Six days in the crater, day three

Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6

This post is part of a slowly unfolding saga of my experience at the Meteor Crater Field Camp that was held from October 17-23, 2010. The field camp was run under the NASA Lunar Science Institute and headed by Dr. David Kring of the Lunar and Planetary Institute.
This post also doubles as my entry into Accretionary Wedge #49: Out of This World, which focuses on extraterrestrial geology and terrestrial analogues. Thanks to Dana at En Tequila Es Verdad for hosting this month’s Wedge!

Tuesday, October 19, 2010.

From above, our deluge of sun hats would appear to run into and froth against the tourist rope corral for a moment before spilling over and around into the area of No Trespassing. Rapidly arriving at a flattened part of the rim, we diffuse and come to rest, idly shifting for something in our backpacks and maybe a better view into the crater. David starts speaking of the plans for the day, and it takes us a few moments to realize we’re looking in the wrong direction. We turn around and he points at a small hill near the side of an access road. Pondering aloud, he wonders What do you suppose that boulder is doing on top of that hill?

Hint: It’s not a geocache.

The plains surrounding Meteor Crater are afflicted with an excess of flatness. Aside from the crater itself, the only relief is from scattered blocks, mounds and low rises of Coconino Sandstone and Kaibab Dolostone. They are blemishes on the otherwise flat patchwork terrain surrounding the crater. Like the boulder on the hill, many large coherent blocks of ejecta excavated during impact were thrown out of the crater and now rest upwards of 300 feet above where they ought to. Three days in, we were no longer tourists; It was time for science. We started work to answer a few relatively simple questions: Where in the crater did those blocks originate? How big are they? What would it take to launch them tens and hundreds of meters to their current position?

To answer those questions, the group split in two. One team examined the ejecta blocks, recording their dimensions and lithology. The other team measured the distance from crater to ejecta, using a physical measuring tape and recording map position and GPS coordinates to cross-check calculations. It was a rather straightforward assignment that also got us thinking in terms of cratering processes. The furthest blocks we studied were a solid five minute walk from the crater. It is easy to lose sight of something that is no longer present, but after the initial impact we would have been scrambling over several additional meters of ejecta the whole trip.

The measuring group stands on the rim of Meteor Crater as the tape is prepared for the trek to ejecta block E-3. Some members of the lithology group are visible on the white block of Kaibab (limestone) in the distance.

Team Tape and Team Lithology knocked out six profiles over the course of the day, including an assessment of the famous three-story-tall House Rock (a.k.a. Monument Rock). Six blocks is a small sample set to be sure, but one that lays the groundwork for a more thorough and complete ejecta study to be conducted over a number of Meteor Crater Field Camps. With a few simplifying assumptions – radial path, ballistic trajectory, 45 degree angle of ejection – our results indicated flight times for these boulders of three to fifteen seconds at ~60-360 km/hr [~15-100 m/s]. These velocities get well above hurricane force winds (though they pale in comparison to the ~12 km/s impact velocity). And we’re not talking about shingles and trees flying around – these are multi-ton boulders getting hucked out in all directions. Given enough force, ejecta can go anywhere. House Rocks might not get very far, but there are loads of examples of ejecta traveling hundreds of kilometers, into the atmosphere, or even off-planet. We only have fragments of Mars as a result of a few impacts into the martian surface sending material into space and eventually to Earth. Simon Wellings (@metageologist) wrote a bit more about the evidence of impacts in his contribution to the Wedge, What came from outer space.

The Apollo Era really brought to light the importance of impact processes on the evolution of planetary surfaces. Apollo missions also proved a challenge to geologists. No lunar material was collected in-situ, which means the provenance of many samples is uncertain. The provenance of regolith (soil) and impact breccia fragments is still the subject of intense debate. Many of these fragments likely have origins in basin forming events (e.g., the Sea of Tranquility). Boulders like those surrounding Camelot Crater in the above photo, are a bit easier to reconcile with their source. Mapping the distribution of ejecta lithology around terrestrial and lunar craters is the ground-truth to theoretical distribution models. Gravity and atmospheric conditions may differ between the Earth and Moon, but the results of an impact are similar across the solar system.

Top: Boulder field at Camelot Crater from the Apollo 17 mission. Panorama compiled by Warren Harold of NASA/JSC. Bottom: Looking outward from the rim of Meteor Crater. Both images are in color.

Learn more about impact cratering processes with the Lunar & Planetary Institute Impact Cratering Lab

Six days in the crater, day two

Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6

This post is part of a slowly unfolding saga of my experience at the Meteor Crater Field Camp that was held from October 17-23, 2010. The field camp was run under the NASA Lunar Science Institute and headed by Dr. David Kring of the Lunar and Planetary Institute.

Everyone is surprisingly awake at 7AM, considering the hard sun yesterday. Maybe it’s the brisk 50°F October air, or perhaps everyone had a long sleep (one pro of a dry campsite). My hunch, however, is on the catered breakfast of fruit, eggs, coffee, juice, oatmeal, and scones that awaits us. A local Flagstaff catering company will be bringing us breakfast and dinner each day, and no one wants to miss out after our first taste yesterday. Even without the extra incentive, the 21 other field camp attendees are highly motivated, intelligent and capable researchers from around the world. And then there’s me, just writing accidental haiku in my field notes…

base of mining slope;
two tear faults up wall expose
best Coconino.

