2058: "Rock Wall"

[Reka]

Title text: I don't trust mantle/core geologists because I suspect that, if they ever get a chance to peel away the Earth's crust, they'll do it in a heartbeat.

There still isn't a post for today's comic? What is with you people?

[rmsgrey]

Pretty sure the frustration level is pretty high for big bang cosmology too - I guess there's less of the "but it's right there" factor, but, on the other hand, it's also a lot harder to test models or do experiments - at least a mantle geologist can study seismic events...

[qvxb]

"Mantle? Bah! Lava tubes!" - O. Lidenbrock and J. Verne.

[Ranbot]

The mantle geologist could move to Newfoundland, specifically, Gros Morne National Park where a piece of old mantle rock is exposed at the surface, called the Tablelands... https://www.internationaltraveller.com/ ... ros-morne/

(And an excellent vacation destination if you like hiking, geology, fjords, and beautiful natural sights in general)

[WriteBrainedJR]

The mantle geologist could move to Newfoundland, specifically, Gros Morne National Park where a piece of old mantle rock is exposed at the surface, called the Tablelands... https://www.internationaltraveller.com/ ... ros-morne/

(And an excellent vacation destination if you like hiking, geology, fjords, and beautiful natural sights in general)

I have so many questions about this. How does the Earth's mantle even become exposed, let alone end up so far above sea level and not, say, a hundred miles from Bumfuque, Middle of Nowhere, Abyssal Plane? How do scientists know that it's really the mantle?

Okay, so two. I have two questions.

[Ranbot]

The mantle geologist could move to Newfoundland, specifically, Gros Morne National Park where a piece of old mantle rock is exposed at the surface, called the Tablelands... https://www.internationaltraveller.com/ ... ros-morne/

(And an excellent vacation destination if you like hiking, geology, fjords, and beautiful natural sights in general)

I have so many questions about this. How does the Earth's mantle even become exposed, let alone end up so far above sea level and not, say, a hundred miles from Bumfuque, Middle of Nowhere, Abyssal Plane? How do scientists know that it's really the mantle?

Okay, so two. I have two questions.

Short answer: plate tectonics
For a longer answer try these links:
https://en.m.wikipedia.org/wiki/Gros_Mo ... ional_Park

https://www.acrosstheblueplanet.com/blo ... tablelands

[Sableagle]

It's not *all* 20 miles thick. Under the Andes and Himalaya it's way thicker than that, and there are a few places ...



... where it's not even 2 miles thick ...



... and some days it's not even 2 inches thick.

[svenman]

Uh, as far as I know volcanism usually involves lava consisting of molten rock that originates from within the earth's crust, not the mantle. But I'm not a geologist.

[Mjb]

Crust thickness is cited at 3-44 miles in general, though they found a 1-mile area a while back. No surprise then that the geologists have been trying to take advantage of the underwater thin spots for decades. The Chikyu was designed to drill into the mantle yet is still only "planning" to do so after over a decade in service. Maybe China will actually get around to it.

[leeharveyosmond]

Core/mantle geologists aren’t the problem. It’s the astronomers you need to watch. They’d abolish the atmosphere if they could.

[da Doctah]

Core/mantle geologists aren’t the problem. It’s the astronomers you need to watch. They’d abolish the atmosphere if they could.

They just need to get over it.

[Pfhorrest]

Yeah "going over it" is one of the things the mantle geologist in this comic explicitly complains she can't do, which astronomers can and frequently do do.

[Ranbot]

Uh, as far as I know volcanism usually involves lava consisting of molten rock that originates from within the earth's crust, not the mantle. But I'm not a geologist.

A geologist would agree with that. (And FWIW, I am a geologist) Lava is not mantle, not physically, not mineralogically, not chemically. Gros Morne National Park is UNESCO World Heritage place in part because the mantle rocks there are not found anywhere else on the surface of the planet.

[Old Bruce]

Uh, as far as I know volcanism usually involves lava consisting of molten rock that originates from within the earth's crust, not the mantle. But I'm not a geologist.

A geologist would agree with that. (And FWIW, I am a geologist) Lava is not mantle, not physically, not mineralogically, not chemically. Gros Morne National Park is UNESCO World Heritage place in part because the mantle rocks there are not found anywhere else on the surface of the planet.

But mostly because it is cool, eh?

[Whatev]

Geocultists, seeking to tear away the walls that protect our world from what lies beyond!

[svenman]

A geologist would agree with that. (And FWIW, I am a geologist) Lava is not mantle, not physically, not mineralogically, not chemically. Gros Morne National Park is UNESCO World Heritage place in part because the mantle rocks there are not found anywhere else on the surface of the planet.

Thanks for the confirmation. There must be different kinds of mantle rocks, though, right? Because I've been doing some googling in order to properly correct the German Wikipedia article on Gros Morne National Park (someone edited in a statement there eight years ago confusing crust and mantle, which had gone uncorrected all that time), and I found that there is at least one other place on Earth where (presumably different) mantle rocks are exposed at the surface, in Oman's Hajar Mountains.

[Ranbot]

A geologist would agree with that. (And FWIW, I am a geologist) Lava is not mantle, not physically, not mineralogically, not chemically. Gros Morne National Park is UNESCO World Heritage place in part because the mantle rocks there are not found anywhere else on the surface of the planet.

...There must be different kinds of mantle rocks, though, right?

I would assume so, certainly there is some change/stratification with depth. Clearly it's difficult to determine [or the comic wouldn't have been made ] Volcanology or mantle studies is not my of field expertise though, and it's been almost 20 years since my college classes in these things, so I'm not going to say anything about different rocks in the mantle, specifically.

