Sunday, September 13, 2015

Circum-Ambulation of Meteor Crater at the Northern Arizona University 3rd Annual Geology Alumni Reunion

Educational panel in the Meteor Crater Visitor Center
Studying geology at Northern Arizona University was perhaps the single best thing I did to promote my career as a geologic educator. Located close to the San Francisco Volcanic Field, the Grand Canyon, Oak Creek Canyon, Monument Valley, the Petrified Forest and numerous other world class landscapes, one couldn't help but become enamored with the fascinating story of earth history on display everywhere. The professors were just as enthusiastic about the place and the small class sizes were ideal for learning. NAU however, has been slow in developing a bond with its former geology graduates but that is quickly changing. Paul Umhoeffer, Professor of Geology at NAU since 1992 and current Director of the Environmental Program, is welcoming back past graduates by arranging an annual three day event in September with gatherings and field trips. This year is the the 3rd ever event with a field trip to Meteor Crater located east of Flagstaff.

This trip was led by Tenielle Gaither (BS, 2008; MS 2011) now working for the Astrogeology Division at the USGS in Flagstaff.

Twenty five of us walked the entire perimeter of the impact crater on a hot September day. We had alumni attend from as early as 1972 and as recently as June of this year!

Location map for Meteor Crater 40 miles east of Flagstaff (copied from Tenielle Gaither's field trip guide).

 Gene Shoemaker's 1959 geologic map of Meteor Crater (copied from Tenielle Gaither's field trip guide).

I had never walked the entire perimeter of the crater so this was a special treat for me. I did once hike to the bottom of the crater with my friend Drew Barringer and you can read about that trip here.

The impact is known to have occurred around 50,000 years ago, an age based on post impact erosion rates and two more tightly constrained dates using surface exposure dating techniques.

This is Whale Rock on the eastern rim of the crater where our first stop was made. The sheer size of some of the ejecta clasts was astounding. This rock is about 30 feet tall as shown with more of its mass beneath the nearby rubble.

More ejecta on the southeast rim of the crater

Some of the old works left by Danial Barringer as he searched for the elusive iron meteorite in the pit of the crater. Scientists now know that most of the bolide was vaporized upon impact with perhaps only 20% of it existing as small fragments.

It was a surprise for me to see so much Coconino Sandstone lying as ejecta on the south rim of the crater as it is located in place in the bottom of the crater. Since some of the bottom-most matrial is lying on the south rim, Barringer suspected that the impactor collided with the earth from a northern trajectory. Other studies yield evidence for impact from the southeast or south, suggesting that it likley impacted at a high angle.

Eolian sand on the south rim

Close-up view of the tear fault located on the north rim of the crater. Whether this is a preexisting fault or one initiated by the impact is not yet known.

Dr. Paul Umhoeffer at Meteor Crater during the 3rd Annual NAU Geology Alumni Reunion

Close-up of the ejecta blanket on the southwest wall of Meteor Crater. It is intriguing to think how high this material might have been lifted upon impact.

Photo of overturned beds within the Moenkopi Formation on the south rim of the crater. See the following image for descriptions, however the light orange sandstone in the left center is the overturned equivalent of the light orange sandstone at right center.

Essentially the same view as above but with annotations showing the location of overturned beds in the Moenkopi Formation. These beds were thrown backwards out of the crater area upon impact. All deformations exposed in the crater occurred within about 10 seconds.

Wide angle view of Meteor Crater from the south side looking north.

Cross-section through Meteor Crater showing elevations above sea level, upturned edge of crater rim, and thickness of breccia in the floor of the crater (copied from Tenielle Gaither's field trip guide).

This is the largest fragment of the meteorite ever recovered from Meteor Crater and is called the Holsinger meterorite. The original bolide was about 100 feet in diameter.

Scenic view of the crater from the south rim

Beds of Kaibab Limestone are progressively oversteepened from bottom right to upper center (beneath the people standing on Barringer Point, the highest point on the rim of the crater).

More massive ejecta boulders from the west rim of the crater

This bench mark has the word METEOR stamped in it

Awesome summertime clouds over northern Arizona

Same, same

Stephen Lang was our Meteor Crater Enterprises guide along the trip. He is not formally trained as a geologist but his knowledge of the crater and the processes that created is astounding and was a special highlight on the trip for me. A job well done Stephen! We need more local guides like you in Arizona!
 
My friend and colleague Marvin G. (NAU Geology BS, '79; MS '85) stands in front of one of the historic buildings located on the southwest corner of the crater. Marvin has also been instrumental in making these geology reunions fun and welcoming!

View to the south of the silica pits, where silica was mined

Close-up of a wall in the silica pits exposing two large red clasts from the Moenkopi Formation. Many thanks to the folks at Meteor Crater Enterprises for hosting the 3rd Annual NAU Geology Reunion Field Trip!

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