Friday, July 3, 2015

The Ant Hill Advantage


So for the last two weeks both Janine and I have been off on separate field adventures. This is not uncommon for geologists in the summer. School ends and we dash off for locations with rocks we cannot get at home. I plan on doing a longer post on the cool things I observed about the insides of old eroded volcanoes soon, but not just yet as the travels are not done!

Ant hill made up of volcanic minerals from the Hopi Buttes Volcanic Field, Navajo Nation, AZ, USA.
I did want to share a fun factoid about how ants help geologists when searching for specific rock types by using some examples from my recent field work. Whether or not you had an ant farm as a kid, its pretty easy to imagine the process involved in building an ant colony. The ants excavate tunnels underground and move the material from their tunnels to the surface. This material, moved grain by grain, adds up and can form a hill. As the ant colony gets larger, they bring up material from deeper underground. This helps geologists because it brings minerals to the surface that may be just out of sight. Because we live on a planet with weather (yay atmosphere!) and liquid water, the surface is constantly changing. The ants kindly bring up minerals and rocks that were buried by soil, wind blown dust, or plants and let geologists know where to look.

For example, in the Hopi Buttes Volcanic Field in the Navajo Nation, AZ, USA the deposits contain a lot of pyroxene. The old volcano insides (dikes, diatremes, etc.) stick up as obvious buttes, but the edges of these rocks are worn down and this makes it hard to figure out just how big the volcanic plumbing system was.
Buttes of black volcanic rock surrounded by red sand and siltstones of the Chinle Formation in the Hopi Buttes Volcanic Field.

To figure out where the contact between volcanic deposits and the old host rock is (in this case, sandstones and siltstones) we could use the ant hills to get a better idea of what rock was below us even in flat areas. If the hills were shiny and black, they were digging in the volcanic rocks! If they were red or pink, those were host rocks. We still dig holes to check, but they give us a head start.
Concentration of pyroxene crystals and small pieces of lava in an ant hill.

Another example was from the Dotsero Maar in Colorado, USA. We were tracking the ash deposits from the eruption (possibly as young as 5,000 years ago, making it the youngest eruption in Colorado). Ash quickly turns to soil and gets covered in plants and other debris. To check the location and thickness of the ash we needed to dig lots of holes. Kindly, ants brought up lapilli (pebble sized pieces of scoria) in areas where there were ash deposits and helped us to know where to dig. As digging in 100 F heat is hard work, we appreciated any advantage we could get.

Digging for ash deposits. We could use the ant hills full of lapilli to know we were in a good spot.

There is actually a long history of geologists using ant hills to find minerals when prospecting. And even one case where other prospectors were known to plant minerals to trick their competitors into following false trails! If you want to read a neat book about prospecting, with proper credit to ants, I suggest Barren Lands which is a good way to learn about kimberlites and diamond mining.

Next week I'm off to Italy, so that will mean even more photos and stories to share later this summer.

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