Basalt is one of the most common igneous rocks and is major constituent of the upper layer of the ocean floors (usually as pillow lava), and hot spot volcanoes (such as the Hawaiian islands). It is a finely crystalline basic volcanic rock which is sometimes glassy. It is recognized primarily by its dark colour and are essentially composed of plagioclase feldspar and pyroxene. Vesicular varieties are typically terrestrial, forming either on volcanic islands or on the continents. When cooled slowly it forms larger crystals and called dolerite or gabbro.