Geography Reference
In-Depth Information
Box 3.1 Hot spots
There are some volcanoes that are not located at plate boundaries.
These are at hot spots. They are probably located at the top of hot
plumes within the mantle (a bit like an ocean or atmospheric circula-
tion eddy). There are over 40 hot spots and many of these have formed
in the middle of plates. For example, at Yellowstone National Park in
the United States there is a hotspot which is responsible for amazing
geysers,
mud pots
and other volcanic features. There is evidence that
there was a massive volcanic eruption there 600,000 years ago depos-
iting ash 12 metres thick up to 1,200 kilometres away. Calculations
suggest that a 'supervolcano' may be due to erupt soon. If it does so
then we will no longer be worrying about climate change but instead
how to deal with a much larger and dramatic environmental
catastrophe.
Where hot spots occur under the ocean and there is a volcano then,
as the plate moves over the hot spot, a series of volcanic islands or
seamounts (underwater mountains that have not reached the water
surface) are formed. The best example of such a system is the Hawaiian
island chain. The island of Hawaii is furthest to the east and is the site
of the most active volcano on Earth at present. In fact the volcano,
Mauna Loa, is the tallest structure on Earth when measured from its
base on the ocean floor, at 10 kilometres tall. Mauna Loa and the whole
island of Hawaii has only taken 1 million years to form which is very
short on geological timescales. Then there are a series of islands, Maui,
Molokai, Oahu and Kauai, in a line stretching more than 500 kilometres
to the west. Each island is made up of an extinct volcano, with the vol-
canoes getting progressively older as you travel west. Further west the
islands are so old and eroded that they disappear below the ocean
water surface to become seamounts. This also happens because the
westerly moving ocean floor becomes deeper towards the ocean
trench.
not subducted below the other. This forces land to rise thereby
creating a large mountain belt such as the Alps where Italy has
moved north into Europe, or the Himalayas where India has col-
lided into Asia. The Himalayan continental collision zone seems to
have shortened the length of continental crust by 1,000 kilometres.
Therefore, here the crust is thickened and compressed and as a
result, the rocks are folded and deformed, crumpled and faulted.
