Stromatolites on early Earth, illustration. Stromatolites are mineralised microbial communities formed from cyanobacteria (blue-green algae). They are formed over thousands of years as the cyanobacteria trap detritus and sediment, forming large living rafts known as microbial mats. The cyanobacteria also secrete calcium carbonate, which causes the mats to mineralise, forming rock-like structures. Stromatolites are known as living fossils, because the process of mat formation and mineralisation continues today.