Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: paul.fr on 12/07/2007 01:21:19
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why is that, even kids always draw mountains with a pointy top. why are mountains not nice and flat on top?
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why is that, even kids always draw mountains with a pointy top. why are mountains not nice and flat on top?
I think that relates more to our image of mountains than the reality of mountains. It also depends on how far away you are when you look at them.
Mountains tops come in all sorts of shapes and sizes.
Clearly, one has plateaus, but they are not individual mountains, but large elevated flat areas.
Table mountain, in Cape Town, has a flat top.
I suspect that most snow covered mountains are likely to be fairly pointy because of the erosion of the water and ice sliding off the side of the mountains; but even then, if the mountain is low enough to have significant vegetation on its top (which will also mean it will be often be covered in soft soil), then it might be able to hold together a bit better.
It will no doubt also depend on the age of the mountain (is the mountain still being pushed upwards - as the Himalayas are, or are the older mountains that are simply being weathered away).
One other factor is that often when the mountain does not have a sharp peak, it is difficult actually to see the peak from a distance, as it is often obscured by lower features of the mountain (this can be true even of peaky mountains, but is probably more true where the sharp peak is not there).
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The sole reason is erosion. All young mountain ranges are pointed when young, rounded in old age. By "young" and "old" I mean 20 million years as opposed to 250 million years.
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I suspect for kids they are all dealing with fine motor skills at young ages and sometimes it is simply easier to go up and come down! Instead of using hand eye coordination to draw with such precise skill! There are few children with such well defined fine motor skills at early ages. When they develop more so does their skill of interpretation with the hand eye thing!
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It tends to fall that pointy peaks are either glacially formed such as the Alps, or are volcanoes, such as Kilimanjaro, whereas flat-tops are formed by rain and stream/river erosion from what was a flat landscape. Some rounded peaks can be the result of glacial activity - terminal morrains and drumlins for example, though these generally don't get much above a few hundred feet high.
There are naturally exceptions to this. The Yorkshire Dales and hills such as Ingleborough and Phen-y-ghent are glacial landscapes, but the combination of being lower and subsequent erosion of the limestone had made them somewhat flat. As has been said, time is the greatest influence on this.
Here is a link to how glaciation erodes the sides of mountians. There are many other features such as cirques and arretes that are also pointy as a result of glaciers.
http://www.fettes.com/Cairngorms/corrie%20formation.htm
Another aspect is the tectonic movement of plates, as George says, that is still forcing mountians higher, causing this rough appearance. But the geology will also affect this - the Black Forest mountains and the Vosges are very young and are being formed by a separation along the Rhine Valley, but they are still very rounded.
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Another question comes to mind - do mountains actually hold the crust of the Earth together to stop it shaking about from the convection currents below?
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Another question comes to mind - do mountains actually hold the crust of the Earth together to stop it shaking about from the convection currents below?
Mountains do not hold the crust together - they are what forms when two pieces of crust collide, and the bits that get pushed together then have nowhere to go but up (rather like the crumple zone on a car body when two cars collide head on).
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OK. But do the mountains not act like tent pegs with their environment? As a sort of firmament that stops or dampens down further vibration from the collision of the tectonic plates?
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OK. But do the mountains not act like tent pegs with their environment? As a sort of firmament that stops or dampens down further vibration from the collision of the tectonic plates?
Mountains may add some local weight to the crust in their immediate region, but I suspect the weight will be slight in comparison to the overall energy driving the tectonic plates. I doubt they can have much of an effect on the motion of the plates, although they may be able to shift the week points so that the cracks created by the strains occur elsewhere (i.e. the quakes would be more likely to occur in the foothills than under the heaviest of the mountains).
The plates are fairly unstoppable, so the energy of the motion must either be geographically shifted, or simply temporally shifted, and in the latter case, it just means you get fewer, but bigger, earthquakes.
