One of my coworkers (a geologist that I do a lot of work for), who shall for the purposes of this blog be called “Tim,” recently went home to Michigan to visit his family. He brought back a very cool little sand sample that he’d scooped up off the beach at Lake Huron. On the site, the sand looked like very unremarkable brown sand, the kind you’d get off of any beach. Under the microscope, though, it had some real character!
When you look at sand through the lens of its characteristics as sediment, the three factors you’re most interested in are:
1) Sorting: If sand is “well sorted,” it means that most of the grains are about the same sign. The sorting of sediment has implications on how the sand was transported. For example, poorly sorted sediment might have come from something like a landslide, where everything got jumbled together, or from glacial till, which just gets pushed around indiscriminately by the glacier. Well sorted sediment indicates that there was probably longer transportation, and normally by wind or flowing water.
2) Size: Is it big or is it small? A lot (but definitely not all) of sediment begins its life as a larger rock, so this can also be an indication of length of transport, or mechanism that created the sediment.
3) Rounding: Is it round or is it angular? The more round a grain of sediment is, the more punishment its taken over the course of its life, which smooths out the rough edges. (A possible metaphor, here?)
The sand that Tim brought back from Lake Huron was pretty fine in size, very well sorted, and very well rounded. The sorting comes from the fact that it was put on a beach by the lake, and goes hand in hand with the particular grain size for that area of the beach. What the rounding means is that these sand grains have been worked a lot, no doubt by the lake, but possibly by other means.
The normal, average sand that those of us in the continental US are used to seeing is primarily made out of quartz. This is because quartz is very tough and very common as minerals go, so there’s lots of it, and it can take a lot of punishment without breaking down. What tells us the most interesting things about sand and where it came from are the other minerals that you can find in it.
In this case, there were very well rounded grains of deep red garnet, and also quite a bit of magnetite. This is some very cool stuff, and not what we see around in Colorado. Garnet comes pretty exclusively from metamorphic rocks, and magnetite is found in both igneous and metamorphic rocks. So the source of this sand was most likely metamorphic basement rock that’s been crunched up and worn down into sand.
Knowing what we do about the geologic history of the great lakes – they formed from the ice sheets melting at the end of the last ice age – this sand may very well have started its life as glacial till, dropped into the bottom of the newly formed lakes during the melting.
There was actually a very surprising amount of magnetite in the sample. Tim separated it out by dragging a magnet through the sand, and it made an impressive black fuzz. This is not necessarily how much magnetite you’d get if you dredged a similar sample from the bottom of the lake. Apparently streaks of black sand are common on the lake shorelines, because once the sand has had a chance to dry, the wind blows the lighter quartz and other mineral grains farther up the beach, leaving the heavy magnetite behind. So it’s very possible that the surprising amount of magnetite Tim brought home was due to this sorting action.