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Basalt from Ethiopia - the columnar jointing is a result of slow cooling, allowing to distribute contraction fractures to arrange in a hexagonal columnar pattern, the geometry that requires the least energy to provide the necessary space when the rock slowly contracts. Rhyolite from Milos island, Greece, erupted approx. 84,000 years ago. Rhyolite is the extrusive equivalent to the plutonic rock type granite - both form from the same magma. Rhyolite lava flows are very viscous. Due to their high viscosity, they only move through laminar flow along sheer planes that form where gas bubbles concentrate. These flow structures are usually visible in the cooled rock. They are also at the Greek origin of the name of the rock, stemming from rheo ("flowing") and lithos ("rock")