What Is a Stromatolite? Rock Built by Slime Over Three Billion Years
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Layered mounds of rock found in ancient sediments were built by mats of microorganisms trapping sediment and growing upward through it, one thin layer at a time. They are among the oldest evidence of life on Earth and their makers changed the atmosphere permanently.
How a mat becomes a rock
The building process is slow and simple. A film of microorganisms, dominated by cyanobacteria, grows on a submerged surface and produces a sticky matrix that traps and binds sediment particles settling from the water. Buried by that sediment, the photosynthetic organisms migrate upward towards the light, establishing a new surface film, and the cycle repeats. Over time this produces a laminated structure of alternating organic and sediment-rich layers, which lithifies into rock preserving the layering. Growth rates are extremely slow, on the order of a fraction of a millimetre a year in modern examples, so a mound half a metre high may represent thousands of years of continuous growth. The resulting shapes vary from flat sheets to domes, columns and branching forms, and the shape reflects the environment, since stronger currents, sediment supply and water depth all influence how the mat grows. Similar structures built by mats that trap without producing distinct lamination are called thrombolites and have a clotted rather than layered internal texture.
What they tell us about early life
These structures carry a substantial share of what is known about the first three billion years of life:
- •They appear in rocks around three and a half billion years old, making them among the oldest widely accepted evidence of life, although the very oldest claimed examples are disputed because non-biological processes can produce similar layering
- •They dominated shallow marine environments for over two billion years, a period in which they were the most conspicuous structures on the planet
- •Their makers carried out oxygenic photosynthesis, releasing oxygen as waste, which accumulated and transformed the atmosphere in the Great Oxidation Event around two and a half billion years ago
- •That oxygen was catastrophic for most existing organisms and made all subsequent complex life possible, which makes this the largest pollution event in the planet's history and the precondition for everything since
- •Banded iron formations, the source of most of the world's iron ore, were deposited as that oxygen reacted with dissolved iron in the oceans
- •Their abundance declined sharply from around a billion years ago, most plausibly because grazing animals evolved and began eating the mats
Where they still grow
Living examples persist in a small number of places, almost all of them hostile to the animals that would otherwise consume them. The best known is Shark Bay in Western Australia, where a shallow bay with restricted circulation has salinity roughly twice that of normal seawater, excluding grazing invertebrates and allowing mats to grow undisturbed into domed structures that have become a protected site and a tourist attraction. Similar communities occur in hypersaline lagoons in the Bahamas, in alkaline lakes in Mexico and East Africa, in hot spring outflows in Yellowstone and elsewhere, and in some Antarctic lakes. They are studied intensively as analogues for ancient conditions, and the comparison has limits, since modern examples grow under specific extreme conditions while ancient ones grew in ordinary shallow seas, so the modern communities are refuges rather than survivals of the original ecology. They are also fragile, and several sites have been damaged by trampling, vehicle access and changes in water chemistry from nearby development.
Why the oldest claims are contested
Identifying the earliest life is genuinely difficult because the structures in question are simple and several non-biological processes can mimic them. Layered and domed textures form by purely chemical precipitation in hot spring and evaporite settings, and deformation of sediment can produce convincing wrinkles and columns. The oldest claimed examples, from rocks in Greenland and Australia, have been argued over for decades, with papers alternately supporting and disputing a biological origin for the same specimens. The criteria used to evaluate a claim include the geological context, whether the shapes are consistent with growth towards light, the presence of organic carbon with an isotopic signature suggesting biological fractionation, and microscopic textures consistent with trapped sediment. None of these is individually conclusive, which is why the question remains open and why the announced date of the earliest life shifts as evidence accumulates. That uncertainty matters beyond Earth, since similar structures are among the features that missions to Mars are looking for.
The takeaway
Microbial mats trap settling sediment, then grow upward through it towards the light, laminating rock at a fraction of a millimetre a year. Their makers released the oxygen that transformed the atmosphere, which was lethal to most existing life and the precondition for everything complex. They declined when grazing animals evolved, and survive today mainly in water too salty for grazers. The oldest claimed examples remain contested because chemistry mimics them.