Activity 4: Molds & Casts
LETS MAKE A FOSSIL #2
Fossils are remains of life from the past. The most common kinds of fossil preservation are molds and casts. The activity will give students the chance to make modern day molds and casts.
Instructional Method: Activity
Goal: To teach students two common fossil types; a mold and a cast.
Objectives: Students will be able to:
Setup: 45 min
Fossils are ancient remnants of once living things. They are our only link to the living past. They tell us of ancient environments and organism habits. Many times they are used to help distinguish periods of geologic time. The absence of fossils in rock can make it difficult to be sure what took place in the past.
Fossils can range in size from 6-foot leg bones to microscopic animal shells. Most fossils we find today are of an organism's hard parts. Hard parts give scientists an idea of how strong the animal was, what may have been feeding on it, what it ate, the speed of the organism, etc. Hard parts are what most people think of as fossils. In reality they are the most rare. They consist of shell, bone, scales, leaves, wood, teeth, claws and horns.
An organism's soft parts are less commonly preserved, but when they are they can tell scientists a lot about the organism. Soft parts can tell the environment and the organism's sensitivity to its surroundings.
Trace fossils are the most common fossils found in nature. They are impressions left in the sediment from once living things. Common trace fossils are footprints or walkways, resting spots, living burrows, feeding burrows, casts and molds. They are not the original parts of an animal. A trace fossil is preserved when mud or dirt that was disturbed by something living hardens and keeps its shape.
There are two main kinds of trace fossils, molds and casts. A mold forms when something is pressed into soft mud and removed by decomposition or pulled out, leaving an impression of the object. A cast is a 3-D example of an object of the past created when a mold fills up with sediment like mud, sand or volcanic ash.
Fossils can form when hard parts of a fossil are replaced by other minerals (see Let's make a fossil #1). This is known as permineralization and petrification. It occurs when fluid flows through rock and dissolves the original animal part while depositing a new mineral in its place. If replacement happens slowly, the original structures in the bone, shell, or plant material can be preserved exactly. A common example of replacement is petrified wood. Original parts are good at telling scientists how the organism lived and what diseases may have affected it.
The following activity allows students to make molds and casts in Plaster of Paris. They will see the difference between the two fossil types and determine which gives them more information, the cast or mold.
Students have just made a cast and a mold. Write on the bottom of each fossil their name and if it is a cast or mold. Have students form two lists to describe what they see in the two 'fossils', hair, shape, texture, size, color, etc. Have students write on paper which fossil gives them the most information, mold or cast (3-D) and why.
To make tracks, try to get animals that you can let walk or slither across the wet plaster. If you do, make sure the plaster is firm enough for the animal to walk on. Insects are easy to find.
Have students trade fossils with another student for observation purposes. Describe what you see in the new fossil and what it is or may have done while it was living, i.e.: walking, swimming, meat eating, vegetarian eating, defense, etc.
Included National Parks and other sites:
Agate Fossil Beds National Monument
Utah Science Core:
4th Grade Standard 4 Objective 1,2