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Depends on what you mean by "shaking about" or "dampens down further vibration"? If you are referring to seismic (earthquake) activity, virtually all active mountain ranges are also active seismic zones. If you are referring to tectonic forces, then yes, you are are partly correct. Mountain zones not only extend upwards, but also thicken the crust downward, which tend to mute tectonic forces somewhat- or as George said, shift the motion geographically. The Himalayas will eventually (over 10's to 100's of million years) stop the tectonic movement of the Indian subcontinent into the Asian continent.
Before plate tectonics was understood, the prevailing theory of mountain building was controlled by isostatic rebound. It was hypothesized that large areas of the crust- large basins- would start to sink into the mantle, probably due to the enourmous weight of sediments being deposited (just imagine the continental shelf of today's Atlantic Ocean, or the Gulf of Mexico). These were called geosynclines. Eventually the geosyncline would heat up due to heat flow and pressure, causing the whole mess to be pushed up back up by the mantle forming mountains. Isostatic rebound does exist, and can be measured in places where glaciers once existed- the weight of the ice downwarps the crust, which then rebounds when the ice melts.
The crust also thickens under converging plates today- certainly as a result of crustal shortening in these zones, probably also enhanced by the subducting plate as it is forced downward into the mantle. With a few exceptions, mountain building can be viewed as a combination of crustal shortening (George's crumpled zones), and heating/pressure forcing the crust (more bouyant) to reach new heights- while erosion tries to bring it all back to level.
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Bass, I was referring to tectonic forces and what you have posted is fascinating and new to me except for the subduction zone formation. The thought that the Himalayas will stop the tectonic movement of the Indian subcontinent is an awe inspiring thought. Thanks for the detailed answer.
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You mean the reason that mountains are pointy hasn't got anything to do with that small bird who is sharpening its beak on the mountain top to show how long eternity is?
I'm gutted.
pete
inthehills
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It is a simple consequence of the fact that erosion at a point in the topography can be (roughly) thought of as a function of the steepness of the terrain at that point and the amount of water that is flowing over that point. Since not much water is flowing near the tops of mountains (water flow at a point is a function of its catchment area, and points near the top of a mountain have a very small catchment area), mountain tops must be steeper in order to be in an equilibrium with the rest of the terrain (young mountain ranges such as the Himalayas are roughly in equilibrium in the sense that the height distribution of the terrain doesn't change much over time). Old mountain ranges tend to have flatter tops due to them having been covered by glaciers in previous ice ages. There are also other mechanisms that affect this, such as burrowing animals and landslide activity; the former makes some slopes convex in appearance while the latter tends to limit the steepness of a long slope, giving mountains an angular look as opposed to getting steeper and steeper near their summit. Stronger rock types can sustain steeper mountain slopes, giving rise to sharp and very steep peaks in glaciated areas where weathered rocks can be transported away quickly.
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Thanks
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why are most mountain tops pointy?
On a recent tour of Canada and Alaska, our guide pointed out that there is a mix of "pointy" and "rounded" mountain tops in this part of the world.
The mountain chain on the west of Canada and USA is formed by the collision of tectonic plates, so the mountains are still actively growing (and eroding).
But the difference in shape is due to the depth of the ice sheet in the last ice age, which covered most of North America
- and, presumably the ice ages before the most recent one, too, although the most recent one has erased most information about previous ice ages.
- The ice sheet (and the boulders it carries) grinds away at the rock, rounding out the valleys and smoothing the mountain sides.
- If the mountain was tall enough to project above the ice sheet, the pointy top remains intact
- If the mountain was a bit shorter, and beneath the depth of the ice sheet, the pointy top is ground down to a more rounded shape.
- Getting up close to rocks exposed by recent glacial retreat showed deep gouge marks where boulders were scraped across the bedrock.
By the end of the tour, I could estimate the depth of the ice sheet by looking at the shapes of the mountain tops.
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why are most mountain tops pointy?
On a recent tour of Canada and Alaska, our guide pointed out that there is a mix of "pointy" and "rounded" mountain tops in this part of the world.
The mountain chain on the west of Canada and USA is formed by the collision of tectonic plates, so the mountains are still actively growing (and eroding).
