October 16 is National Fossil Day

How are fossils formed? For fossil formation to take place, a series of fortunate events must occur. If any part of the series is missing, we will never see the fossil! In fact, fossilization is a rare occurrence. Nature tends to lean toward recycling itself, which includes just about everything from plants and animals to rocks and minerals.

Animals, dead or alive, are usually food for other animals. From insects to dinosaurs, an animal could be someone’s lunch! Any part of the animal’s body that isn’t consumed is usually scattered about, and these leftovers make great food for bacteria. In addition, these leftovers are exposed to the elements: sun, rain, and even the soil itself all help to breakdown and decompose the sturdiest of bones, shells, and wood.

If we are ever going to see a fossil, some very specialized events must intervene to ward off the natural process of decomposition. This is the fossilization process known as permineralization.

The following is the most common scenario for fossil formation:

To start with, an animal, insect or plant must die in water or near enough to fall in shortly after death. The water insulates the remains from many of the elements that contribute to decomposition.

As time passes, sediments bury the exoskeleton. The faster this happens the more likely fossilization will occur. Land and mudslides help. River deltas are also good for the quick accumulation of sediments, which further insulates the fossil from decomposition.

The sediments themselves have a huge influence on how well the fossil turns out. Very fine-grained particles, like clays, allow for more detail in the future fossil. Coarse sediments, like sand, allow less detail to show.

As the sediments continue to pile on, the lower layers become compacted by the weight of the top layers. Over time, this pressure turns the sediments into rock. If mineral-rich water percolates down through the sediments, the fossilization process has an even better chance of preserving our ancient animal or insect. Some of the minerals stick to the particles of sediment, effectively gluing them together into a solid mass. Over the course of millions of years, this compacted mass of rock can dissolve away and replace the outer shell forming the fossil’s shape.

As the continental plates move around the earth, crashing into each other, mountains are formed. Former sea floors are lifted up and become dry land. Thanks to the movement of the plates, fossils that were once buried under hundreds to thousands of feet of rock come closer to the surface and await discovery!

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