Ryan AndersonMar 19, 2009

Planetary Surface Processes Field Trip: Day 5

Meteor Crater, Walnut Canyon, and Red Mountain

This article originally appeared on Ryan Anderson's "The Martian Chronicles" blog and is reposted here with permission.

(This is day 5 of a week-long planetary geology field trip to Arizona. Get caught up with days 12, 3, and 4.)

Meteor Crater
Meteor Crater Meteor Crater is the best preserved (and the first recognized) impact crater on Earth.Image: Ryan Anderson

Today was a long and awesome day. We started out at Meteor Crater, the youngest and best preserved impact crater on Earth! Our guide today was Shawn Wright, a colleague from the Hawaii field workshop, among other places. He showed us infrared images of the crater taken from an airplane and we walked around the rim trying to identify the main compositions detected. Meteor Crater is especially nice for this because it excavated into three distinct layers: the red Moenkopi siltstone (the surface of the surrounding plains), the yellowish Kaibab limestone (normally beneath the Moenkopi), and the white Coconino sandstone (below the Kaibab).

Back in the early 1900s, people were trying to dig and find the iron meteorite that they thought was buried under the crater (it turns out the meteorite was blasted into thousands of pieces upon impact). Luckily, the mining work carved a notch in the rim that lets you see the three units of the crater where they have been overturned by the impact. When a large impact occurs, it lifts up the ground and forms an “overturned flap” at the rim. You can see in the picture that the Moenkopi goes from relatively solid-looking to very fractured-looking, and is then overlain by blocks of Kaibab and Coconino.

Reversed stratigraphy
Reversed stratigraphy At the rim of the crater, the impact has reversed the sequence of layers. The red Moenkopi would normally be on top but here it is overlain by blocks of Kaibab limestone and Coconino sandstone that have been excavated by the impact.Image: Ryan Anderson
Shocked sandstone
Shocked sandstone The shocked sandstone still preserved very fine cross-bedded layers, but can be crumbled into a power with your hand.Image: Ryan Anderson
Crossbeds at Walnut Canyon
Crossbeds at Walnut Canyon Crossbeds at Walnut Canyon are essentially fossilized sand dunes from when Arizona was a coastal desert. The direction that the layers are tilted tells us that the prevailing winds blew from north to south, although the various sets of layers in this image actually reflect several wind directions.Image: Ryan Anderson
The interior of Red Mountain cinder cone
The interior of Red Mountain cinder cone The layers are from different stages of the eruption that deposited cinders with slightly different composition or weathering properties. The bizarre shapes are due entirely to erosion, mostly by water.Image: Ryan Anderson

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