## Friday, October 3, 2008

### The Question

If water is the transport medium of sediment, the grain size of sedimentary deposits most closely indicates the _______.

I've narrowed down the possible choices to these two (the other two were obviously wrong):

1. Average velocity of the water from the time of erosion until deposition.

2. Velocity of the water at the moment the sediment settled to the bottom.

Either one makes sense to me, in light of what we talked about in class. The Hjulstrom diagram, which was sort of the basis for this lesson, has both erosion and deposition values, so my initial instinct was to go with the first answer. Also, sediment can reach the bottom and still move along it, so it seems that the velocity of the water at the moment the sediment settled to the bottom would only account for grains that were actually picked up and dropped to the bottom. Of course, that is only one way in which sediment moves due to current. Once dropped to the bottom, grains can continue to roll along the bottom and be transported.

Hmmm...I may have just figured this one out. Yay, blog!

NOTE: This is a take-home test in which I have been given full permission to consult books/other people. I'd prefer it if I wasn't given the answer...just some discussion that might help me come to it on my own. Thanks!

Jerry D. Harris said...

1. Average velocity of the water from the time of erosion until deposition.

And, as a result (in general terms), it tells you something about how far from the sediment source the deposit is. Given that water velocity is a function of the gradient down which it flows, and given that the farther from elevated areas, the flatter things typically get, deposits of various clast sizes are often (though not always!) a function of distance. Exceptions include various kinds of catastrophic events, but those would also be relatively clear in the deposit.

2. Velocity of the water at the moment the sediment settled to the bottom.

Well, the minimum velocity, at any rate -- flowing water can continue to move, but as it slows, it loses the competence to carry clasts of certain sizes (and densities and surface area/volume ratios). So sediment can get dumped when the water slows down and loses the energy needed to pick up and carry bigger clasts.

Amanda said...

That all makes plety of sense. So, it seems as though both answers are correct, in a way. But which is more closely indicated?

I had to hand in the test a little while ago and I went with the first answer.

This test was pretty frustrating because I understand the concepts, but the questions still seemed really difficult. I can't imagine what it was like for people struggling with the concepts.

For instance, we had a question that asked where, on a given map, we would likely find volcanoes and deep earthquakes. There was a subduction zone indicated. There were two points along the subduction zone (and a few others that obviously weren't correct). One was just a tiny bit further inland than the other. In class, we talked solely about the fact that subducting plates melt and cause volcanoes and that they also cause earthquakes as they are subducted. The book also covered that. But nowhere in lecture or the book was it indicated how far from the subduction zone those phenomena occur. This wouldn't be so bad (it definitely encourages some critical thinking) except the map didn't have a scale on it, so there was no way to know how far from the subduction zones the points were.

Very frustrating.