Curling stone used at the Olympics. The two different granite varieties making up the stone can be … [+]
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Ailsa Craig is a small island in the Firth of Clyde, an inlet along the west coast of Scotland. It has a troubled history. Claimed by the Scots, invaded by the Spanish Empire, it was once a save heaven for smugglers and hermits. After the lighthouse of the island, built between 1883 and 1886, was automated, the island became uninhabited and may of little interest nowadays, if not for its peculiar geology and the strange Olympic sport known as Curling. Since 1851, Ailsa Craig provides the rocks for the best curling stones in the world
Geologically speaking, the island is all what remains of a long extinct volcano. Sixty million years ago a volcano erupted on the supercontinent of Pangaea, feed by a magma plume rising up in Earth’s mantle. The plume ripped apart Pangaea, separating Greenland from Scotland, and opening the primordial Atlantic Ocean. As the ocean basin became wider, the volcano was cut off from its magma source. After the eruption ended, erosion removed the volcanic edifice, exposing the former magma chamber beneath the volcano.
The island of Ailsa Craig.
Wikipedia/Johnny Durnan/CC BY-SA 2.0.
The geology of Ailsa Craig comprises three types of magmatic rocks, or more specific three granitoid-varieties, characterized by their mineral content. The Ailsa Craig Common Green, the Ailsa Craig Red Hone, and the Ailsa Craig Blue Hone. The distinctive coloration of the latter rock, a grey-white spotted with bluish dots, results from the presence of riebeckite and arfvedsonite, two amphibole minerals rich in sodium and iron. Riebeckite-granite is an extremely rare granite variety and Ailsa Craig is the only spot worldwide where it can be found in large quantities. Ailsa Craig Red Hone is a coarse-grained variety of the fine-grained riebeckite-granite, named after the large crystals of reddish feldspar in the rock. Almost the entire island is composed of this type of granite, crossed by a few dikes of olivine-dolerite. The olivine-dolerite is known as Ailsa Craig Common Green, named after the olivine, green silicate crystals rich in magnesium and iron, making up the rock. The riebeckite-microgranite and olivine-dolerite are used for the production of curling stones. Almost 70 percent of all curling stones used worldwide come from here.
The uniform, fine-grained texture of the riebeckite-microgranite lends the rock a high elasticity. Curling stones made from Ailsa Craig granite display a high resistance to impacts, as experienced during a curling game. The minerals composing the granite, mostly quartz and feldspar, have a low thermal conductivity. This is important, as a too cold or too hot surface would negatively affect the thin water film on which the curling stone slides. Granite is also very resistant towards erosion and weathering, and will not change its properties over time.
