1.8 Billion Years Ago
THIS STONE is a piece of the oldest rock formation in Ireland, known as the Annagh Series, from the coast of north Mayo. About 1.8 billion years ago, this rock solidified at the root of a mountain range. By this time there had been life on earth for 1.7 billion years, and photosynthetic organisms were gradually building up free oxygen in the atmosphere.
After this rock was formed, it would take another 1,270 million years for multi-cellular life forms to appear. That will be the subject of the next station.
THE ANNAGH SERIES ROCKS started as granite, which cooled slowly at the root of an ancient mountain range. Like the mountains of today, it was formed when two plates of the earth’s crust collided. The mountains have long since eroded away and the granite has been squeezed at immense temperature and pressure by later geological events, which have metamorphosed it into gneiss (pronounced ‘nice’).
Rocks identical to the Annagh Series gneiss have been found in Nova Scotia, Canada. They are in fact part of the same formation, which was split in two by the break up of a former continent to form the Atlantic Ocean. This started relatively recently in the Jurassic period, a mere 200 million years ago. North America and Europe are still drifting apart at a rate of a few centimetres every year.
Microorganisms had been living in the oceans for over a billion years when this rock was formed, where they play an important part in the process of continental drift. A rain of dead plankton falls to the ocean floor, where it forms a layer of fine mud or ‘ooze’. This ooze lubricates the oceanic plate where it slides under continental margins, as it does beneath the Andes in South America. As it does so, it carries a great deal of water with it. This water reacts with the basal rocks of the continents and causes the rock to melt at much lower temperature. It then flows upwards through cracks in the crust and erupts in the form of volcanoes.