Igneous rock the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. They are formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and fine-grained. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example. The molten rock, with
St. Michael's Cave receiving almost 1,000,000 visitors a year. The cave was created by rainwater slowly seeping through the limestone rock, turning into a weak carbonic acid which gradually dissolved the rock. Through this process, tiny cracks in The Rock's geological fault grew into long passages and large caverns over thousands of years. The numerous stalactites and stalagmites in the cave are formed by an accumulation of traces of dissolved rock deposited by water dripping from the ground above. In 1974 a Neolithic bowl was discovered in the cave, one of many examples which prove that the cave was known to prehistoric humans.