Accretionary Wedge #37: Sexy geology and discrepant outcrops, and the answer to Tuesday's mystery photo

Photo by Mrs. Geotripper

This month's Accretionary Wedge (#37) is sponsored by Lockwood at Outside the Interzone, and involves "sexy geology". To be more precise, it is about "geology that makes your heart race, your pupils dilate. Rocks and exposures that make you feel woozy and warm. Structures and concepts that make your skin alternately sweaty and covered with goosebumps. Places you've visited, read about, or seen photos of that make you feel weak-kneed, and induce a pit in your stomach".

Geotripper checks out a really cool outcrop (photo by Mrs. Geotripper)

There are all kinds of such outcrops and choosing is hard, so I settled for the last outcrop I saw that brought on a visceral response. It was last Saturday, while I was checking out a possible field trip route in Southern California. The outcrop also happens to be the answer to Tuesday's mystery photo. Read on!

The Monterey Shale is usually white; why is it gray here?

The Monterey Shale (or Modelo Formation) is a deepwater deposit of shale, sand and diatomaceous earth that is exposed widely along the California Coast. It was deposited in Miocene basins adjacent to the San Andreas fault, and the rock is unusually rich in organic material. When buried deeply in the crust, long slow heating causes the organic material to simmer into petroleum and natural gas. As such, the Monterey has been the source rock for billions of barrels of California oil.

So what was the mystery rock? I was educated as a geologist, not a teacher, so it took a few years before I heard about the concept of discrepant events in education. A Discrepant Event is something that "surprises, startles, puzzles, or astonishes the observer. Often, a discrepant event is one that does not appear to follow basic "rules of nature" and the outcome of a discrepant event is unexpected or contrary to what one would have predicted". Well, Tuesday's mystery rock wasn't a discrepant "event", it was a discrepant outcrop. The result looks like something familiar, but is far from normal.

I suggested that readers follow their intuition, and that their answers would be "sort of half right". Most respondents suggested that they were looking at obsidian (above), and some kind of tuff or scoria (below). They were sort of half right. The problem is that these rocks are not volcanic at all. Brian, at my Google+ page maybe came the closest by guessing that these were fulgerites, fused rocks caused by a lightning strike.

Much of the Monterey Shale is composed of silica, and much of the silica is saturated with oil. Perhaps you might wonder what would happen if the rock ignited? It could have been a lightning strike. It could have been a forest fire. In any case, the rock burned underground, and reached temperatures sufficient to melt the shale into an obsidian-like rock. In places, gas bubbles produced the vesicular texture that resembles tuff or scoria. Oxidation of the rock produced a rainbow of red, purple, orange and yellow rock (the quarry across the road apparently is producing colorful decorative stone). Native Americans have made use of the glass for thousands of years for spear and arrow points.

So, is this rock igneous, sedimentary or metamorphic? Some sources refer to combustion metamorphism, others refer to fused shale. No one talks about magma. In any case, I found the rock fascinating. Even downright sexy.

More information on this cool outcrop can found here. It is located on Grimes Canyon Road just a few miles south of Fillmore, California.