The Coconino is an eolian quartz sandstone; white, fine-grained, occasionally massive but often with cross-beds. If you’ve never heard of it, perhaps you’ve seen the Coconino cliffs of the Grand Canyon or Zion National Park. At Meteor Crater, the Coconino is the lowest unit excavated by impact, extending from 90-300 meters below the surface. The crater center is buried under ~100 meters of lake sediment, and a mine shaft is the only portal to the original crater floor. Remnant mine talus piles on the crater floor hint at the intense shock buried 100 meters below, where some sandstone was altered to vesiculated glass; It floats! The major occurrences of Coconino ejecta still present around the rim are generally not shocked to glass, but are no less interesting. The Coconino in the photo below is ‘fuzzy’ because it has been pulverized to rock flour, though relict bedding is preserved.

Overturned Coconino SS ‘rock flour’ with relict cross-bedding in northwest wall of Meteor Crater. This coherent ejecta block was originally 90+ meters beneath the surface and now rests ~15 m above the surrounding terrain.

Over on the south side of the crater, heterogeneous shock distribution resulted in relatively unshocked Coconino in contact with the rock flour variety. A useful reminder on the importance of context!

Continue reading

Six days in the crater: day one

Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6

This post is based on field notes and memories supplemented by background reading material from the Meteor Crater Field Camp that was held from October 17-23, 2010. The field camp was run under the NASA Lunar Science Institute and headed by Dr. David Kring of the Lunar and Planetary Institute.

Twenty-two of us are spread out between three Chevy Suburbans, and it’s strange having legroom on a geology expedition. Not that there’s far to go. We are camped out in an RV park a mere 5.5 miles from our field site: Barringer Meteorite Crater. This is the first day of the first ever Meteor Crater Field Camp, and we are making the first trip first thing in the morning to my first visit to any crater, ever. Everyone’s ready to get started, and we don’t have long to wait. Our first stop is approximately fifteen feet outside the gates of our campsite, and we step out of the vehicles after realizing the stop wasn’t because of a forgotten water bottle or notebook.

View to south from RV Park

Sunset from Meteor Crater RV Park, panning from south (left) to west (right). I’ve ‘underlined’ the approximate extent of Meteor Crater’s rim (view large!), which is flanked by silhouettes of volcanic mesas.

At 5.5 miles out there’s not much of the crater to see and so we huddle around Dr. Kring, our fearless leader. He asks us to imagine standing at this exact spot 49,000 years ago* as Meteor Crater formed. “What would you see?” he asks us. Slowly we find our voices: A streak of light and a flash. A fireball. Wind whipping across the plains. A massive volume of ejected material rushing toward you. Then nothing because you are dead.

We learn we wouldn’t even last that long. At this distance the shock wave would reach you in less than a second, turning you inside out before the wind and heat simultaneously flash-cooked your body as it tumbled away in pieces with the desert sage. Oh well. Someone luckier and further away would have seen quite the show.

49,000 years later, as we pile back into the vans, it’s mostly sunny and warming past 50°F on the high desert plains of Arizona.

[*On the basis of thermoluminescence, 26Al, 10Be, and 36Cl studies, it's generally agreed that Meteor Crater formed 48- to 49-thousand years ago. More recently, updated 36Cl reference material argues for an older age of 60- to 65-thousand years.]

Hiking on rim Meteor Crater

Uneven terrain on the crater rim

On the docket for our first day at the crater is a 3.7 kilometer (2.3 mile) hike along the rim, ~1.8 km clockwise before lunch, then retracing our steps counterclockwise. There’s House Rock (a.k.a. Monument Rock), the largest (intact) boulder on the crater rim at ~10 meters tall. Pile on two more House Rocks and you’d have a reasonable estimate of the meteorite diameter. And maybe at one time there really was another House Rock or two on top. Surface exposure age dates from the top of House Rock are younger than the 49,000 year formation age.

It wouldn’t be surprising if House Rock had initially been covered by ejecta. Erosion and weathering is evident everywhere at the crater. A gully running downslope beneath the crater museum rapidly cut through three layers of authigenic breccia, both exposing and halfway eroding a projectile fragment over a two year period. The breccias are so friable that when we later head into the crater, we aren’t allowed to touch or even go near the left side of the gully where the projectile is exposed.

On our CCW hike back we generate a little erosion of our own by scrambling down the crater wall a bit from House Rock to see something (else) awesome. We start out on Kaibab on the crater rim and walk down through Moenkopi, and end up back at Kaibab. The top slice of bread in the Kaibab-Moenkopi-Kaibab sandwich is ejected, overturned Kaibab. The bottom slice is in-situ Kaibab. And the meaty Moenkopi center is where it gets interesting. Look at the photo below. Both members of the Moenkopi are visible here. The pale reddish brown Wupatki (at left) is a massive sandstone underlying the dark, reddish brown fissile siltstone called the Moqui. The surrounding pale/tan blocks are dominantly Kaibab (limestone/dolostone), though most shown here are loose boulders.

Fold hinge in Moenkopi Fm

MC ejecta morphology

Idealized Meteor Crater ejecta morphology. Erosion has exposed the Moenkopi fold hinge. Image by David A. Kring.

Originally horizontal beds were uplifted during impact and now dip away from the crater. In the center of the above image, the Moqui member – the surface unit at the time – takes a sharp turn and winds up parallel to the crater wall. This is an exposed portion of a fold hinge, where a flap of target material was overturned to create an inverted stratigraphic sequence (see diagram).

There’s so much more to see at the crater. Sand fields, tear faults, shocked quartz…and we’re still just getting started!