Irene Preston's Historical Writing
ASPECTS OF GEOLOGY
SCOTLAND AND THE GREAT GLEN FAULT
The Great Glen Fault is the remnant of a collision of continents way
back in the Earth's past about 400 million years ago. The fault
stretches from Inverness to Fort William. The land north of the fault
was once the shoreline of a vast northern continent called Laurasia. The
land south of the fault was another vast continent called the Baltic and
separating these two continents was a vast ocean called the Iapetus. It
began to form as early as the Pre-Cambrian, 1000 million years ago and
was closed by the late Silurian, around 400 million years ago.
To understand the forces that caused the continents to collide we must
study an aspect of geology called Plate Tectonics.
The outer layer of the earth is not quite the continuous skin it was
thought to be. It is in fact made up of separate pieces called plates.
Each plate is a solid slab of lithospere, crust plus the uppermost part
of the mantle, and is 18-20 km thick. The magma below the plates moves
in a circular motion called convection cells. This theory is based on
the way liquids are known to behave when heated from below. Heat from
the earth's interior is said to cause rising convection currents which
supply material to spreading ridges. Subduction zones are seen as the
cooling and descending parts of the cells. The plates move about the
Earth's surface at the rate of a few centimetres a year. There are
continental plates and oceanic plates, with a leading edge called
margins which are either destructive, eliminating the land back into the
magma, or constructive, when fresh land is formed along a ridge, called
sea-floor spreading.
About 500 million years ago during the Ordovician period the spreading
of the Iapetus Ocean stopped and the ocean began to close. The gradual
merging caused a vast amount of volcanic activity and regional
metamorphism. By the early Silurian most of the Iapetus had been
subducted under the southern Baltic Continent leaving only a narrow
ocean. By the late, Silurian 400 million years, ago it had totally
disappeared.
Only minor deformation occurred but a vast range of mountains was formed
and is known as the Caledonian Orogeny.
The action of one plate affects another so the closing of the Iapetus
Ocean brought another ocean to the opposite shores of the Caledonian
Continent. This ocean was called the Rheic and its fate was to be
similar to the Iapetus, and it began to close about 275 million years
ago bringing in its wake volcanic activity and a vast intrusion of
molten magma. The magma cooled to form an enormous mass of granite
called a batholith that today lies beneath Cornwall and the Scilly
Isles. It is only identified on the surface as the tors and moors. The
heat from the granite intrusion gave rise to the flow of hot
mineral-rich fluids along joints and sedimentary bedding planes. As the
fluids cooled rich mineral veins were produced such as tin and copper,
and were probably mined before the Roman occupation.
Most of the Southern continents began to merge into one landmass called
Gondwanaland, situated in a desert zone. This period is known as the
Devonian but as the plates moved towards the Equator the climate was
most suitable for warm shallow seas and the growth of corals and many
shell-covered creatures. This period eventually gave us the limestone
very prominent in Derbyshire. In the northern part of the continent the
mountain ranges eroded depositing large quantities of sand, pebbles and
grits. Huge deltas formed at the shoreline and produced what we now call
the millstone grits. The mineral-rich deltas were invaded by a variety
of giant horsetails and soft barked trees. At this time the sea level
fluctuated and regularly flooded the deltas and vegetation. This cyclic
flooding is called rhythmic sedimentation but commonly called a
cyclothem, and lasted for several million years. Eventually this
carbon-based material produced a substance we call coal. This vast
period is known as the Carboniferous and is one of the most interesting.
The large continent of Gondwanaland moved north across the Equator and
joined with the northern continents forming one vast continent called
Pangea. As it passed the Equator the climate became very dry and arid
giving rise to desert conditions. This period is known as the Permo-Triassic.
Continental crust consists of a light granitic crust resting on the
denser basaltic plate material below. When two continental plates
converge one slides under the other. The basaltic rock sinks into the
mantle and melts into the magma but the lighter granitic crust is not
dense enough. The edge nearest the subduction zone is forced upwards and
distorted to form a mountain range. This process is called orogenesis
and a single phase of mountain building is called an orogeny.
Sea-floor spreading takes place along a ridge, an area of weakness
probably due to a stretching of the oceanic plate as the plates on
either side are subducted. The opening of a ridge is continually filled
by rising magma producing small pockets of basaltic lava. As they come
into contact with the sea water they cool rapidly and form pillow lava.
100 million years ago the South Atlantic Ridge began to form and
separated South America from Africa. 60 million years ago the spreading
of the Mid-Atlantic Ridge had reached far enough north to begin
splitting Northern Europe from Greenland. Magma rose into the separating
continental edges producing volcanic activity and basalt lava flows.
When this happens on land the sudden cooling causes the basalt to
contract and crack forming large vertical six-sided columns known as
columnar jointing. They can be seen in Ireland and are known as the
Giant's Causeway, and are also seen on the Isle of Staffa near Mull. The
highest parts of an oceanic ridge may stand above sea level and form
islands. Iceland, the Azores and Tristan da Cunha are all part of the
Mid-Atlantic Ridge and have active volcanoes with basaltic lava flows
The mountains along this ridge are larger than any mountains on earth.
About 25 million years ago the African plate started to move northwards
into the European plate. This also pushed India into Asia and the result
of these movements was the formation of the Alps and the Himalayas.