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
Smoo Cave behind after dissolution of the surrounding dolostone. The cave was formed along two geological lines of weakness by a combination of erosion from the sea and an inland underground stream which has formed the innermost chambers. Upstream of the Allt Smoo which runs into the cave, impermeable quartzites have been faulted against the Durness Limestone, causing the stream to sink down into the carbonate rock soon after it has crossed the contact between the two different rock types. The cave is unique within the UK in that the first chamber has been formed by the action of the sea, whereas