Physical Geography and Geofiction

Geopoeia
Jump to navigation Jump to search

THIS PAGE IS UNDER CONSTRUCTION. (More information will be added very slowly.)


plate tectonics

The following animation shows the history of plate tectonics on Earth from 600 million years ago until now. Yellow areas are land; light blue areas are shallow seas (usually continental shelf); darker blue is sea. The black and red lines are plate boundaries. Red lines represent subduction - that is, plate boundaries where one plate slides under another. This is one of the main sources of orogeny (mountain-building; see below), earthquakes, and volcanism.

Tectonics.gif

mountains

Mountains are formed in events of orogeny lasting (usually) tens of millions of years, and ground down by erosion after that. The height of major mountain ranges (in meters) can be estimated by means of the following formula:

Formmountains.png

in which O is the length of the period of orogeny (mountain-building) and A is the time passed since active orogeny ended, both measured in millions of years. The vast majority of major mountain ranges have heights that do not deviate more than 15% (up or down) from the result of this formula. Mountain ranges are higher (i.e. max. 15% higher) if they are formed on top of older ranges, consist of particularly hard rock types, and/or experienced relatively little erosion because of climate circumstances; they are lower (i.e. max. 15% lower) if they were formed in basins or other low-lying areas, consist of particularly soft rock types, and/or experienced relatively much erosion.

In exception to the above, major mountain ranges formed by very recent and/or ongoing collision of continents can result in much higher mountains, especially if the orogeny takes place in a region that experienced orogeny in the past (and therefore, was elevated already). In extreme cases (like the Himalayas) this may add up to two/thirds (67%) to the result of the above formula.

The maximum height of mountains is limited by gravity. On a planet with Earth-like gravity, mountains cannot be much higher than the highest peaks of the Himalayas. (Higher mountains would just sink in the mantle because of their own weight.) On planets with less gravity, higher mountains can in principle occur (depending on plate tectonics and/or other causes of orogeny); on planets with more gravity, mountains will be lower.

Links