Warren Dunes, briefly

In July my friend visited Notre Dame to run in the Sunburst (Half-)Marathon, and while he was around we took a trip to Warren Dunes State Park. Dunes occur over a broad area in the eastern shore of Lake Michigan (Sleeping Bear, Silver Lake, Cowles Bog, etc.), with Warren Dunes being one of the largest and most popular southernmost parks.

Peak ridge of a sand dune

This was my second trip to a sand dune field (my first was nearby Cowles Bog), and I’m consistently surprised by the variety of environments in such a small area. Some near-shore areas are almost all sand:

People scrambling up, also note group of three on horizon at right

But a few hundred meters down the shore, heavily vegetated sand dunes appear to be securely anchored in place:

Of course, looks can be deceiving and a closer inspection revealed exposed roots in some areas – a result of dune migration, or just short-term erosion?

Other than a few birds, trace fossils, and a heck of a lot of mosquitoes in the boggy inland area, there wasn’t much visible life on the dunes. That is, until we saw this guy (he was hard to miss)

The mosquitoes were so bad that we took the first opportunity we came across to get out of the bog and back into dune territory. Unfortunately, it turned out to be a steep face of a sand dune. We had to take a few breaks on the way to the top…

Here I used photo-documentation as an excuse to pause.

It was so steep that in some areas, even though he was so far away his footsteps would cause sand near me to collapse. My best estimate from maps of the area are that it was ~70 ft to the top, with this photo being taken ~halfway up. We did a quick 4 mile hike around the park to see some highlights, but I’m looking forward to a return trip next summer to learn a bit more about the area and spend some time on the beach.

Tracing.

What did you do today? Yesterday? The past three years?

Tracing crystals. It’s one of the boulders I’ve been pushing uphill since the beginning of my graduate research. Why? What is it all for? Three little words: crystal size distributions. CSDs are a method to quantify rock textures from thin sections. Let’s look at one of my samples, Apollo 17 basalt 75015,52:

You can readily describe the overall texture of a thin section, and estimate phase proportions and morphology, etc. The coarse-grained nature and large crystals are obviously the result of slow cooling. Ilmenite (FeTiO3) makes up the majority of the opaque phase and varies in morphology from generally euhedral, lath-like crystals to skeletal and almost amoeboidal in some areas. CSDs are a way to (attempt to) address questions such as:
– Whether these phases all one population of crystals (i.e. a single crystallization sequence with larger crystals being the earliest formed) or a mixed population (perhaps large skeletal ilmenite crystals formed prior to eruption or elsewhere during flow and were later mixed with smaller euhedral ilmenite crystals) or were they affected by some other process;
– How fast the sample cooled, and whether it was constant or variable
– Whether this basalt is related to other basalts from the same area.

Crystal size distributions are an objective method of assessing crystal size, shape, abundance, and distribution. It’s basically a three step process. Step 1 is to make a mosaic of the thin section (as above). Step 2 involves tracing individual crystals (we use a tablet), which takes anywhere from a couple of days to a couple of weeks. I trace different phases on separate layers in Photoshop, with the final result looking like this:

Gray is the sample area, blue is pyroxene, yellow is plagioclase, and black is ilmenite.  For this sample I traced ~1500 individual crystals.

The rest is easy. We have a slew of programs to measure length, width, area, and position of the crystals (NIH ImageJ), estimate crystal habit short/intermediate/long axis dimensions, and population density (CSDCorrections).

See the website of M.D. Higgins for more information on quantitative analysis by CSDs.