The 40,000-Mile Volcano | Hacker News Reader

The 40,000-Mile Volcano

nytimes.com

108 pointshvo34 comments

>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.

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.

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.

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

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.

> Though long enough to circle the moon more than six times

Why do journalists do this?

Also, why is everyone missing this?

Ocean levels fall sharply in such bitterly cold periods as water is tied up in massive continental ice sheets. In a paper, she proposed that the reduced pressure on the ridges might let them erupt far more frequently. As a result, more carbon dioxide would spew into the ocean and, eventually, into the atmosphere, trapping more heat and warming the planet.

In short, according to this hypothesis, the ice sheets would eventually grow large enough to initiate their own destruction, refilling the oceans. It was a radical idea that has stirred debate.

>> Though long enough to circle the moon more than six times

>Why do journalists do this?

Good question. You see stuff like this all the time. "That's three trillion gallons of water, or enough water to fill up five million Olympic-sized swimming pools." As if people can somehow visualize five million swimming pools any better than three trillion gallons. Drives me bonkers.

> Why do journalists do this?

I agree, this doesn't really help convey the size of the number. It assumes readers know the circumference of the Moon, when the size relative to Earth would be more useful.

The number 40,000 stood out to me because the Earth is roughly 40,000 kilometres in circumference. Of course, this is 40,000 miles, so it's roughly 1.6x the circumference of the Earth.

I don't really know what the circumference of the moon is. Or, at least I didn't. Combine the facts above with the ones in the article, and you discover the circumference of the moon is roughly 1/4 that of the Earth.

Nice, same thought process here.

> Also, why is everyone missing this?

Because people avoid this topic due to religious convictions about the way the world's climate works. We have poisonously politicized climate science to the point that reasonable scientific discourse can no longer take place.

One interesting aspect of the article.. for HN crowd.

>Now, scientists have inaugurated a major new effort. Off the West Coast, they have wired up a highly active ridge with hundreds of sensors and cameras, as well as cables that flash the readings to shore. The ocean observatory is to operate for at least a quarter century, replacing sporadic glimpses with continuous scrutiny. This month, the surge of data is hitting the Internet.

" With extreme care, humans and robots have measured temperatures as high as 780 degrees."

Degrees of what?? The article is read worldwide and it could be using literally any temperature scale. Anyone has any idea?

Lava typically liquefies at ~700°C, and 780°F is only 415°C, so I guess that is in Celsius.

OOI has a pretty neat data portal for those interested at: https://ooinet.oceanobservatories.org/

>“It’s a whole new perspective on how the Earth works,” said Daniel J. Fornari, a senior scientist at the Woods Hole Oceanographic Institution on Cape Cod, Mass.

It's really interesting to see a scientist say that we don't really know how Earth works. I thought the debate was over.

No geologist would ever (or likely will ever) say that we "really know how the Earth works". I say this as a geologist, and I can't imagine any other geologist disagreeing.

The natural sciences are incredibly complex. You're not dealing with nice simple interactions, and we can't run experiments on the time, spatial, or energy scales needed to directly test hypotheses in the way you would in an experimental science.

Instead, we observe.

We try to understand the present to understand the past and the past to understand the present.

When you're observing an uncontrolled system, it's very tough to isolate one variable from another. Geology is often accused of not being "quantitative enough". What people don't realize is that the quantitative part is usually stupidly easy. It's choosing the right physics to model that's hard. We usually don't fully understand exactly which physical process are at work.

Nicely put. This how I always think about economics: we've got plenty of numbers and models, but meaningfully large experiments are are to design without risking a lot of livelihoods. Science is brutal at scale, and I'm ever impressed with the techniques to make sense of the noise.

Next thing, your going to do is tell me that climate models are speculative. But you can't fool me -- I know that geologists are bought and paid for by the oil companies.

I read a recent article in Science, summarized here (http://science.sciencemag.org/content/349/6252/1032.short), on tomographic analysis of Earth's mantle using data recovered during large earthquakes. The large plumes that, for example, create volcanic islands like Hawaii turn out to be ~600km wide, and extend to ~3000 km below the surface.

It was not known previously how deep these plumes go. Many thought they were much smaller and shallower.

It's in some sense surprising that even basic facts like this are the current frontier.

Just in terms of geology, you could search HN links from the past year and find lots of areas of ongoing research and disagreement, for instance about the make-up and structure of the earth's core. And if you broaden the idea of "how Earth works" to include things like climate, then the number of areas of ongoing research becomes quite vast.

"Listen, I don't know why you do it, but if you keep ramming your head into the wall like that, you can hurt your brain quite badly."

"Hah, what do you know, scientist? You already admitted you don't know how the brain works!"

"Oh, you're damn right, what was I thinking? Please carry on."

I would say it is far more unusual to see a scientist saying that we do really know how the Earth (system) works. You may well encounter people saying that we have a good understanding of certain processes, for example plate tectonics, but that is not the same thing.

Absolutely amazing!!!

Way cool indeed. And this is my first comment.....

> "Way cool indeed. And this is my first comment....."

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