21 June 2008
After our trip to the mine tailings, we headed to the remains of an ancient 40 km impact crater. The crater is totally invisible, but the rocks tell the story plain as day. Our first stop was just outside the town of Gypsumville. We drove through swampy, bumpy back roads into the middle of nowhere and stopped next to an unassuming patch of rock and gravel.
At first it didn’t look like much, but then we noticed that the rocks were all mismatched. Almost a dozen different types of rock were all jumbled together. More importantly, they were mostly igneous rocks from deep down in the earth. We were standing on the remains of the crater’s central uplift: a mountain formed when the rock of the earth’s crust rebounded from the huge impact.
After picking up some samples we drove into Gypsumville itself and stopped at a gravel pit near town. In the pit, the rocks looked almost like rust-colored cement: pebbles and rocks of all types, fused together into semi-coherent layers. Rocks like this are called breccias and are common in large impacts. The ones we looked at were formed as near-molten debris collapsed back down the wall of the newly formed crater.
Finally we drove to the Gypsum mine. The ancient crater acted as a basin to catch water, which then evaporated, leaving huge gypsum (calcium sulfate) deposits. Here is a chunk of gypsum crystals that I just picked up off the ground:
The day after Gypsumville we visited a patch of dead, orange soil in the middle of the forest that was at first glance similar to the mine tailings site. But this time it was natural. We were at a salt spring, where water containing salt and iron oozes from the ground, killing the plantlife, staining the rocks red, and providing an abode for mats of bacteria and algae.
Other than being all-around cool, this site is also interesting because the iron deposited by the springs almost totally masks the spectrum of the underlying carbonate rocks. One of the big mysteries in Mars science is how the climate changed, and if Mars once had a CO2 atmosphere, you might expect to find carbonates. However, very few carbonates have been found on Mars. The salt springs showed that just a little bit of iron is enough to hide their presence.
After the salt springs, our final stop was a nearby limestone quarry. In the quarry, we could see “fossilized” salt springs in cross section.
Portions of the wall showed tunnels in the limestone stained with iron, just like the salt springs, and the minerals nearby were specially interesting: clays and silica, two minerals indicative of water and found on Mars by the rovers and orbiters.
Here Melissa is gathering some silica sinters for her research. The greenish-gray stuff is clay, and the rounded, reddish tube at the edge of the rocks, going into the water, is a well-preserved spring “chimney”.
And so, we come to the end of the Great Canadian Adventure. Among other things, I learned that Manitoba is very flat, has a very healthy insect and arachnid population, and is huge. We clocked more than 2000 kilometers driving all over the place on this trip.
You would think that I would have had enough driving and time spent donating blood to the local insect population, but you would be wrong. Yesterday I drove from New York to Michigan, and today I will be leaving for Michigan’s upper peninsula with my family. I will be entirely cut off from the internet, so this will be my last post for a week or so. Keep checking back, though. Briony and Melissa may have some blog posts up their sleeves.