That is not true. You are mixing up Tibet and plateau. The Himalayas have many many foothills (and these “hills” are usually several thousand feet high)
The Himalayas start at the Salt Range. You have a series of Plateaus and mountains of ever increasing height.
The Himalayas actually explode oiut of the ground, its surreal you are one moment driving aloing the Punjab plains and then you see mountains literally exploding out of the earth.
Would it though? If we’re modelling the mountain as a less dense object floating in a more dense fluid, so long as gravity is non-zero, it’s going to sink until the mass of fluid displaced by the object is equal to the mass of the object itself. Since the object is less dense than the fluid, this happens with a bit of the object sticking out.
A lower gravity would probably affect the rate at which the system returns to equilibrium after a displacement, but not the equilibrium position itself. If gravity is pulling less hard on the object, then it’s also pulling less hard on the fluid, which is what creates the pressure in the fluid and therefore the buoyancy force which pushes the object back up.
It seems like the unsourced half-heard factoid would need to make some assumptions beyond just the “object floating in fluid” model to declare a maximum height for mountains, though. Otherwise you could get an arbitrarily tall mountain by giving it an arbitrarily deep root, or making it arbitrarily steep on top. Some assumptions like, “mountains can only be so steep” and “a mountain’s root can only get so deep before it gets absorbed into the mantle” need to be made.
You mean Chimborazo?
I find the “distance to the centre of the Earth” measure kind of lame. Like “if I assume the large-scale shape of the Earth to be different than it conspicuously is, then such-and-such is the largest local deviation from that shape”.
Might as well declare that the Earth “should be” a perfect sphere except with a 30km lump right over Everest, and then posit the Everest is the bottom of the deepest valley on Earth.
I think you’re right & I was goofed up.
Agreed. The “distance from the gravity well” sounds cool, though, for the reason mentioned. Could we add another – “distance from the Moho discontinuity”?
I’m not a geologist, but Earth was more geologically active in the past due to a hotter interior. And mountains sinking because of their weight sounds like it would be fairly slow. Perhaps that could have resulted in some taller mountains for a short time because they’d rise faster and farther before they got pulled down by their own weight?
What you’re describing is basically the idea of isostasy, which says that high topography must be balanced by a root of crust extending into the mantle. Isostasy does ostensibly put maximum height on mountain ranges, since there are limits to the amount of weight that the crust at the bottom can support and how much heat they can be exposed to at depth.
The only problem here is that under the “geo 101” version of isostasy, the Himalayas are simply too big. If they were in isostatic equilibrium, their root would extend too far down and would be supporting too much weight. As it is, in certain conditions magmas are forming within the Himalayas from the pressure alone. So as it turns out, the Himalayas are not in equilibrium. The current theory is that the force of India pushing against Asia is holding the Himalayas up and allowing them to stay out of equilibrium. If India suddenly stopped moving northwards, the Himalayas would rapidly subside until they reached equilibrium. When I took a tectonics class a few years ago, this was apparently very much still a contentious issue (and presented as an example of how plate tectonics is still a relatively young and incomplete theory).
first, the belief that the india-asia is the first/only major crumpling since pangea broke up has been corrected. second, it doesn’t really matter where one’s base begins. for all you know, the initial contact between india and southern china may have been undersea. nah, it’s the peak that’s farthest upwards away from sea level or the geoid that gets the gold. it’s pointless postulating which mountain range may have been highest, even if one can measure with reasonable accuracy the the original base height and the amount of sediments that was eroded.
i will instead postulate that the highest mountains existed when the earth has not yet formed oceans, nor did it have an eroding atmosphere. this is roundabout the time when it was continuously bombarded with meteors, the peaks must have been sheer and jagged like those on the moon, and may very well have been as high or higher than olympus mons on mars (8 miles above the martian plane.)
This is probably necessarily true. Since slab pull (the pull of a subducting oceanic plate) is the major driver for plate movement, a continent-continent convergent boundary probably can’t form on its own (since continental crust doesn’t subduct). In the case of the Himalayan boundary, there was oceanic crust to the north of India (the old Tethys Sea) that was completely subducted under Asia. There’s some debate as to whether there’s still some oceanic crust clinging to the Indian Plate somewhere deep in the mantle that continues to drive the collision (and thus hold up the Himalayas), or if the ridge push from the mid-ocean ridges is enough to sustain the boundary once formed. I’m not quite sure what bearing this has on the question of what are the highest mountains, though.
