DR MORLEY READ / SCIENCE PHOTO LIBRARY DR MORLEY READ / SCIENCE PHOTO LIBRARY
Volcanic plug in the crater of the extinct Pululagua volcano on the western slopes of the Andes, Ecuador. A volcanic plug is formed when lava solidifies in the vent of an extinct volcano. Over time the less resistant rock of the volcano's cone erodes, leaving just the plug. This volcano, with a crater measuring 3 kilometres in diameter, has been extinct for roughly 2500 years.
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