Geo 365: March 20, Day 79: The South Side

Looking south into Hole-in-the-Ground, I have always presumed that the sparse ponderosa pine forest was due to the effects of more shade, thus slower evapo-transpiration, on that north-facing slope. But while writing up Fort Rock a few days ago, I realized that the south-facing amphitheater there indicated prevailing winds from the south. I had known that for decades, but never made the connection to Hole-in-the-Ground. Another contributing factor to the ability of pines to colonize in that area may be that snow drifts deeper there. That is, it may not simply be an area of slower moisture loss, but it could be that it receives more moisture on average than the rest of the crater.

A comment I received yesterday links to an older post, shortly after we returned from this trip, by Cujo 359, AKA Intrepid Companion. I didn't think to get a series of photos suitable for stitching into a panorama, but he got a really nice one, showing the full extent of this feature in one image. In addition, he wrote up a really good compare-and-contrast between the above and Meteor (Barringer) Crater in Arizona. As he points out, the two are of similar size, share a surprisingly similar form, are of similar ages, but have very different origins.

Below is a cross-section of Hole-in-the-Ground, taken from USGS Circular 838, Road Log 6, Mile 25.1.

This is based on two drill holes, indicated on the left interior of the crater by I and II, in addition to a geophysical survey. Here, phreatic explosions blew out pre-existing bedrock, and a portion of the new magma, then the walls slumped in to create the pit. At Meteor Crater, there is no underling magmatic rock, and the layers of country rock blasted out by the impact are folded over- literally upside down- like blankets in a neatly made bed. Another thing I found interesting is that the layer of faulted basalt illustrated yesterday is marked as "Paulina Basalt Flow No. 2," indicating it came from Newberry Volcano, something like 40 miles away. I had no idea flows from there had made it that far in this direction.

Photo unmodified. August 20, 2011. FlashEarth Location.