|What speed do mountains grow at?|
|Question Date: 2018-11-27|
Mountains do not grow like we might think of
living organisms as growing, however mountains
can change size. Mountains form through a
process called orogeny, or the building of
continental mountains by squeezing, crumpling, and
folding Earth's crust. Mountains are formed by
the movements of the vast tectonic plates that
make up Earths exterior, as well as Earth's
climate and processes of erosion.
Mountains are changing sizes all over Earth's
surface. Both mountain ranges and the peaks in
them are increase in height and volume at
different rates. Some mountains are rising really
quickly, like the Himalayas (7 mm per
year), though the Mount Everest peak in the
Himalayas is only growing about 4 mm per year.
The Himalayas are forming because the
Indo-Australian Plate has been actively smashing
into the Eurasian Plate for 50 million years,
causing long-lived crustal thickening. Other
mountains are rising very slowly, like the
Rocky Mountains. These mountains were formed
by the subduction of Pacific oceanic crust beneath
North America from 80 to 55 million years ago. The
Rockies continue to rise due to buoyant forces
(think marshmallow floating on hot chocolate) and
erosion, but not as quickly as the Himalayas.
Still other mountains are no longer rising at all,
like the European Alps. These mountains formed
when Africa collided with Europe about 55 million
years ago. Any rise that may occur is being
balanced by wind and water eroding the mountains
down. Therefore, the Alps are at a steady
Mountains can also change depending on
landslides, volcanic eruptions, gain/loss of thick
ice sheets, a several other geologic processes.
The rate of rise is therefore controlled by the
forces causing the crust to increase in height
(for example: plate tectonics, climate, erosion)
relative to the forces causing the crust to
decrease in height (for example: erosion, ice
weighing down crust).
The speed at which mountains grow depends on
rates of continental collision and erosion, as
well as isostasy, which could essentially
be thought of as the buoyancy of the crust.
When two pieces of continental crust are pressing
against each other, the crust piles up to form
mountains. This is how they grow. Erosion
weathers the mountains down, and makes them
shorter and less steep. The important thing to
realize about mountains, is that like icebergs,
they are essentially floating on the mantle,
and there is a lot of them beneath the surface.
When the top of a mountain is eroded, the top part
gets lighter, so it floats a little higher.
This is what we call isostasy. Because of
isostasy, mountains aren't shortened as fast
as they would otherwise be by erosion. The balance
of all these factors determines how fast mountains
change their height. Some, like the
Appalachians are getting shorter, and some
grow a few millimeters or centimeters per year.
The rate at which mountains grow
depends on the
difference of how fast they are being pushed up
and how fast they are being worn down. Mountains
form and grow in
several ways, some
faster than others.
At the same time,
erosive forces such as wind, rain, and
glaciers are continually breaking up the rock
and reducing the height. In some cases,
erosion is occurring faster than growth and a
mountain range is "growing" at a negative rate
(i.e., it is getting shorter); in others, the
growth is greater and the mountains are getting
taller. In either case, the change is pretty small
compared to the size of the mountains:
the Adirondacks are growing at ~0.1 inches
per year, and Mount Denali at ~0.4 inches per
year. (An earthquake a few years ago cause a big
loss for some peaks in the Himalayas, ~23 inches.
This isn't a recurring rate though, and the
Himalayas are otherwise growing. ) (Some
additional explanations on mountain growth is on
here, and here is an
interesting site with a lot of information
on geological processes related to mountains.)
The rate of mountain growth depends on geological
factors in the region. For instance, Mount
Everest grows by 4mm per year. On the other
hand, some mountains have been measured to shrink!
Some factors that play a role in this process
1) how much the tectonic plates in the region
move against each other to increase the height of
2) rates of erosion
3) frequency of earthquakes, and so on.
It seems that generally, geologists quote
growth rates of less than 1cm per year.
That depends on the mountain. The fastest-growing
mountains that aren't volcanoes grow at rates of
centimeters per year. Volcanoes that are active
can grow faster if they have enough lava to erupt.
Earth's land and sea are all sitting on tectonic
plates that move slowly:
Some sources state that in the North Atlantic, the
rate of movement is only about 1 cm (about 0.4 in)
per year, while in the Pacific it amounts to more
than 4 cm (almost 2 in) annually, while others say
that plates, in general, travel from 5 to 10 cm/yr:
Plate Tectonics - A Scientific Revolution -
Academic Home Page.
Earth's land and sea are all sitting on
tectonic plates that move slowly.
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