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Mull is constructed rather like a multi-tiered wedding cake. Thick layers
of basalt lava sit on top of a complicated basement of much older rocks
which peep out around the edges of Mull rather like the silver base does
on the lowest wedding cake tier. Geologists love Mull because it has such
a long and interesting history (its oldest rocks on Iona are about 2000
million years old), and it has unique structures and rocks found nowhere
else in the world (such as the Loch Ba Ring Dyke and felsite).
Mull has not always been in its present position and form. Over geological
time it has undergone enormous changes. Mull's oldest rocks formed way
south in the southern hemisphere and Mull (in common with the British
Isles) has gradually drifted northwards to its present day position. (In
the process of continental drift.) Its rocks preserve details of the climatic
zones that it passed through on its northward journey. For example, if
you stand on the rocky shore opposite Inch Kenneth at Gribun you will
be standing on sandstones deposited in a desert region at the same sort
of latitude and rather like the Persian Gulf today.
Most of Mull is made of lava poured out of fissure volcanos when the
North Atlantic was forming and Mull was torn apart from its 'neighbour'
Greenland as the vast super-continent which once joined North America
and Europe divided. The molten lava which erupted from about 60 to 50
million years ago forms Mull's 'stepped' tablelands. Into these, at a
later stage, intrusions of other 'rocks formed by fire' took place, forming
the mountains of Mull's famous Central Igneous Complex. Volcanic explosions
and intense earthquakes shook Mull at that time and one of the old
faultlines,
the Great Glen fault is still occasionally active. (Nothing catastrophic!)
Mull's final shape has largely been carved by huge glaciers which only
melted away from Mull 10,000 years ago leaving deep 'U' shaped valleys
between the mountains and long glaciated lochs both freshwater and marine.
Mull doesn't have any precious gems at the surface, but geologists know
that deep below Mull, in the earth's mantle there must be priceless minerals
at this spot, but the temperature at that depth would melt any drills
probing for them! Mull with its incredible history and unique rocks and
intrusions is a 'Mecca' for geologists who travel from all over the world
for field work.
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