Prompted by reports of the terrible earthquake in Nepal and specifically the following:
Am I correct in gathering from this that Everest (and other Himalayan peaks) will get higher and higher until the plates involved come to a grinding standstill? Have any geophysicists roughly calculated the height Everest and others could reach and the timescale involved? Is it possible that Everest could eventually snatch the prize of tallest mountain in the solar system from Mars’ Olympus Mons? (I’m assuming that tectonic activity has ceased on Mars but I could well be wrong about that.)
There’s more involved in mountain height than just whether the creation process is ongoing. Olympus Mons is as high as it is mostly because Mars’ gravity is 38% of Earth’s. There is a point where the sheer mass of the mountain is so high that it can’t find a stable way to be taller, and that point depends on what the strength of local gravity is.
You have to consider erosion. Even though the mountains are uplifting at 1.8 inches/year, erosion is wearing them down at some similar rate. If the erosion rate is equal to the uplift rate, the mountains are said to be in dynamic equilibrium and overall they won’t get any higher, although the vagaries of erosion will still lead to individual high peaks like Everest continually emerging and collapsing.
Whether the Himalayas themselves are in dynamic equilibrium is AFAIK not really known. Uplift is easy enough to measure these days, but average erosion is tricky to figure precisely, especially in the high mountain ranges where it takes the form of infrequent but catastrophic landslides. At any rate though, even if the rate of uplift is exceeding the rate of erosion and the mountains are indeed growing on average over time, that rate is going to be much, much lower than the 1.8/year number.
The other thing is that the exact mechanism of the uplift isn’t entirely understood. According to the sort of Geology 101 version of isostasy the Himalayan plateau is actually taller than it should be able to be and still be balanced by a root of crust sticking down into the mantle. The general idea is that the force of the plate collision is “holding” them up, but the exact physics of that aren’t completely understood.
That’s not quite right. Olympus Mons is just a gigantic shield volcano. It’s huge, but it’s not especially steep. If it existed on Earth, the crust root required to balance it would have to stick further down than you can have crust material, so you’d get rapid subsidence (not to mention that pesky erosion.) On Mars the crust extends much further down, and so it’s possible for all 25 KMs of Olympus Mons to be in isostatic equilibrium.
As for why it is so huge, what they think happened is that it was a hot spot volcano not unlike Hawaii. With Hawaii, as the crust moved along on top of the stationary hot spot, it created a chain of short-lived volcanoes that left the chain of islands and seamounts behind. Picture. On Mars, though, plate tectonics has ceased so it’s as if the material in all the Hawaiian Islands had erupted in the same spot, leaving a massively huge shield volcano. Because of the thicker crust and lack of erosion, the stuff could just keep piling up.
Not quite. The *theoretical limiting factor of a mountain range is that the crustal root gets too far down into the mantle and melts not the crush strength of the rock.
But the Himalayas don’t seem to have a root, or at least not one large enough to balance the high topography. Like I mentioned, it seems that the same forces that are driving the Asian-Indian collision (namely, the piece of oceanic crust that’s attached to the Indian Plate and sinking into the mantle) also allows the mountains to be taller than they should be by simple isostasy. If it weren’t for the fact that the Himalayas do exist it might be something geophysicists argued about being possible or not.
If the forces driving the collision stop (like if the slab of oceanic crust breaks off) then the uplift will cease and the mountains will rapidly subside.
And, again, the factors creating the Himalayan Plateau in general are separate from what makes Everest the current high point. At some point the very topmost reaches of Everest will likely collapse and some other mountain will be the high point for a while. That’s and entirely different process from the fate of the Himalayan Range as a whole, and one that plays out over much shorter time scales.
*Well technically it just starts to deform in a plastic manner, but it ceases to be a coherent part of the crust.
