An Un-well
September 7, 2022 1:40 AM Subscribe
If you dig a hole at the bottom of the sea, will the soil at the bottom be dry or wet?
So, obviously the sediment and whatever would be all mixed with water, but if you get down below that, and somehow managed to keep the seawater from coming in, would there be a point where the soil (?) would be dry again?
I have been googling about aquifers and bedrock, but I don't understand how the earth's crust works. I have found this question, which kind of gets into it, but I still don't really understand. Does the water table extend all the way down to the bottom of the earth's crust? Do we know?
So, obviously the sediment and whatever would be all mixed with water, but if you get down below that, and somehow managed to keep the seawater from coming in, would there be a point where the soil (?) would be dry again?
I have been googling about aquifers and bedrock, but I don't understand how the earth's crust works. I have found this question, which kind of gets into it, but I still don't really understand. Does the water table extend all the way down to the bottom of the earth's crust? Do we know?
So, I'm not a expert and could definitely be wrong, but my understanding is that at some depth under the ground (whether or not that ground is under a ocean), there is a layer of impermeable rock which prevents water from seeping through. What depth that kind of rock is found at depends highly on local geology, so I don't think there's a general answer for "under the ocean"
posted by wesleyac at 5:02 AM on September 7, 2022
posted by wesleyac at 5:02 AM on September 7, 2022
The mantle is hot enough to melt rock, so the deeper into the crust you go, the hotter it gets. Even if some particular part of the seabed is structured in such a way that liquid water never stops seeping in from above, there will always be some depth below which water is past its critical point and can't reasonably be thought of as wet any more.
posted by flabdablet at 6:14 AM on September 7, 2022
posted by flabdablet at 6:14 AM on September 7, 2022
Best answer: Here's the surprising answer: you can find water not only deep within the Earth's crust, but even hundreds of kilometres into the mantle itself.
Geologists have found that there is probably as much water beneath the surface of the crust as in the oceans. There's more detail and a very helpful illustration in this article at Quanta Magazine.
Here are a couple of published papers if you want to really dig in:
https://academic.oup.com/nsr/article/7/1/224/5514012
https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-020-00379-3
If you were to drill down into the crust from the bottom of the ocean, you'd find the level of moisture would decrease as you got into denser rocks of the crust. But then you'd find quite a bit of water again in a thick layer of the mantle, even though it's extremely hot. So wet, dry, wet(ish).
On geological timescales, this water even mixes and recirculates with the water in the oceans . If you scooped all of the oceans into space tomorrow, over millions of years water would trickle back into the oceans from reservoirs deep below the Earth's crust.
It's quite unexpected and very neat!
posted by mcbaya at 6:35 AM on September 7, 2022 [20 favorites]
Geologists have found that there is probably as much water beneath the surface of the crust as in the oceans. There's more detail and a very helpful illustration in this article at Quanta Magazine.
Here are a couple of published papers if you want to really dig in:
https://academic.oup.com/nsr/article/7/1/224/5514012
https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-020-00379-3
If you were to drill down into the crust from the bottom of the ocean, you'd find the level of moisture would decrease as you got into denser rocks of the crust. But then you'd find quite a bit of water again in a thick layer of the mantle, even though it's extremely hot. So wet, dry, wet(ish).
On geological timescales, this water even mixes and recirculates with the water in the oceans . If you scooped all of the oceans into space tomorrow, over millions of years water would trickle back into the oceans from reservoirs deep below the Earth's crust.
It's quite unexpected and very neat!
posted by mcbaya at 6:35 AM on September 7, 2022 [20 favorites]
Best answer: It entirely depends on location and the nature of the sediments.
With increasing depth the pressure increases and the grains that form the sediment are forced closer together - the pore spaces that the water would fill become smaller and the water is squeezed out.
If the sediment is made of grains that, due to their shape and size, cannot be forced closer, it remains wet as long as the permeability is greater than zero.
Eventually, though, you'll hit a rock horizon that has no permeability - a massive limestone or the igneous basement for example. That will be dry.
Of course beneath that impermeable layer there can easily be fossilized sediments that contain water from before the impermeable layer was formed. They will be under very high pressure and will be very hot. Digging into them would ruin your day.
The depths at which an impermeable layer is reached are entirely dependent on where you dig. Dig on the shallow side of a continental shelf and it could be pretty shallow. Dig on the abyssal plain of a chunk of oceanic crust that been locked off from tectonic movement for a few million years and it could be very deep.
The mantlar water is really potential water. Those minerals aren't "wet", but if the pressure on them were to be released they would break down and produce water as a by-product.
posted by thatwhichfalls at 7:20 AM on September 7, 2022 [4 favorites]
With increasing depth the pressure increases and the grains that form the sediment are forced closer together - the pore spaces that the water would fill become smaller and the water is squeezed out.
If the sediment is made of grains that, due to their shape and size, cannot be forced closer, it remains wet as long as the permeability is greater than zero.
Eventually, though, you'll hit a rock horizon that has no permeability - a massive limestone or the igneous basement for example. That will be dry.
Of course beneath that impermeable layer there can easily be fossilized sediments that contain water from before the impermeable layer was formed. They will be under very high pressure and will be very hot. Digging into them would ruin your day.
The depths at which an impermeable layer is reached are entirely dependent on where you dig. Dig on the shallow side of a continental shelf and it could be pretty shallow. Dig on the abyssal plain of a chunk of oceanic crust that been locked off from tectonic movement for a few million years and it could be very deep.
The mantlar water is really potential water. Those minerals aren't "wet", but if the pressure on them were to be released they would break down and produce water as a by-product.
posted by thatwhichfalls at 7:20 AM on September 7, 2022 [4 favorites]
It's quite unexpected and very neat!
What a peculiar way of saying "an endless source of terrifying nightmares that makes regular thalassophobia look like a disinclination for the deep end of the public baths"
posted by some little punk in a rocket at 5:54 AM on September 8, 2022 [1 favorite]
What a peculiar way of saying "an endless source of terrifying nightmares that makes regular thalassophobia look like a disinclination for the deep end of the public baths"
posted by some little punk in a rocket at 5:54 AM on September 8, 2022 [1 favorite]
« Older Language Learning Theory: Recognition of Words and... | Tabletop games for ages 7 through 70 Newer »
This thread is closed to new comments.
posted by Ursula Hitler at 3:04 AM on September 7, 2022 [1 favorite]