IN SEARCH OF SNOWBALL EARTH
The author
of this piece is not a geologist but is fascinated by the story of the Earth
told by looking at its rocks. Geologists should read no further but should
consult "A Guide to the Geology of Islay " (published by Ringwood Publishing)
by David Webster, Roger Anderton and Alasdair Skelton, all skilled and
experienced geologists with a special knowledge of Islay.
About 650
million years ago, the evidence of the rocks indicates that the whole or most
of the Earth was encrusted with ice. Exposed rocks of that age in various parts
of the world offer the evidence. One of those places is Islay the southernmost
Scottish island facing the Atlantic Ocean.
If you go to
Islay by boat you may arrive at the harbour of Port Askaig, directly opposite
the southern end of the island of Jura. When you leave the boat, park your car
for a while in the car park and look upwards at the cliff facing you. In that
cliff is to be found evidence for “Snowball Earth”.
If you walk
over to the cliff and look carefully at the rocks in it, you will find the
evidence. These rocks are not volcanic. They are not lava or granitic. They
have no structure. Rocks which have been laid down over a long period on the
surface normally have done so in successive layers or beds. Often the beds can
be readily seen. If the rocks have spent long periods buried under the surface
of the Earth, they will have been exposed to heat and pressure which will have
altered the minerals which compose them. Often the pressure, which may result
from continental plate movement, will have resulted in the minerals arranging
themselves in lines at right angles to the pressure. When these rocks
ultimately return to being exposed on the surface, you can see these lines.
Geologists call this phenomenon "cleavage".
The rocks on
the Port Askaig cliff have no visible bedding or cleavage. Geologists can see
that this is typical of the detritus left by the movement of glaciers which
have melted leaving behind the detritus which they have accumulated. If the
glacier has left the land to float on the sea surface, this detritus will be
ultimately be deposited on the sea bed.
Geologists
also discover the age of many ancient rocks by measuring the radioactivity in
certain component minerals. The age of the Port Askaig rocks is about 650
million years.
Geologists
can also learn the approximate position of rocks when they appeared on the
surface of the earth. One way of doing this is by measuring the alignment of
crystals in the rocks relative to the magnetism of the earth. Such studies have
led geologists to conclude that about 650 million years ago Scotland was not
far from the South Pole. By continental plate movement, Scotland has proceeded
northwards to its present location over that unimaginably immense period of
time. At that time England was located on another distant continent They were
not to be united until more than 200 million years had passed.
If you
examine the cliff carefully you will see that it contains red stones of
different sizes. These stones are red granite. There is no red granite in or
near Islay, so the stones have come from somewhere else. 650 million years ago,
Scotland was physically nearer Scandinavia than it is now. Some believe that
these red granite boulders and pebbles could have come from there.
This
glacially deposited rock has come to be known as the Port Askaig Tillite. There
are other exposures of it in Islay, mainly exposed at or near the seashore on
the east coast. At a shoreline burn mouth about a mile south of Port Askaig, the tillite has been eroded by the sea.
Pebbles from it are lying on the stone beach and can be picked up and examined.
The rocks
exposed on the surface in Islay include formations which preceded the
glaciation and others which followed the glaciation. However unlikely this may
sound, surface exposure of rock formations deposited millions of years apart is
made possible by the lateral movement of continental plates tilting and folding
rock formations.
The rock
type which underlies central Islay is limestone which is older than the Port
Askaig Tillite. It can easily be seen on the surface because of the lush
vegetation which the calcareous soil produces.
The neighbouring quartzite rock
produces thin unfertile soil and vegetation. The sharp division between the
rock types is readily seen by the contrast between the green grass of the
limestone vegetation and the brown moor and heather of the quartzite vegetation.
This ancient
limestone is exposed in the hills near Loch Lossit. As the exposed rock becomes
younger, it alters gradually to tillite. The lower or earlier part of the
tillite contains fragments of limestone likely to come from the local limestone
bed rock. This can be compared with the red granite rocks in the older part of
the tillite found at Port Askaig.
You can also
see the rock formations which immediately followed the glaciation. Good
exposures are found at the shoreline between the Caol Isla and Bunnahabhain distilleries. These formations also comprise limestone.
After a long period of glaciation, water which has been trapped on land by the
ice is released into the sea, causing sea levels to rise. In turn this causes
deposits on the seabed of material which includes calcium carbonate, and
becomes limestone over long periods of time.
A feature of
these rock formations is Stromatolite Fossils. Stromatolites are a primitive
form of bacterial life which live on carbon dioxide, water and sunlight. They
discharge oxygen as a waste product.
Their large numbers caused large scale discharge of oxygen to Earth’s atmosphere
when it was oxygen poor, and allowed the development of more sophisticated
forms of life which required oxygen, and ultimately to human beings. Stromatolite
fossils are readily found in the shore rocks south of the Bunnahabhain distillery. They may have relevance
to the current search for life elsewhere in the solar
system.
Grateful
thanks to David Webster for two days of fascinating onsite education in the
lively autumn weather of Islay.
Walter
Semple
26th
October 2017.
When someone comments that this is probably the best AGM they have attended..ever!!! It is encouraging to feel that whilst such formal meetings can usually be a boring affair that no one wants to attend we have managed to make the ordeal light and fun to come along and enjoy.
