Mountain
A mountain is a
landform
that extends above the surrounding
terrain in
a limited area. A mountain is generally steeper than a
hill, but
there is considerable overlap, and usage often depends on local custom.
Mountains cover 54% of
Asia, 36% of
North America, 25% of
Europe, 22%
of South America, 17% of
Australia,
and 3% of Africa. As a whole, 24% of the Earth's land mass is mountainous.
Also, 1 in 10 people live in mountainous regions. All the world's major rivers
are fed from mountain sources,[verification
needed] and more than half of humanity depends on
mountains for water.[1][2]
The adjective montane is used to describe mountainous areas and the
things associated with them.
Heights
Mountains are generally given as heights above mean
sea level.
The
Himalayas average 5 km
above sea level, whilst the
Andes average
4 km. Most other mountain ranges average 2 – 2.5 km. The highest mountain on
Earth is
Everest, 8,848 m
(29,028
feet), set in the world's most significant mountain range, the
Himalayas.
Other definitions of height are possible. The peak that is farthest from
the center of the
Earth is
Chimborazo in
Ecuador. At
6,267 m above sea level it is not even the tallest peak in the
Andes, but
because Chimborazo is very close to the equator and the Earth bulges at the
equator, it is 2,150 m further away from the Earth's centre than Everest. The
peak that rises farthest from its base is
Mauna Kea
on Hawaii,
whose peak is over 9,000 m above its base on the floor of the
Pacific Ocean.
Even though Everest is the highest mountain on Earth today, there have been
much taller mountains in the past. During the
Precambrian era,
the
Canadian Shield once had enormous mountains 12,000 m in height that are
now eroded down into rolling hills. These enormous mountains formed by the
collision of
plate tectonics much like the Himalaya and the
Rocky Mountains.
At 26 km
(Fraknoi et al., 2004), the tallest known mountain in the
solar
system is
Olympus Mons, located on
Mars.
Characteristics
The altitude of mountains means that the tops exist in higher cold layers
of the atmosphere. They are consequently often subject to
glaciation and
erosion
through frost action. This produces the classic mountain
peak shape. Some mountains have
glacial lakes, created by melting glaciers; for example, there are an
estimated 3,000 in
Bhutan.
Sufficiently tall mountains have very different climatic conditions at the
top than at the base, and will thus have different
life zones
at different altitudes on their slopes. The plants and animals of a zone are
somewhat isolated when the zones above and below are inhospitable, and many
unique species occur on mountainsides as a result. Extreme cases are known as
sky
islands.
Cloud
forests are forests on mountain sides which attract moisture from the air,
creating a unique
ecosystem.
Mountains are not generally favored for
human
habitation; the weather is harsher, less food is available, and there is
little level ground suitable for
farming. At
very high altitudes, there is less
oxygen in the
air, and less protection against solar radiation (UV).
Acute mountain sickness (caused by
hypoxia - a lack of oxygen in the blood) affects over half of lowlanders
who spend more than a few hours above 3,500 metres. Despite some biological
adaptation by peoples who have lived on mountains for hundreds or thousands of
years, babies' average
birthweight is reduced by 100 grams for every 1,000-metre gain in
altitude. Thus, many stores lining the many mountain ranges known to cause
these sicknesses provide oxygen tanks and higher SPF sunscreens.
Most mountains of the world have been left in their natural state, and are
today primarily used for
recreation. Some mountains are very difficult to climb, and offer
spectacular views. Some people therefore enjoy the sport of
mountaineering. Mountains are also the site for the sport of
downhill skiing. People engaging in these activities often stay at
mountain resorts built for the purpose. it touches the sun
Geology
A mountain is usually produced by the movement of lithospheric plates,
either orogenic
movement or
epeirogenic movement. The compressional forces, isostatic uplift and
intrusion of
igneous matter forces surface rock upwards, creating a landform higher
than the surrounding features. The height of the feature makes it either a
hill or, if higher and steeper, a mountain. The absolute heights of features
termed mountains and hills vary greatly according to an area's
terrain.
The major mountains tend to occur in long linear arcs, indicating tectonic
plate boundaries and activity. Mountain creation tends to occur in discrete
periods, each referred to as an
orogeny.
The orogeny may last millions of years, and the uplifted region is being
eroded away, producing valley-and-peak
terrain,
even while the uplift is taking place. Two types of mountain are formed
depending on how the rock reacts to the tectonic forces – block mountains or
fold mountains.
The compressional forces in continental collisions may cause the compressed
region to thicken, so the upper surface is forced upwards. In order to balance
the weight, much of the compressed rock is forced downwards, producing
deep "mountain roots". Mountains therefore form downwards as well as upwards
(see isostasy).
However, in some continental collisions part of one continent may simply
override part of the others, crumpling in the process.
Some isolated mountains were produced by
volcanoes,
including many apparently small
islands that
reach a great height above the
ocean floor.
Block mountains are created when large areas are widely broken up by faults
creating large vertical displacements. This occurrence is fairly common. The
uplifted blocks are block mountains or
horsts. The intervening dropped blocks are termed
graben:
these can be small or form extensive rift valley systems. This form of
landscape
can be seen in
East
Africa, the
Vosges, the
Basin and Range province of Western
North America and the
Rhine valley.
Where rock does not fault it folds, either symmetrically or asymmetrically.
The upfolds are anticlines and the downfolds are synclines; in
asymmetric folding there may also be recumbent and overturned folds. The Jura
mountains are an example of folding. Over time, erosion can bring about an
inversion of relief: the soft upthrust rock is worn away so the anticlines are
actually lower than the tougher, more compressed rock of the synclines.
Local definitions
Some authorities define a mountain as a peak with a
topographic prominence over a defined value: for example, according to the
Britannica Student Encyclopedia, the term "generally refers to rises
over 2,000
feet (610
metres)".[3]
The
Encyclopædia Britannica, on the other hand, does not prescribe any height,
merely stating that "the term has no standardized geological meaning".[4]
United Kingdom
In
England and Wales the
Department for Environment, Food and Rural Affairs has defined "mountain"
(as a mass
noun) as all land over 600 metres, for the purposes of
right to roam legislation. This is a close metric equivalent of 2,000 feet
(610 meters) . [5]
The
Land Reform (Scotland) Act 2003 does not appear to draw this distinction,
and in
Scotland the term "mountain" is more subjective, often being used for
hills exceeding 3,000 feet (914.4 m) listed as
Munros, as well
as many lower hills which are distinctive or mountainous. In the United
Kingdom the term "hill" is commonly used for all hills and mountains,
regardless of height.
