Stromatolites are layered rock-like structures formed by the metabolic activity of microorganisms. These formations are created through the trapping, binding, and cementation of sedimentary grains by vast colonies of microbes, most notably cyanobacteria. The name itself comes from the Greek words for ‘layered’ and ‘rock,’ which accurately describes their appearance. They are considered organo-sedimentary structures, meaning they are a product of both biological processes and the deposition of sediment.
These structures represent some of the most ancient records of life on Earth, with fossilized examples dating back approximately 3.5 billion years. Found in the rock record on every continent, they provide a window into the planet’s distant past. The study of both ancient and modern stromatolites offers insight into the early evolution of life and the development of Earth’s ecosystems.
The Formation Process
The creation of a stromatolite begins with a microbial mat, which is a multi-layered sheet of microorganisms, primarily composed of bacteria and archaea. These mats form on submerged or moist surfaces, held together by slimy substances secreted by the microbes. Within these communities, cyanobacteria play a prominent part. These microbes use photosynthesis to convert sunlight into energy.
The sticky, filamentous cyanobacteria trap fine sediment particles that are suspended in the surrounding water. This newly deposited layer of sediment would block sunlight, so the microbes migrate upward through the fresh sediment to reestablish a new living layer on the surface. This continuous upward movement is a response to the dual needs of avoiding burial and accessing sunlight for photosynthesis.
Over extended periods, this cycle of sediment trapping and microbial migration builds layer upon layer. The structure is solidified through a process of cementation. The metabolic activities of the microbes can alter the water chemistry around them, causing minerals like calcium carbonate to precipitate out of the water and bind the sediment grains together. This process hardens the layers into the solid, laminated rock structure of a stromatolite.
Historical Significance
The historical importance of stromatolites is tied to their ancient origins. These structures are the earliest widespread, macroscopic evidence of life on the planet. For billions of years, during the Archean and Proterozoic eons, stromatolite-building microbes were dominant life forms, shaping the geology and biology of the early Earth. Their abundance in the fossil record peaked about 1.25 billion years ago.
The photosynthetic activity of the cyanobacteria within stromatolites drove a profound change to the planet’s environment. Photosynthesis releases oxygen as a byproduct, and for billions of years, these microbial communities steadily produced enormous quantities of it. Initially, this oxygen was absorbed by dissolved iron in the oceans, creating massive geological deposits known as banded iron formations. This process prevented oxygen from accumulating in the atmosphere for a long time.
Once the oxygen sinks in the oceans and on land were saturated, free oxygen began to accumulate in the atmosphere around 2.4 billion years ago. This episode is known as the Great Oxidation Event, and it fundamentally transformed Earth’s atmosphere from one devoid of oxygen to one containing it. This atmospheric shift was detrimental to the anaerobic organisms that had dominated the planet, but it set the stage for the evolution of new, more complex life forms that could utilize oxygen for respiration.
The proliferation of stromatolites eventually declined. Their decrease in the fossil record coincides with the appearance of more complex, grazing organisms in the Cambrian period. These new life forms fed on the microbial mats, disrupting the delicate process of layer formation. The rise of these grazing creatures restricted stromatolite formation to specific environments where predators could not survive.
Modern and Fossil Examples
Today, living stromatolites are rare and are found primarily in harsh environments where the conditions prevent grazing animals from surviving. A well-known location is Shark Bay in Western Australia, a hypersaline marine environment. The high salt content of the water deters most creatures that would otherwise consume the microbial mats, allowing the stromatolites to grow undisturbed. These modern examples appear as dome-shaped, rock-like structures rising from the shallow waters.
Another significant location for modern stromatolites is Cuatro Ciénegas, a desert basin in Mexico known for its unique spring-fed pools with highly variable water chemistry. Some living examples can also be found in specific freshwater locations or other saline lakes around the world.
Fossilized stromatolites are found globally and offer a direct look into Earth’s deep past. A well-known fossil site is the Gunflint Chert, located along the northern shore of Lake Superior in North America. The rocks here preserve stromatolites from the Paleoproterozoic Era, showcasing the distinct, finely layered patterns that characterize these ancient structures.
These fossilized forms can vary in shape from flat mats to large, complex columns and domes that can be meters in diameter. When cut and polished, these fossils reveal the intricate laminations created by the cyclical growth of the microbial mats billions of years ago. The preserved structures in locations like the Pilbara region of Western Australia are among the oldest, providing a connection to the earliest life on our planet.