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Lava rocks!

Two types of lava can form from the same volcano. These samples came from an eruption in Antarctica that occurred about 25,000 years ago. They were collected by WHOI geoscientists Adam Soule and Mark Kurz and Joint Program student Andrea Burke during a Polar Discovery expedition in 2007.The kind of lava on the left is called pahoehoe (rhymes with “joey-joey”). It is smooth on the surface. Inside, it is riddled with round bubbles formed by trapped gas that was trying to escape the molten lava. Bubbles near the surface are small because the rock cooled quickly there. Deeper down, the rock stayed hot longer and the bubbles grew bigger. The kind on the right is called ‘a’a (“Ah! Ah!” as you might say if you tried to walk on it barefoot). It is spiky on the surface. Inside, the hot lava was so stiff that few bubbles could form.

(Photo by Chris Linder, Woods Hole Oceanographic Institution)