It's Interesting To Me How Much People Struggle To Intuit Differences Of Scale. Like, Years Of Geology

It's interesting to me how much people struggle to intuit differences of scale. Like, years of geology training thinking about very large subjects, and I'm only barely managing it around the edges.

The classic one is, of course, the mantle- everybody has this image of the mantle as a sort of molten magma lake that the Earth's crust is floating on. Which is a pedagogically useful thing! Because the intuitions about how liquids work- forming internal currents, hot sections rising, cool sections sinking, all that- are all dynamics native to the Earth's mantle. We mostly talk about the mantle in the context of those currents, and how they drive things like continental drift, and so we tend to have this metaphor in mind of the mantle as a big magma lake.

The catch, of course, is that the mantle is a solid, not magma. It's just that at very large scales, the distinction between solids and liquids is... squirrely.

When cornered on this, a geologist will tell you that the mantle is 'ductile'. But that's a lie of omission. Because it's not that the mantle is a metal like gold or iron, what we usually think of when we talk about ductility. You couldn't hammer mantle-matter in to horseshoes or nails on an anvil. It's just a rock, really. Peridotite. Chemically it's got a lot of metal atoms in it, which helps, but if you whack a chunk of it with a hammer you can expect about the same thing to happen as if you whacked a chunk of concrete. Really, it's just that any and every rock is made of tons and tons of microcrystal structures all bound together, and the boundaries between these microcrystals can shift under enormous pressure on very slow timescales; when the scope of your question gets big enough, those bonds become weak in a relative sense, and it becomes more useful to think of a rock as more like a pile of gravel where the pebbles can shift and flow around one another.

The blunt fact is, on very large scales of space and of time, almost everything other than perfect crystals start to act kind of like a liquid- and a lot of those do as well. When I made a study of very old Martian craters, I got used to 'eyeballing' the age based on how much the crater had subsided, almost exactly like the ways that ripples in the surface of water gradually subside over time when you throw a rock in to a lake. Just, you know. Slower.

But at the same time, these things are more fragile than you'd believe, and can shatter like glass. The surface of the Earth is like this, too. Absent the kind of overpressures that make the mantle flow like it does, Earth's crust is still tremendously weak relative to many of the planet-scale forces to which it is subject- I was surprised, once, when a professor offhandedly described the crust as having a tensile strength of 'basically zero;' they really thought of the surface as a delicate filigreed bubble of glass that formed like a thin shell, almost too thin to mention, on the outside of a water droplet. On human scales, liquid is the thing that flows, and solid is the thing that breaks. But once stuff gets big or slow or both, the distinction between a solid and a liquid is more that a liquid is the thing that doesn't shatter when it flows. And it all gets really, really vague, which I suppose you'd expect when you get this far outside the contexts in which our languages were crafted.