Morphology - Shape and Structure
Stromatolites can form into 1 of 2 forms:
During the Proterozoic stromatolites within each of these 2 forms would range from low relief mounds, bulbous masses or long slender columns with branching structures.
*Stromatolites reached their peak of diversification during the Proterozoic before grazing dwindled their numbers. Modern stromatolites are not this diverse.
*Stromatolites reached their peak of diversification during the Proterozoic before grazing dwindled their numbers. Modern stromatolites are not this diverse.
What causes their different shapes?
The reason why we have two main stromatolite shapes is because of water energy. If the water energy is low then a columnar stromatolite will develop. If water energy is high, a dome shaped stromatolite will develop.
A handful of other variables can affect their growth and shape as well:
A handful of other variables can affect their growth and shape as well:
- Amount of sediment supply
- Proximity to photic zone (i.e. the deeper they are in the water, the less photosynthesis they can perform)
- Amount of oxygen available
Lamination - The Inside of a Stromatolite
On the outside stromatolites look like any ordinary rock, but if we look at the inside we see a succession of layers that were made by the cyanobacteria. These layers can vary in shape but once the first layer is made every layer on top of that will follow a similar form. In other words, if a stromatolite begins as a mound, it will stay a mound . If it starts out with a coney shape, it will stay a coney shape. A stromatolite's lamination structure is concentric. Because of this, stromatolites cannot start out with dome shaped layers and then sprout into a cone shape.
If we take a closer look at the layering, you can see that the layers are not exactly uniform. Some sides of a layer may be "wavier" than the other or more "bumpy". The reason for this is due to the fact that cyanobacteria are actual living bacteria. They do not focus on making even layers, they just try to get the most sun and this can result in slightly uneven growth rates around the stromatolite.