PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY
Fossil stromatolites. Cross-section through a rock, showing circular structures that are fossilised stromatolites. The rock is quartz, with inclusions of kerogen (black), a fossil fuel-like deposit of organic origin. Each of the circular layers began as a covering of blue-green algae (cyanobacteria), which captured grains of sediment and became mineralised. As more layers were added over thousands of years, a stromatolite, or rocky mound, gradually formed. Fossilised stromatolites have been found within rocks as old as 3.5 billion years. This sample is from the Barbeton Greenstone Belt, South Africa, and was photographed in 1999.
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