Stromatolite

Tiny living things make these rocks.

They live in shallow water. These tiny life forms make a sticky mat. This mat traps sand and bits of rock.

The sand sticks to the mat. Then, more layers grow on top. This makes the rock look like it has stripes. These rocks can grow to be very tall. They tell us about life from a long time ago.

Some of these rocks are very old.

Stromatolites are layered rock structures.

They are made by tiny living things. These include cyanobacteria, which are small life forms that use sunlight for food.

These tiny life forms make a sticky mat. This mat traps sand and other rocky bits. The bits stick to the mat. Over time, more layers grow on top. This creates a striped look called lamination.

Stromatolites can grow to be a meter or more tall. Some are even found in very deep water.

They are very important to science. They give us a record of the oldest life on Earth. Many fossilized stromatolites are billions of years old. They were very common long ago. They reached a peak about 1.25 billion years ago. Later, they became rare. This might be because new animals began to eat them. Today, you can find living ones in salty water. This salt helps protect them from being eaten by animals.

They are also found in some fresh water.

Stromatolites are layered rock structures that tell a story about Earth's history.

These formations are created by tiny living things called microorganisms. Some of these include cyanobacteria, which use sunlight to make food. Other microbes like sulfate-reducing bacteria also help build them. These tiny life forms live together in sticky mats. These mats are called biofilms. As these mats grow, they trap sand and other rocky bits. This creates the layered look we see today.

The way they grow is a slow, step-by-step process. First, the microorganisms produce sticky compounds. These compounds act like glue to hold sand and minerals together. This forms a thin layer called a microbial mat. Over time, new layers of sediment settle on top of the old ones. The calcium carbonate in the water can cement these layers into limestone. This building up of layers is called accretion. Because they need sunlight, many stromatolites grow upward in dome shapes.

Scientists use fossils to study how life began. Many fossilized stromatolites are billions of years old. They were a major part of the fossil record for the very first life on Earth. These structures reached a peak about 1.25 billion years ago. After that, they became much less common. By the start of the Cambrian period, they had fallen to only 20% of their peak. Some researchers think this happened because new animals began to eat them.

There are many different shapes that stromatolites can take. Some look like cones, while others are shaped like columns or branches.

You can find them in many places around the world. Modern living stromatolites are often found in very salty water. One famous place is Hamelin Pool in Shark Bay, Western Australia. The high salt levels help protect them from being eaten by animals. You can also find them in freshwater places like Lake Van in Turkey.

Stromatolites are different from other structures called thrombolites. While stromatolites have clear layers, thrombolites have a clotted texture. You might also hear the name oncolites. These are rounded, layered structures that are similar to stromatolites. Even though they look like rocks, they are actually made by living colonies. They show us how tiny cells can work together to change a whole planet. By making oxygen, cyanobacteria helped prepare Earth for bigger living things.

Stromatolites are layered, biochemical structures formed in shallow water. They are a type of microbialite, which is a rock structure built by microorganisms. These formations are created through the trapping, binding, and cementation of sedimentary grains. This process happens within biofilms, specifically microbial mats. These mats are produced by various microorganisms, most notably photosynthetic cyanobacteria. Other microbes, such as sulfate-reducing bacteria and Pseudomonadota, also contribute to their formation.

The growth of a stromatolite is a precise, step-by-step biological process. First, microorganisms like cyanobacteria produce adhesive polysaccharides. These sticky compounds form a layer over the microbial mats. As water moves over these mats, debris and sedimentary grains become trapped in the sticky layer. Next, calcium carbonate in the water acts as a cement. This mineral binds the trapped particles together into thin layers called laminae. Over long periods, these layers accrete, or build up, to create the characteristic banded pattern.

Stromatolites exhibit many different shapes, known as morphologies. They can appear as conical, stratiform, domal, columnar, or branching structures. The domal shape is particularly common. This vertical growth is necessary for the organisms to reach sunlight for photosynthesis. Scientists also distinguish stromatolites from thrombolites. While stromatolites have a clear, layered lamination, thrombolites have a clotted, non-layered internal texture. Another related structure is the oncolite. These are layered, spherical structures that can range from a few millimeters to several centimeters in size.

In the history of Earth, stromatolites are vital records of ancient life. They are major constituents of the fossil record from the Precambrian era. These structures reached a massive peak in abundance about 1.25 billion years ago. However, their numbers declined significantly after that time. By the start of the Cambrian period, they had fallen to only 20% of their peak abundance. One theory suggests this was due to the "Cambrian substrate revolution," where new grazing creatures began eating the microbial mats. Another hypothesis involves protozoa, like foraminifera, which may have disrupted the mats through bioturbation.

Modern stromatolites are actually quite rare. Most living examples are found in hypersaline environments, such as marine lagoons or salty lakes. High salinity levels are important because they prevent animals from grazing on the mats. A famous example is the Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia. In these extreme conditions, specific organisms like the archaeon Halococcus hamelinensis thrive. This organism has special genes to repair DNA damage caused by intense UV radiation.

Beyond salt water, stromatolites also exist in freshwater settings. In Turkey, large microbialite towers up to 40 meters high have been discovered in Lake Van. These towers are made of aragonite and grow from the precipitation of calcite. In Mexico, the Laguna de Bacalar features extensive living microbialites. These can form reef-like beds that rise several meters vertically. Other freshwater examples include Lake Salda in Turkey, where the structures are made of hydromagnesite.

The impact of stromatolites on our planet is profound. Cyanobacteria within these mats were instrumental in changing Earth's atmosphere. Through continuous photosynthesis, they used sunlight, water, and carbon dioxide to create food. This process released oxygen into the primeval atmosphere. This phenomenon is known as the Great Oxygenation Event. By priming the environment with oxygen, these tiny organisms helped pave the way for the evolution of more complex eukaryotic life.