Basalt is a fine-grained dark coloured volcanic rock. The crystals in it are so small that they cannot be seen with the naked eye. These crystals are small because the rock has cooled down quickly from molten lava and the crystals did not get time to grow (unlike granite).
Basalt is often found in volcanic cones or in lava plateaus . Volcanic cones are formed above vents, or pipes, which go down into the earth's crust. Plateaus, such as the Antrim Plateau shown on this map, were formed above fissures or enormous cracks in the earth's crust. In this case the fissures opened up when the North American plate started to move away from the Eurasian plate and the Atlantic Ocean began to open up in the Tertiary Period, about 60 million years ago
Some basalt, especially in the top of an eruption, had small air bubbles called amygdales. These little bubbles eventually had other crystals deposited in them to give white speckles in the basalt. This is called amygdaloidal basalt.
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