2: Orbicular Rock

Orbicular rocks have orbicules, i.e. spheroidal structures, within the even-grained matrix. The orbicules have a core made of country rock fragments or concentrations of mineral grains. The diameter of the orbicules varies from one centimetre to almost half a metre. The orbicules are unique and often flattened or otherwise irregular. Orbicular rocks were formed from magma deep underground. Conditions must be exceptional at the rock’s moment of creation. They may entail the fast cooling of molten rock, a fast-flowing lava stream, or a lack of oxygen. A competing theory suggests that orbicular rocks formed during the solidification phase of the rock due to the seepage of potassium. The majority of orbicular rock occurrences are boulders whose source, the bedrock they broke away from, is unknown. Finland has around twenty known bedrock occurrences; for instance, the orbicular rock deposits in Viitasaari and Maaninka, which are now protected.