulders, the two climbed to a high ridge that looked down on the area. From there, the boulders appeared to fan out from a chair-shaped hollow on the steep mountain slope just below them. They had just found Arizona’s second largest known land slide compared to the largest; The Grand Canyon. After a very comprehensive study their results were published in 2004 in the journal Geomorphology. They named the Landslide after ASU Professor Mel Marcus who died while on a field trip to Colorado in 1997.

Graphic Representation of Marcus Landslide
About the Landslide

Geologists believe the Marcus Landslide occurred in the McDowell Mountains about 500,000 years ago. A portion of the mountain top on the east-central summit of the McDowell Mountains suddenly collapsed into a huge rock avalanche. An estimated 194 million cubic feet of rocks, vegetation and grus filled soil (eroded granite particles) flowed eastward down the side of the mountain and onto the valley floor for about 1 mile. The resulting landslide rubble is about 1,650 feet wide and up to 100 ft deep above the valley floor. The landslide debris is enough to fill six ASU Sun Devil Stadiums!

The event probably started as a mass of sliding rock before quickly transforming into a debris flow falling to the valley floor 1,200 feet below. Walls up to 90 feet tall define the sides and toe of the slide mass. The debris flow transported several hundred very large boulders, weighing up to 12,000 tons, to the valley floor. The energy released during this single landslide event was the equivalent of energy released in one atomic bomb!

Outline of Marcus Landslide
Why Did This Happen?

Granite rock in this area formed from molten magma which intruded overlying rock and then hardened. Granite masses can fracture into perpendicular joints, exposing square edges that become rounded through erosion. This process accounts for the large, spherical boulders and formations found across the landscape. Granite further breaks down into grus, coarse grain fragments the size of sand or small pebbles that can act like ball bearings on a slope. Granite may also erode into clay, which contracts when dry and expands when wet. Expanding clay and rounded grus particles create a slippery surface, which set the stage for massive instabilities on steep slopes, like that of East End Peak.

During the ice age the mountains here in Arizona were not covered with ice. However, the local climate was much cooler and wetter than today. Freeze-thaw cycles weakened joints in the granite rock masses and hastened erosion. Coupled with steep slopes and a trigger event – like heavy rain or an earthquake – a major landslide became possible. Although we don't know exactly what the trigger event was the landslide debris that you are walking on is the result of a rock avalanche we now call the Marcus Landslide.


References:
"The Marcus Landslide, An Ancient Landslide in the McDowell Mountains, Arizona" by Brian F. Gootee, AZGS Research Scientist
"A large landslide on the urban fringe of metropolitan Phoenix, Arizona" John Douglass, Ronald I. Dorn, Brian Gootee
"Marcus Landslide Virtual Field Trip McDowell Mountain Regional Park, AZ" http://alliance.la.asu.edu/slides/introduction.html
http://www.protrails.com/trail/623/phoenix-scottsdale-mesa-marcus-landslide

Permission Granted:
I wish to thank the City of Scottsdale and the land managers of the McDowell Sonoran Preserve for their permission to publish this EarthCache.
Logging Requirements

To log this EarthCache you must physically visit the coordinates which are located on the final loop of the Marcus Landslide Trail. Email to me answers to the following questions before posting your log. Do not post your answers with your log. You are welcome to post photos however, do not post pictures that will give away any answers.

1) At the EarthCache coordinates you will find two viewing tubes. Describe to me what you see when you look through the upper tube and then what you see through the lower tube. How are the views different?
2) On your hike around the trail loop you will see many weathered granite boulders. Tell me why some boulders, or parts of some boulders, would be more eroded than others.
3) Describe the soil content where the eroded boulders rest on the ground.
4) What is the strongest piece of evidence that you have observed that convinces you that a landslide took place here?

Additional Hints (No hints available.)



Advertising with Us