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BASALT - A VOLCANIC ROCK | |
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The Central Coast is predominantly sedimentary rock: sandstones and shales. But in late Jurassic time, about 150 million years ago, there were some extrusions of volcanic rock - basalt.  DYKES These occur as tubes of basaltic dolerite that flow up through the surface rock beds. DIATREMES These are the 'roots' of short-lived explosive volcanoes, and are not associated with a volcanic cone. Hot lava flows up from deep below the earth's crust and explodes violently when forcing its way through the surface rocks. The lava is shattered by the explosion into small pieces, known as breccia. This is popularly called 'blue metal'. There is a good example of a diatreme in a deep gully on private land behind Woy Woy Tip near Brisbane Water National Park. BASALTIC LAVA FLOW At Kulnura, north west of Gosford, there is an extensive lava flow of black basalt. When this basalt pushed through in Jurassic time it was hot, but as it cooled slowly it formed columnar basalt. This is quarried at Kulnura, crushed and used as 'blue metal'.  
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