Irene Preston's Historical Writing
GEOLOGY OF PLATES
Volcanoes at constructive plate margins tend to have quiet eruptions
with great outpourings of runny lava, basalt flows. This is largely
because of the composition of the lava which is low in silica. Iceland
has many gentle-sided volcanoes produced by this process such as the
Island of Surtsey. Island arcs are made by single stationary plumes
shooting upward from fixed positions. As the plates move over them
volcanic chains are formed.
Thermal plumes are vertical columns of upwelling material from the
mantle, 100-250 km in diameter that rises from beneath a continent or an
ocean. It can be perceived at the earth's surface as a hot spot. Thermal
plumes carry enough energy to move a plate and they may be found at both
plate boundaries and plate interiors.
Volcanoes at destructive plate margins have a lava rich in silica,
called viscous, which doesn't flow easily. It is violent as pressures
build up inside vents and blasts away any solidified lava blocking them.
Ash and dust can form cauliflower-shaped clouds above the volcano and a
flood of hot ash can flow down its slopes. This happened in Washington
when Mount St Helens blew away half its side in 1980.
There was little erosion to the land mass 135-65 million years ago when
it was 40/45 degrees north of the equator. There was a world-wide rise
in the sea level known as the Cretaceous Sea. Sediments of coccoliths,
skeletons of algae, formed the chalk cliffs of Dover. 1000 metres took
30 years to deposit 1 mm of chalk. Sponges on the sea floor were buried
and became nodules of flint used by early man to make his arrow heads.
Flint nodules also make a decorative frontage to buildings when broken
in half as their shiny surface reflects the sun. This process is called
knapping.
Geologists use the term orogenesis for periods of mountain building,
whereas a single phase is called an orogeny. One such phase took place
about 280 million years ago during the late Carboniferous and early
Permian periods. This orogeny was called the Hercynian or Variscan, both
German names, and was caused by the closure of the Rheic Ocean bringing
southern Europe into contact with the Caladonian continent forming a
vast land mass called Pangea.
The effects seen in the south west of England are folds in Cornwall,
Devon and the southern parts of Wales and Ireland. The compression of
Devonian and Carboniferous rocks against the edge of the Caladonian
continent formed the great batholith of granite and the serpentine
Lizard. The Cornwall batholith stretches from Devon and Cornwall out
into the Atlantic Ocean and covers some 40 miles by 25 miles. This is
small compared to South Africa and America, and the Malayan batholith is
300 miles long. The coastal batholiths of British Columbia, Canada and
Peru are over 1000 miles long and 120 miles across. Between 225 and 65
million years ago Pangea separated into smaller continents during the
Mesozoic which includes the Triassic, Jurassic and Cretaceous periods.
The south Atlantic started to open separating South America from Africa.
During the Cretaceous period the mid-ocean ridge of the North Atlantic
began to develop. A shallow sea extended into the western interior of
North America. At the beginning of the Mesozoic era in the northern
hemisphere Europe and Eurasia began to split from Gondwanaland. This
formed the Tethys Ocean and the highest world-wide sea-level during the
late Jurassic and early Cretaceous periods. This area is now covered by
the Mediterranean Sea.
With the separation and rotation of Africa towards Eurasia the Tethys
Ocean regressed bringing the orogeny of the Alps, Rockies and Andes.
The Devonian period was during the Palaeozoic era and part of the great
northern continent of Laurasia which included North America, Greenland
and Europe combined. The other great continent in the south included
India, Africa, South America, Australia and Antarctica. Whilst Laurasia
was in equatorial heat during the Carboniferous period Gondwanaland was
lying near the South Pole covered with ice glaciers 280 million years
ago. During the lower Permian period North America, Europe and Asia
joined Gondwanaland making one whole land mass called Pangea.
Britain's oldest rocks are the Lewisian gneisses, the Isle of Lewis.
They are exposed in the Hebrides and along Scotland's North-West coast.
Two separate orogenies known as Scourian, 2,700 million years ago and
Laxfordian, 1,800 million years ago. The gneisses must have come from
even older rocks to provide sediments to metamorphose.
The Alpine orogeny took place about 25-30 million years ago when the
African plate moved northwards but Britain was only on the fringe and
the centre was in southern Europe.
The Himalayan orogeny brought India into contact with Asia. Where the
Mediterranean Sea is now there was an earlier sea call the Tethys.
When an oceanic plate meets a continental plate it thrusts beneath the
lighter continental and is subducted into the mantle. As the Atlantic
began to open during the Tertiary period 65 million years ago Britain
was placed near a proper ocean or an active plate margin with sea-floor
spreading and subduction zones. A sea is a water-covered continental
land mass such as the North Sea.
The San Andreas Fault is between 15 and 20 million years old and is
still active along the edge of a conservative plate.