This summer I visited Newfoundland. I’d heard lots about its
attractions - people, music, scenery, food, icebergs, whales, puffins.
But I was surprised at what a world-class fascinating place it is for geology. Gros
Morne National Park, on the west coast, is a case in point.
One of the landmarks in the area is the Tablelands mountain
range, a reddish lump of rock over 700 meters high, and totally barren of plant
life.
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Part of the Tablelands mountain range.
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For once, the lack of vegetation isn’t because humans
cleared it all. It’s because the rock itself is toxic to plant life. The area
is a UNESCO-designated World Heritage Site because it’s unique. The Tablelands
consist of rock from the earth’s mantle. This was discovered by geologist Bob
Stevens in the 1970’s and it was an important factor in the acceptance at the
time of the tectonic plate theory.
The earth is made up of layers: core, mantle and crust.
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Cutaway diagram of Earth's internal structure (to scale)
with inset showing detailed breakdown of structure (not to scale)
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Everywhere (almost) the mantle is covered by the crust. The
Tablelands is unique in exposing a large chunk of the mantle. The mantle contains
a lot of metals: iron (accounting for the rust-red colour), magnesium, chrome,
mercury, platinum, nickel. And that's why it's toxic to plants.
Over time, the effect of water on the rock creates
serpentinite, a lovely texture like – of course – a snake skin.
Tectonic Plates
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| This map shows 15 of the largest plates. |
This theory was hotly disputed as late as the 1970’s but has
now been generally accepted. The earth’s crust consists of a number of plates which
slowly float around on the surface of the mantle. Occasionally (very
occasionally) they crash into each other and then drift apart again. Right now
the North American plate is moving North-West at a rate of about 2.3 cm (not
quite an inch) every year.
The plates don’t necessarily stay intact through this
process. Newfoundland was formed by two plates colliding. They were more or
less the earlier versions of the North American and Eurasian plates. When they
drifted apart, the European bit remained stuck to the (new) North American
plate. So the Western half of Newfoundland is geologically similar to the rest
of North America, and the Eastern half is similar to Ireland in the Eurasian
plate.
