Formation of Rhyolite

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Rhyolite is a kind of igneous rock formed from the cooling of ash and lava on the ground surface. 

About 140 million years ago, a volcano near Sai Kung erupted violently. Acidic ash and lava cooled quickly and formed the rhyolite of the North Ninepin Island. 

Hexagonal columnar joints were formed during the rapid cooling.

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