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Preparation
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In the previous investigation, students discovered that living things can die and decay. Soft parts are more likely to decay than hard parts. For this reason, the most common fossils are bones, teeth, shells, and the woody stems of plants. For a fossil to form, an organism must be buried quickly so that any oxygen is cut off and its decay slows down or stops. In this investigation, students continue to think about how fossils form. Fossils are rarely the original unchanged remains of plants or animals. Fossil formation begins when an organism or part of an organism falls into soft sediment, such as mud. The organism or part then gets quickly buried by more sediment. As more and more sediment collects on top, the layer with the organism or part becomes compacted. Minerals from water then move into the pore spaces between sediment particles. The sediment cements together and becomes rock with the organism or part inside. Sometimes, open pores in the rock let water and air reach the organism or part, causing it to decay or dissolve. What is left behind is a cavity in the rock where the organism or part was. This empty space is called a mold. A mold shows the original shape and surface of the organism or part. Sometimes, sand or mud fills a mold and hardens, forming a cast of the original organism or part. A cast is a replica of the original organism. Petrification happens when mineral solutions remove the original organism or part and replace it with new minerals. The replacement of the original materials is generally a very slow process. The result is a nearly perfect mineral replica of the original organism or part. In this investigation, students simulate how fossil molds, fossil casts, and petrified fossils form. They make a mold by pressing a shell or bone into clay. A mold is left behind after the shell or bone is removed. Students then fill the mold with plaster of Paris, which dries and forms a cast of the shell or bone. Finally, students place sponges into Epsom salt solutions. They observe as the solution travels through the sponges. After the sponges have dried, they see salt crystals that formed inside the holes of the sponges, forming a petrified fossil of the sponge.
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Images to be viewed by the class: For the instructor:
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Use disposable craft sticks for mixing the Plaster of Paris. Be sure to mix the Plaster of Paris yourself. Do not pour unused portions of Plaster of Pairs, or water mixed with plaster of Paris into the sink or drain. Dispose of them in the trash instead. Students should wear safety impact goggles when cracking the hardened Plaster of Paris. Monitor students around the hot water to make sure they do not burn themselves. The hot water should not be boiling. Review the investigation for your specific setting, materials, students, and conventional safety precautions. |
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