GRANITES
 Granites
literally form the backbone of Cornwall stretching from Dartmoor
in Devon westwards to the Isles of Scilly, and with their associated
minerals have underpinned the Cornish economy for hundreds of
years. They are still alive and kicking, furnishing China Clay,
building stones, great climbing pitches for budding mountaineers
and spectacular scenery, and not so helpfully radon gas.
See
the Granite geological timeline >>
The
granites started life around 300 million years ago. The first
sign that things were really cooking deep down was the intrusion
of lamprophyres, strange rocks derived from the earth's mantle.
The easiest place to see an example of these rocks is on the beach
at the south end of Prisk's Cove near Mawnan Smith. The rocks
are very weathered - and so would you be if you started life in
the high pressure and temperature conditions of the mantle and
were now expected to be in prime condition on a beach roamed by
cows!
 Soon
afterwards the main bodies of the granites started to rise up
one after the other (give or take a million years) - like gigantic
hot air balloons - into the overlying rocks. As they rose, they
pushed finger-like into the surrounding slates (killas)
and swallowed great chunks. You can still see these foreign slate
bodies or xenoliths within them, no xenophobia about the Cornish
granites! The best places to see giant tapering fingers of granite
in the slates are at Megilligar
Rocks near Porthleven, or Porthmeor and Wicca Pool
on the north coast of Penwith. It was here that Henry De La Beche,
who was to become the first director of the British Geological
Survey, sketched the jagged granite boundary more than 150 years
ago.
| What
was happening on the surface above the granites? Most of the
overlying strata has been removed by erosion but there is
evidence at Kingsand
and Withnoe, that the granites literally blew their top in
the form of violent volcanoes like Vesuvius or Mount St Helens
in the US of A in more recent times. Here rhyolites, the fine-grained
version of granite, can be seen as volcanic neck and lava
flows. |
 When
the rising granite balloons reached as far upwards as they could
go they started to solidify and form a crystalline mass. This
initial crystallisation caused the remaining molten rock to change
composition and become increasingly volatile. Every so often the
pressure got so much that the volatile mineral rich components
escaped through fissures within and beyond the granites. This
spectacular geology can be seen at places like Cligga
and Roche Rock, not to mention the mineral lodes and elvans,
which criss-cross many parts of the county.
 A
long period of more than 200 million years separates the formation
of the granites and associated mineral lodes from today. During
much of this huge span of time, which saw the dinosaurs come and
go and flowering plants evolve, the climate was tropical and dominated
by monsoons. The resulting downpours caused weathering deep into
the ground rotting the granites. This effect was particularly
marked in the granites around St Austell where the feldspar minerals
were transformed into kaolin, more popularly known as China Clay.
The
Eden Project is located in one of these deeply weathered
pockets from which all the clay has been removed leaving only
the unaltered granite.
 During
the Ice Ages the rotten rock was further broken up by the growth
of ice crystals repeatedly forming in the freezing conditions.
During the thaws in the arctic summers the rotted material became
so waterlogged that it slurped down slope leaving the stronger
unweathered parts of granite to stand proud as tors. Excellent
examples occur throughout the region, like the Stowe's
Hill on Bodmin Moor and Pulpit Rock on the Isles of Scilly.
Click
here for a list of Photograph credits
Granites
| The Lizard | Killas
| Coastline
|
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