How Do Fossils Form

How Do Fossils Form

It’s an exciting feeling to see a fossil in a museum. But how are these fossils formed? What are the different ways that a fossil can be saved for us to see? Well, to answer this, there are four major ways in which a fossil can be preserved. They are: petrification, molds, impressions, amber and sedimentary fossils. Below, find a description of each:


Petrification occurs when a living object is slowly turned to stone of a huge number of years. Petrification is sometimes called “permineralization” because it is brought about mainly by minerals. Minerals seep through the organic matter is an object, filling it completely. Then the organic matter rots away, but a mineral version of the fossil is left. This process usually works best in the fossilization of trees. Some of the most famous petrified trees are in California, and contain huge rings that describe ancient eras.


Molds are literally molds of an animal. Sometimes animals became trapped in mud, dirt or clay. Then their bodies deteriorated, leaving behind their shape and size in the ground. When the hole created by this is discovered, it is known as a mold. A mold can be created in two ways. An organism can deteriorate and leave a hole showing details of its body. Or a hollow object, such as a shell, can become filled with matter. When the object deteriorates, the matter filling it is left behind as a mold.


Have you ever seen a dinosaur’s footprint? These are formed when mud, clay or silt containing an imprint made by an animal turns to stone. This is an example of an impression, or the impression that an animal leaves in soft matter. These fossils are useful in determining weight and structure of ancient animals. Sometimes, even toenails and pores can be seen!


Some fossils are preserved in amber. Amber is a sap-like substance from trees. It is sap that has dried over hundreds of years. Because tree sap is so sticky, it is possible for bugs and even small animals to be trapped within it. In time, the sap hardens to amber and a perfect specimen of the creature is preserved. Amber fossils are plentiful, and are sometimes worn as necklaces and bracelets today!

Sedimentary Fossils

The sea bed contains perhaps the most fossils on the earth. This is because the soft ground under the sea is made of sedimentary rock, or rock that is composed of layers of land. When sea creatures die, they drift to the bottom of the ocean and are covered with a layer of sand. In time, a volcano or mudslide, etc.,may cover the surface under which they are buried. In this way, a new layer is added, and the fossil is preserved in layers of time.

Therefore, fossils made in this way are sometimes referred to as “sedimentary fossils.” While there are many of these fossils, they are often very hard to get to. Often, they are dug from ground that was once underwater. In fact, fossils in sedimentary layers are useful in indicating when land was above and below ground.

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