ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY
Quartzite rocks covered in lichens. Quartzite is a metamorphic rock whose colour ranges from white (pure quartz, seen here) to black. It is formed by the metamorphism of a pure sandstone, the original quartz (silicon dioxide) grains being recrystallized and interlocking. Lichens are specialized organisms comprising a mutualistic symbiosis between a fungus and an alga. Lichens are highly sensitive to pollution, the presence or absence of certain lichens are used as indicators of pollution levels in the atmosphere. Photographed in British Columbia, Canada.
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