The famous igneous rock formations at Peggy’s Cove are mostly granites and granodiorites which were formed in the middle to late Devonian Period of the Palaeozoic Era – that is about 385 million years ago. More specifically, they comprise coarse-grained biotite-muscovite granite cut by dykes of pegmatite and fine-grained sugary-textured granite called aplite. They are part of the South Mountain Batholith of Nova Scotia.
The rock is mainly composed of clear crystals of the mineral quartz, white crystals of feldspar, and biotite crystals which are black. Feldspar also comes in the form of pink crystals – and when both pink and white crystals are present, the white tend to be plagioclase and the pink to be orthoclase. Granodiorites have a higher proportion of the pink plagioclase to the white orthoclase form of feldspar.
The rock of Peggy’s Cove formed 15 km beneath the Earth’s surface as a mass of molten magma when a temperature of 800 °C was generated by the collision of two ancient landmasses called Meguma and Avalon. That collision caused Meguma to fold like an accordion and become forced on top of Avalon to create a mountain range similar to the Himalayas. All the pressure and heat that built up beneath the mountains as a result of these Earth movements, converted solid rock to semi-liquid magma. As the magma rose slowly upwards through the Earth’s crust, it cooled and crystallised into a mass of solid granite that remained buried until millions of years or erosion gradually removed the overlying 15 km of rock and revealed it as we see it today.
Two of the photographs (9 & 10) in the Gallery below show large inclusions known as xenoliths. These are fragments of rocks that fell into the molten magma from the rocks that surrounded the magma chamber, becoming partially melted themselves and incorporated into the newly-forming granite.
A geological guide to Peggy’s Cove, Nova Scotia [http://www.novascotia.com & http://www.peggyscovearea.com]
Nova Scotia Geological Highway Map ISBN 0-9737982-5-4
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