Groundbreaking discovery finally proves rain can move mountains | Hacker News Reader

Groundbreaking discovery finally proves rain can move mountains

phys.org

103 pointslelf19 comments

Study of Himalayan erosion.

"When a cosmic particle from outer space reaches Earth, it is likely to hit sand grains on hillslopes as they are transported toward rivers. When this happens, some atoms within each grain of sand can transform into a rare element. By counting how many atoms of this element are present in a bag of sand, we can calculate how long the sand has been there, and therefore how quickly the landscape has been eroding," Dr. Adams said.

He's referring to beryllium-10, which is mostly generated by cosmic ray interactions with nitrogen and oxygen in the atmosphere. The Be-10 subsequently washes out of the atmosphere with precipitation and tends to bind in surface soils. It has a half life of 1.39 million years so it is useful for dating various geological phenomena.

https://en.wikipedia.org/wiki/Beryllium-10

You can see how this beryllium isotope was used in the original paper "Climate controls on erosion in tectonically active landscapes" (linked by sradman in comment https://news.ycombinator.com/item?id=24812181) within the Materials and Methods section of the paper.

It is beautiful how the study of the very small (cosmic ray interacting with nitrogen and oxygen) can add so much to the study of the very large (monsoons and the Bhutan Himalaya).

"...scientists have also believed rain can erode a landscape quickly enough to essentially 'suck' the rocks out of the Earth, effectively pulling mountains up very quickly."

I do not understand this, unless the point is that rainfall, by eroding valleys, reduces the weight of a mountain range, allowing tectonic forces to raise the ridges and peaks faster than otherwise.

I'm confused as well. Another possibility is the "up" is relative? If there is a mountain already buried in looser soils, rain could wash away and expose the mountain, which will appear to be going up, because everything else is going down.

Crust floats on the mantle. Mountains are areas where the crust is thick and it goes down into the mantle further than less thick areas. As the weight from the top is removed, that can push up areas from down below.

We tend to think of mountains as stable things with the constant stress underneath, rather than a layer of congealed fat on top of a warm stove (or, a lava lamp). But the latter is a better model for geologic behaviour over long periods of time.

This is correct. The earth is ductile at long time periods, and mountains maintain isostatic equilibrium as the tops are eroded away. The article quote is absurd. Source: my earth science PhD.

I love your username. I spend a lot of time with Census Tigerfiles. GEOID is life!

Thanks. There's another meaning of geoid, which is the shape of mean sea level of the globe. The height of sea level varies according to the local strength of gravity. When they first put up satellites that could accurately map sea level, it gave us the amazing maps of the sea floor. Basically, the sea surface subtly mimics the features of the sea floor.

I think I recall being thaught that water followed the gravity pull which was greater around the equator than the poles which would explain why with the poles melting, the equatorial regions would be the most affected by the subsequent seas rise. Is this wrong? Where can I see a good sea map? Thanks!

Guess I just went and looked at a Geoid map and I was lied to, EU is in big trouble!

Maps are here:

https://topex.ucsd.edu/grav_outreach/

Yes, water does follow the gravity. I'm not a climate person, and it's a complicated system, but as I understand it, new melt would be distributed evenly by mass around the globe, which means a larger volume of water at the equator due to higher temperatures there. My hunch in that changes in wind patterns and currents will be the more important effect though.

According to an article I read recently, the reason that equatorial reasons will see the greatest rise in sea level is that ice currently exerts a significant gravitational pull. As ice melts, that gravitational pull is reduced, causing sea levels to fall in the vicinity of the melting ice and to rise far away from the source of the melting.

Yes, you're right, it is the isostatic unloading that they're referring to - but in regards to the 'sucking', they seem to mean that the semi-localised unloaded of the top of crust (via rain, river incision) creates a zone of lower lithostatic pressure, and so that area ends up getting pushed up to maintain isostatic equilibrium. Not truly a 'sucking', but as an analogy, I think it's OK. It's more easily seen as part of the critical wedge angle for fold and thrust belts.

Does anything truly 'suck'? When you drink a milkshake with a straw, the milkshake is pushed up your straw by the atmosphere..

Exactly, good point. It is convenient short-hand term, and in the case of climate forcing tectonic activity, it works well, I think.

Fair. 'sucking' does work to describe the unloading. I guess the idea that the _speed_ of the erosion creates suction, is what bothered me.

The paper Climate controls on erosion in tectonically active landscapes [1]:

> The ongoing debate about the nature of coupling between climate and tectonics in mountain ranges derives, in part, from an imperfect understanding of how topography, climate, erosion, and rock uplift are interrelated. Here, we demonstrate that erosion rate is nonlinearly related to fluvial relief with a proportionality set by mean annual rainfall.

[1] https://advances.sciencemag.org/content/6/42/eaaz3166

"Earthshattering" would be a better word

Groundbreaking works too :)

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