Deposition of Sediment
Okay, so now you know how particles fall through water. Let’s take a look at how stuff comes out of water: specifically, how it washes up on river beds and shores.
Why Should I Care About Falling Dirt?
If deposition of sediment--roughly translated from science-speak as “Falling Dirt”—isn’t really a title that jibes your interest, imagine this scenario: A murderer drops all of his evidence into a big lake, thinking the evidence will never be found and he will totally get away with it. One day, some of the evidence starts to wash up on shore. You, an ace forensics expert, are put on the case. Because you learned about particle sorting, you can predict exactly where each type of evidence will wash up on shore, track it all down, and find the killer. Good job, particle sorting!
Here’s another scenario: You, a geologist, find a type of rock washing up on shore that doesn’t normally wash up in that area. You know that it shouldn’t be there based on where that rock type is normally found and your knowledge of how rocks like this should sort. You investigate, track back the source of this rock, and find a giant landslide just beginning to crumble! You’re able to get the right resources to this area to prevent the damage from getting much worse and potentially hurting a lot of people.
Convinced yet? How about a third scenario. You, a geologist, are searching for a rare mineral resource that has important implications in the future of technology (or medicine, or whichever field you think is pretty important). Based on what you know about particle sorting, you’re able to locate trace amounts of this mineral in streams and washed up on shores, eventually leading you back to the main source of all that mineral, just like the gold miners did during the Gold Rush. The mineral then goes on to change the world, and you’re a superhero!
So, falling dirt can be pretty cool.
Here’s another scenario: You, a geologist, find a type of rock washing up on shore that doesn’t normally wash up in that area. You know that it shouldn’t be there based on where that rock type is normally found and your knowledge of how rocks like this should sort. You investigate, track back the source of this rock, and find a giant landslide just beginning to crumble! You’re able to get the right resources to this area to prevent the damage from getting much worse and potentially hurting a lot of people.
Convinced yet? How about a third scenario. You, a geologist, are searching for a rare mineral resource that has important implications in the future of technology (or medicine, or whichever field you think is pretty important). Based on what you know about particle sorting, you’re able to locate trace amounts of this mineral in streams and washed up on shores, eventually leading you back to the main source of all that mineral, just like the gold miners did during the Gold Rush. The mineral then goes on to change the world, and you’re a superhero!
So, falling dirt can be pretty cool.
How Does Dirt Fall?
We learned in the last sublesson how particles fall when they’re falling straight down in mostly still water. In that case, dense particles sink quickly, because the force of gravity is better able to “outweigh” the buoyant force pushing them up, and small particles sink quickly, because they have less buoyant force pushing them up.
Now we’ll be talking about how particles come out of a moving stream of water, like when waves wash up on shore. The rules here are just a little bit different:
So, dense particles will sink more quickly. This makes sense based on what we know about density, and it’s the same as the rule for particles falling straight down. But now large particles sort out more quickly! What gives?
The reason for this difference is that it’s not quite the same process. Stuff falling through a small amount of moving water—like a wave crashing on the shore—isn’t the same as small stuff “falling through the cracks,” like we saw with our ball pit example, and it’s also not the same as gold panning, because buoyant force isn’t so important here. Buoyant force doesn’t matter much now because we’re only talking about a very thin layer of water. Again, like a wave crashing on the shore:
Now we’ll be talking about how particles come out of a moving stream of water, like when waves wash up on shore. The rules here are just a little bit different:
- Dense particles settle quickly.
- Large particles settle quickly.
So, dense particles will sink more quickly. This makes sense based on what we know about density, and it’s the same as the rule for particles falling straight down. But now large particles sort out more quickly! What gives?
The reason for this difference is that it’s not quite the same process. Stuff falling through a small amount of moving water—like a wave crashing on the shore—isn’t the same as small stuff “falling through the cracks,” like we saw with our ball pit example, and it’s also not the same as gold panning, because buoyant force isn’t so important here. Buoyant force doesn’t matter much now because we’re only talking about a very thin layer of water. Again, like a wave crashing on the shore:
The particles aren’t sinking like they were in gold panning, they’re being carried along by moving water. They will be dropped off when water starts moving slow enough that it can’t hold onto the particle anymore.
This has to do with kinetic energy, which, as you learned in Chemistry, is just a fancy way of saying “the energy that something has when it is moving.” The faster something is moving, the more kinetic energy it has. The faster water is moving, the more kinetic energy it has. The more kinetic energy something has, the better it can hold onto big, dense, heavy stuff. That’s right, I said heavy. And I don’t mean dense. I mean massive. Something that weighs a lot. Kinetic energy is all about mass and velocity (speed). If the speed of the water is not high enough to pull away the heavy stuff from the shore, that heavy stuff gets deposited. So, heavy stuff gets deposited farthest inland, where the water is fasted, and small stuff gets deposited farther out to sea, where water is slowest.
This video will recap what I just said:
This video will recap what I just said:
And that is everything you need to know about particle sorting! If you can keep straight what happens under what conditions, and even better why it happens, then you’re gold! (And therefore you will sink rapidly….)
Summary
You should understand:
- How and why the deposition of sediments at riverbeds and on shorelines is different from particle sorting in still water: deposition on shorelines has to do with what water is able to carry away based on its kinetic energy, and particle sorting in still water has to do with buoyant force.
- That the largest, densest particles will be found furthest upshore and the smallest, least dense particles will be carried further out to sea.
Learning Activity
Content contributors: Alan Li, Emma Moulton