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Comment by cmsmith | Hacker News Reader

>It turned out that all of those eruptions, from the Pacific, Atlantic and Arctic Oceans, took place from January to June. The cause, she proposed, is Earth’s slightly elliptical orbit around the sun. That changes the strength of the sun’s gravitational pull on Earth during the year and, as a result, the magnitude of the tides that squeeze the planet.

I felt fairly informed amount about underwater geology, but the idea that the crust down there is so delicate that solar tides can cause eruptions was new to me.

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The way this works is that the melting 'point' for basalt is actually a sloping line in pressure-temperature space[0]. At a constant temperature, the solid-liquid phase transition[1] is such that above a certain pressure, basalt is solid and below that pressure, basalt is liquid.

At the mid-ocean ridges, some parts of the very shallow crust are ever so slightly in the solid/high pressure side of this line. With a decrease in pressure due to sea level change, the phase transition is crossed and the rock melts. This is called 'decompression melting'.

Compare this with 'decompression freezing' of water: when you open a really cold beer bottle, ice often forms in the beer. The phase transition for H2O is such that liquid is stable at higher pressures than ice (I think this relates to ice being lower density but I don't know). Opening the beer lowers the pressure, causing some water to freeze.

This same process and some similar ones is associated with a lot of volcanic eruptive phenomena. For example, landslides on volcanoes often cause explosive eruptions because the decrease in pressure causes exsolution of gases from the magma, leading to an explosive reaction. This has been hypothesized to be a big part of the St. Helens eruption, as a small eruption lead to a collapse of part of the volcano, decompressing the deeper magma chamber and leading to a much larger eruption.

On a larger timescale, it's been shown that volcanism on Iceland was suppressed during the last ice age and had a post-ice-age resurgence.

[0]: Often called the 'Clapeyron slope'.

[1]: In reality there is range of these lines bounded by the 'solidus' and 'liquidus', with a crystal mush solution akin to ice water in between.

>"Compare this with 'decompression freezing' of water: when you open a really cold beer bottle, ice often forms in the beer. The phase transition for H2O is such that liquid is stable at higher pressures than ice (I think this relates to ice being lower density but I don't know). Opening the beer lowers the pressure, causing some water to freeze.."

This is fascinating! This past New Years, I had two beer bottles in a row freeze into beer slushies when I opened them. We had left them in the freezer probably longer than we should have. I really appreciate the explanation. I tried to explain the weird situation to a colleague the next week but he didn't believe me. Thank you.

Also, with the pressure off, some of the CO2 in the beer escapes as a gas and, thus, goes from higher pressure to lower pressure, and that makes the CO2 colder and, thus, should contribute to the beer freezing.

I would guess that the word Delicate isn't really the case, more "fine tuned". They say the volcanos are about a mile deep, so based on this calculator - http://www.calctool.org/CALC/other/games/depth_press , thats about 2360PSI. I am not a physicist, but it seems that if that pressure were to decrease because of a greater pull on the water from the Sun, then it would be less able to "contain" the magma underneath. unfortunately, I don't know what the actual deviation in pressure down there might be with the fluctuations of the earth's tides.

This also reminds me of this - http://blogs.discovermagazine.com/d-brief/2014/10/01/antarct...

If the icecaps melt, and the water migrates into the oceans, then the pressure on the seafloor increases by some amount, which might further decrease seafloor volcanism?

The pressure at the bottom of the ocean is just equal to the weight of water above you. So every 1ft of tides produces about half a psi of pressure change (or 1 meter and 1000 Pa). My guess would be that the water doesn't matter so much, but what they're talking about is tidal bulges in the earth's crust and mantle.

Yeah, that is bizarre.

How much do these volcanoes contribute to ocean acidification and over what time scales?

If the cycle is that the melting glaciers cause the volcanoes to stop which causes an ice age, then human global warming melting should cause the volcanoes to cool down even more. So there's a chance the planet will rescue us from ourselves.

Oh thank god. I was worried I'd have to change my consumption habits.

There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.

So the sea level changes involved with the last ice age were around 400 feet, and we'd probably need a similar change in sea level to see a similar impact on undersea volcanoes right now.

The smaller hiccup with that rescue plan is that we don't really want to see sea levels go up 100+ feet; even if that "fixes" global warming, that was one of the consequences of global warming we were trying to avoid.

The larger hiccup is that the undersea volcanoes may not be contributing enough CO2 that even if they stopped completely it may not have a significant impact on our trajectory, and even if they were a large enough contributor, we may not have enough ice left to raise the sea levels enough to significantly affect their output.

Won't tides effect all parts of the earth not just the crust?

Of course, but it's the crust that has seismic events that can be triggered from subtle changes.

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