Spoiler:

FWIW, the main way the mantle is studied is by seismic refraction of waves from natural earthquakes. Waves refract in materials of different densities according to Snell's Law including those passing through the planet. So we can infer where the mantle changes in density and assume the density change is a different type of rock/chemistry, but it's very hard to say for certain what that rock/chemistry actually is. That's when the mantle scientists turn to their Diamond anvils in the lab.

Generally though the closest sort of lava to actual mantle is believed to be the "ultramafic" type... See: http://www.geologyin.com/2014/12/how-to ... -into.html
But as very hot mantle-derived material moves through the earth's crust there is [typically] a lot of crust melting, mixing, fractional cooling, re-melting, re-mixing, repeat, repeat, repeat, before it reaches the surface as lava, if it even gets there. In a very overly-simplified way, you can think of the grades of igneous rocks (see link above) as degrees of "contamination" of crust materials into mantle material, but even ultramafic has some crustal "contamination," and would only represent mantle nearest to the crust.

Also related to the comic... removing the Earth's crust in the way the comic suggests would also remove the pressure that makes the mantle what it is. Materials under great pressures, like those deep inside the earth, do not behave chemically as they do at the earth's surface. If removing the crust was hypothetically possible, a researcher would really be examining the effect of removing pressure from mantle material, which would certainly be useful data, but not mantle in it's actual state. Kind of like studying a wild animal by shooting it.

...I found that there is at least one other place on Earth where (presumably different) mantle rocks are exposed at the surface, in Oman's Hajar Mountains.

Yes, sorry, I was being hasty and I overstated... I should have said Gros Morne is one of very few places in world mantle-derived rock is exposed at the surface.

(edited for typos)

[Sableagle]

Lava is not mantle, not physically, not mineralogically, not chemically.

Not even in Iceland? Those are (allegedly) photographs of the mid-Atlantic rift.

[Pfhorrest]

Pretty sure the stuff that comes out of oceanic rifts is crustal rock, albeit melted, since it goes on to form the crust.

IANAG but I'd expect that mantle rock is generally made of denser substances than crustal rock, which is why crustal rock floats on it.



A tangential thought this reminds me of that I thought of recently: are there surface waves of the atmosphere, like there are of the oceans? Like, if you could see air as opaque, and you were in space, would you see ripples and waves undulating across the surface of the planet like it was a water-world?

[ucim]

A tangential thought this reminds me of that I thought of recently: are there surface waves of the atmosphere, like there are of the oceans? Like, if you could see air as opaque, and you were in space, would you see ripples and waves undulating across the surface of the planet like it was a water-world?

Yes, there definitely are. They are called gravity waves (which water waves are also called), and while I don't have any videos handy, they are on the net. Look for time lapse videos of looking down on clouds from mountaintops. Yes, some of them are cloud formation and dissipation, but you will also see gravity waves (you need the clouds to make them visible).

Some of the videos are pretty awesome.

Jose

[WriteBrainedJR]

Pretty sure the stuff that comes out of oceanic rifts is crustal rock, albeit melted, since it goes on to form the crust.

Yep. If it was mantle, there would be a lot more than two places on Earth with exposed mantle rock. The ocean floor, and anyplace that used to be ocean floor, would be lousy with it.

[Ranbot]

Lava is not mantle, not physically, not mineralogically, not chemically.

Not even in Iceland? Those are (allegedly) photographs of the mid-Atlantic rift.

No, it's not mantle. Iceland is mainly basalt-type lava/rocks. Iceland is believed to be a the result of a mantle plume or "hot spot" (See also Hawaii) which adds a lot of heat to the crust, but by the time that hot material/magma makes it's way to the surface it undergoes so much fractional cooling/melting and mixing with other rocks that what comes out at the surface is a fundamentally different chemically than mantle. Fractional cooling and melting (or fractional crystallization) removes different minerals at different temperatures and pressures as the magma migrates up... think of it like a (edited for a better analogy) geologic sieve; separating minerals, leaving them in behind through miles of rock.

There some places on the ocean floor (where the crust is the thinnest) that have ultramafic lava flows, that's the closest to mantle we get at the earth's surface.

[ucim]

I couldn't find the particularly impressive display I remembered (of cloud gravity waves) but here are a few nice ones:

https://www.youtube.com/watch?v=drG58wDE65M

https://www.youtube.com/watch?v=YGg0SxRYAvA

https://www.youtube.com/watch?v=9S5gXnjzwsc

https://www.youtube.com/watch?v=99zyfD4-nWs

Here's a planetwide visualization:
https://www.youtube.com/watch?v=6SqMCIKV364

Something to keep in mind - the boundary between cloud and non-cloud is not necessarily a fixed boundary, as clouds form and dissipate too. Standing lenticular clouds for example "stand still", but the air is moving very quickly through them after being tossed about by a mountain range. When the air is rising, droplets condense as the air cools (as the pressure drops), and when the air is falling, the droplets evaporate. So, the lenticular clouds show where the air is doing what, but they themselves illustrate a standing wave in a medium that is hardly stationary. Gliders look for them (they show where to find lift) but small powered planes find them dangerous (due to strong up and downdrafts). Gliders are built differently from powered planes. Each to xis own.

Jose

[Wee Red Bird]

There still isn't a post for today's comic? What is with you people?

Sometimes the Friday one is so late, I've finished for the weekend before it comes out. On a Monday it can get confusing if the Monday strip is out first thing. Maybe not today, as it's afternoon, but sometimes.

[richP]

Time to update this comic? Now that NASA put a seismometer on Mars, the most frustrating field has to be Martian mantle geology. Still trapped on this side of a rock wall, but now you live many many miles away from the wall (so far away that nobody has yet visited the wall in person).