The Moine rock sequence comprises of sedimentary rocks that have
been deformed and altered after exposure to the high temperatures
and pressures associated with massive upheavals in the earth's crust.
However, although the Moine rocks on Ardnamurchan were subjected to
temperatures of several hundred degrees centigrade and were buried
several kilometres down in the earth's crust, in western Ardnamurchan
they have, unusually, remained relatively undeformed. As a result,
the original structure of these one-time marine sediments is still
visible in present-day rock exposures.
The Moine rocks are the oldest rocks on Ardnamurchan and may be regarded
as the foundation of the Ardnamurchan peninsula.
Between 245 and 60 million years ago were the Triassic, Jurassic
and Cretaceous periods of geological time. No Cretaceous-aged rocks
outcrop on Ardnamurchan but there are Triassic and Jurassic-aged rocks;
these would once have been widespread over the peninsula, covering
large areas of the Moine, but volcanic activity and erosion has, for
the most part, removed or obscured these dinosaur-aged rocks.
60 million years ago, as the earth's crust between Europe and Greenland
split apart, creating the north-east Atlantic ocean, the area which
is now the Ardnamurchan peninsula was one of five major volcanic centres
along the Scottish west coast. Even though the peninsula is of less
rugged and mountainous terrain than other British volcanic centres
developed at this time, it demonstrates clearly the main geological
features typical of such centres.
Ardnamurchan was active as a volcanic centre for about 1 million
years. The early activity was heralded by the eruption of volcanic
ash through numerous fissures scattered across the landscape. It is
likely that this ash covered much of the peninsula. After the ash
came huge volumes of basalt lava, which again erupted through fissures
creating an extensive and thick lava plateau.
Today, only fragments of the lava plateau remain with the thickest
section (100 metres) at Ardslignish. Once the lava plateaux had formed,
the volcanic activity became more focused and additional lava and
volcanic ash was erupted through the vents and craters of large, traditional
cone-shape volcanoes. Volcanic ash would have been blown clear of
the vent, but coarser lava or agglomerate, comprising pebbles, cobble
or boulder sized blocks of rock would have remained within the confines
of the crater, which may have been several kilometres in width. The
presence of large areas of agglomerate on the peninsula are testament
to both the size of the craters and the prolonged and violent nature
of the eruptions.
In addition to the volcanic activity occurring above ground, there
was also volcanic activity below ground. Molten rock was being forced
or intruded into cracks and voids that were forming beneath the ground
surface and also within the volcanic cone. In particular, intrusions
called cone-sheets and ring-dykes were produced at this time. Though
these are essentially circular features, with diameters ranging from
a few hundred metres to over several kilometres, it is rare for them
to actually encompass the full 360o. The Ardnamurchan peninsula is,
however, renowned for its well developed examples of these ring-structures.
It was by studying the distribution of Ardnamurchan's cone-sheets
and ring-dykes that geologists were able to identify the three separate
centres of volcanic activity on the peninsula.
The first volcanic centre was around Ben Hiant. From here the focus
of activity moved several kilometres westwards to around Aodann. Here,
only the volcano's eroded roots and a small volcanic vent (west of
Kilchoan) remain along with sections of earlier plateau lava and agglomerate
erupted from the previous volcanic centre.
From around Aodann the volcanic activity centre shifted north-eastwards
to Achnaha, where the third centre developed. Large ring-structures
are visible here and an obvious and dominating one is called the Great
Eucrite.
Also associated with the volcanic activity was the development of
dykes and dyke swarms. Dykes form when magma is intruded upwards into
long, linear, vertical cracks in the earth's crust, typically created
through crustal stretching. Dyke swarms represent particularly large
numbers of these features occurring close together and parallel to
one another.
Once the volcanic activity ceased, the extinct volcanoes were subject
to millions of years of erosion by water and ice, revealing their
roots and the older rocks of the area and producing the present day
landform.
