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!

Wednesday, September 02, 2015

Scenes from the August 9, 2015 Vermilion Cliffs and Highway 89A Flood Event

2018 UPDATE: The Arizona Department of Transportation (ADOT) posted on their blog site how the boulders on the road were removed. See the posting here.

On August 9 of this year, a large storm cell dumped significant moisture on the south facing Vermilion Cliffs near the small settlement of Cliff Dwellers. A passing motorist shot this video which shows huge rooster-tail waves running over the highway. A local news report had this to say about the large runoff event.

On August 22 I was  at the site of the flood, preparing for an 8-day river trip in Grand Canyon. I drove to the culvert area for this unnamed wash and shot pictures of the debris that was still present near the roadway.

Flaggers Wesley and Aaron work from 7 PM until 8 AM directing traffic to one lane across the damaged portion of the road (shown).

Bulldozers have been here in the ensuing 12 days since the flood, moving rocks off of the highway. They are quite large for the relatively shallow gradient of the stream here.

The dozers have cleared away much of the debris on this, the upstream side of the culvert. But the infilling inside the culvert is impressive.

Damaged fence post near boulders

Some boulders were too large to move and so they were cracked into smaller pieces

Vermilion Cliffs in the background is where the storm dumped its load. The edge of the cliff is a drainage divide and so here is the extent of the drainage area of the flood.

The downstream side of the culvert beneath US Highway 89A, four miles west of Cliff Dwellers

The main stem of the flooded wash, which is a tributary to Soap Creek Canyon. Later in the day, we ran Soap Creek Rapid and saw a few new boulders in the channel of the Colorado River, but these were from the opposite tributary, Jack Ass Canyon.

The culverts size simply could not handle the water an debris in this event and so the flood water jumped the channel. This is an undisturbed portion of the debris fan.

Another view of the waterway of the flood

Boulders casting an early morning shadow across the mud laden floodplain. It's been a great summer for monsoon rain and floods in the Southwest but perhaps just normal with respect to other seasons. What is different is that people are traveling with video cameras now and so once far away events are recorded and put on the web. On August 7, Diamond Creek flooded in Grand Canyon and you can read a boatman's account of the 10,000 cfs flood here.