But the difference in shape is due to the depth of the ice sheet in the last ice age, which covered most of North America
- and, presumably the ice ages before the most recent one, too, although the most recent one has erased most information about previous ice ages.
- The ice sheet (and the boulders it carries) grinds away at the rock, rounding out the valleys and smoothing the mountain sides.
- If the mountain was tall enough to project above the ice sheet, the pointy top remains intact
- If the mountain was a bit shorter, and beneath the depth of the ice sheet, the pointy top is ground down to a more rounded shape.
- Getting up close to rocks exposed by recent glacial retreat showed deep gouge marks where boulders were scraped across the bedrock.
By the end of the tour, I could estimate the depth of the ice sheet by looking at the shapes of the mountain tops.
This is correct. The sharpest peaks are the ones that remain a nunatak for a sufficient amount of time. You can clearly see this in eg Patagonia with Cerro Torre, or on Antarctica with eg Ulvetanna and surrounding mountains. Mountains that were located near the edge of an ice sheet were never completely covered in ice and their peaks remained intact while their sides got steeper. The results of this can be seen on eg Baffin Island, on Greenland and in Norway. The mountains farther inland are rounder and the ones closer to the coast are sharp. It must also be pointed out that the rock type must be right. The tallest and sharpest mountains in the world generally, predominantly consist of granite because this rock is extremely resistant to wear and contains very few cracks or other weaknesses compared to sedimentary or metamorphic rocks. Metamorphic rocks can also sustain sharp mountains but not as reliably. In the European Alps this is illustrated when we compare the Valais Alps with the Mont Blanc massif. Mont Blanc contains the most jagged peaks in the alps while the Valais Alps are almost as sharp, but not quite (Matterhorn is a very sharp peak in the Valais Alps, but there are other mountains in the Mont Blanc massif that rival or exceed this, eg Aiguille Noire de Peuterey, Petit Dru, Grandes Jorasses, Aiguille de la République or Dent du Géant). The Valais Alps are known for loose rocks while the Mont Blanc massif is a climber's mecca due to its rock quality -- thanks to the plutonic granite that contains few weaknesses and weathers mainly by breaking off in big chunks like the peels of an onion. Strong rocks don't break off as quickly when the glacier undercuts the face, allowing it to get steeper. If the rock type is too weak the mountain will get covered in glaciers and not steepen. Eg volcano summits (although this is caused by the the fact that volcanoes are not formed like regular mountains but instead are born with a certain steepness limited by the angle of repose; however, volcanoes also tend to be very weak which is why they generally can't be eroded to become very steep except for certain circumstances). Huge mountains such as Denali or those in the Himalayas cannot be made too steep without collapsing, regardless of rock type (FYI Denali is made out of granite but its sides are still of only modest slope due to the immense slope heights). We can see that the maximum average steepness of a mountain slope with a certain height decreases when we increase the face height, irrespective of rock type. The highest vertical faces in the world cap out at around a mile in height, eg Great Trango Tower. The highest walls in the world that can still clearly be labelled walls (steepness > 45 degrees) are those of Nanga Parbat (Rupal face), Dhaulagiri (W face) and Annapurna I south peak (SW face). They are each around 4500 m in height. There are clearly steeper walls at around 3000 m (Lhotse, Jannu or a subsection of the previously mentioned walls).
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I looked back at my holiday snaps, and found one that illustrates the point...
This photo was taken in College Fjord. The more distant peaks are very pointy, while the lower peaks closer to the waterline are more rounded, and would have been ground down by the glaciers forming the moving ice sheet.
We did get to see Denali (previously known as Mt McKinley), through a lucky break in the clouds.
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By definition a peak is pointy, whereas a plateau is flat. There are plenty of plateaus higher than what passes for mountains in Britain (it is arguable that we only have about three genuine mountains, and the huge Mexican Plateau is higher than any of them!) but who wants to climb a plateau when you can drive across it? So the goal is to climb a peak, and it just happens that the highest bits in most countries are peaks.