I doubt it. The reason why Olympus Mons is so freakin’ huge is because Mars’ plate tectonics have shut down. It’s most likely a hot spot like Hawaii, but unlike on Earth where plate tectonics whisks the erupted lava away from the hot spot, on Mars the lava just keeps piling up, forming gigantic shield volcanoes like Olympus Mons. The thinner atmosphere and lower gravity helps, but the tectonics issue is the big one.
In contrast, in the early Earth before oceans or continental crust, heat flow would have been extremely high and plate tectonics (if it could even be so-called then) would have been very active. Big accumulations of lava would move laterally quickly and reach isostatic equilibrium rapidly, so huge tall shield volcanoes would have been unlikely (and obviously no continental crust means no big collisional ranges). Maybe the impact event that formed the moon might have caused some temporary “topography” that was higher than the Himalayas, but I don’t know if that really counts.
I think it’s fair to say that the Himalayas are the highest mountains known to have existed, but that probably says more about how hard it is to really hang a number on the paleoaltitude of other mountain ranges than anything else.
^
nice. í have one comment though. the granite masses that eventually formed the continents must have differentiated and cooled earlier than the basalt/peridotite ocean beds around them. those granites must have been huge blobs maybe stratospheric in height before isostasy and erosion could work on them.
As far as age goes I came across this on Wikipedia, about cratons, the most ancient, stable portions of continents.
It’s not as if the cratons just suddenly congealed out of nothing, though. How exactly the continents formed is another sort of grey area in plate tectonics, but I believe the current idea is that you started to get ocean-ocean subduction zones that formed island arcs, which eventually would collide with other island arcs eventually forming large enough landmasses to allow for the differential melting that creates granitic rocks. I don’t think anything in that process would necessarily create bigger mountains than those that exist in the Phanerozoic world.
180 million is very short, about the time it takes an entire basaltic ocean plate to completely subduct. but then, basalt really just cycles between crust and mantle so it’s the same stuff over and over again.
maybe not. island arcs tend to erode very quickly and as mentioned earlier, whole oceanic plates can subduct completely. nothing in the giga-year range preserves them. i remember one seminar way back in the 80s (Dr. Brian Skinner, australian) he said there are more epithermal gold and porphyry copper deposits being formed now than in any other time because now is when a lot of volcanic island arc systems are in full “blast.” a german professor challenged that position, mentioning basically what i wrote above. skinner said even if there were a lot of arc systems that have eroded, there would have been a chemical imprint --way back from archean. no way, he said (note: skinner was a geochemist while the german was a tectonics man.)
Does the ocean water that surrounds Mauna Kea help support it? At 33,000’ it seems to be somewhat taller than what the earth can theoretically support.
I dunno, you tell me. The difference between 29,000 and 33,000 in geological terms may be insignificant. A typical jet passenger airplane flies at about 35,000 ft. give or take, but it’s nowhere near “space”. “Space” is defined at something like 50 miles above sea level which would be 264,000 ft. The point being, it’s all, in relative terms, quite close to the surface.
There isn’t necessarily a single height that is the maximum the Earth can support, it’s just a matter of whether the weight of the mountain, the “normal” crust thickness, and the root extending into the mantle will cause the rock at the bottom to buckle or go plastic. It depends on the kind of rock we’re talking about and, as in the case of the Himalayas, there are ways of getting around it.
In the case of Mauna Kea, a mountain can be much taller on oceanic crust because the crust is normally thinner. So, to grossly oversimplify, Mount Everest sticks up about 8 kilometers over “normal” continental crust thickness, and so if it were in isostatic equilibrium would need to be balanced by an 8 km root. With a “normal” continental crust thickness of around 45 km, the root would go down 53 km and have 61 km’s of rock weighing down on it. Mauna Kea sticks about 10 km above the seafloor, but oceanic crust is about 7 km thick, so it’s root would only go 17 km deep and only support 27 km of rock (although oceanic crust is about 20% denser). Mauna Kea could become much higher than Mount Everest except that as the Pacific Plate moves to the northwest, Mauna Kea will eventually go extinct in favor of newer volcanoes to the southeast like Mauna Loa and the Lo’ihi Seamount and others yet formed.
What effect does the increasing distance of the moon (and thus the slowing of the Earth’s rotation) have? A billion or two years ago, would its proximity have promoted plasticity in the crust meaning lower mountains?
This is all very interesting - why is that? The weight of the water?
No, it’s because they’re mainly composed of basalt, which is denser than your average continental rocks.
The water is over the basalt because of the density difference (and thus “elevation” difference), not the other way around.