Lessons in Evolutionary Biology

Grades 6-8: Age of the Earth

Age of the Earth

Learning Experience

We will discuss the age of the earth and how scientists have learned how old Earth is. This is important for understanding biological evolution because life on Earth has appeared and developed over a very long geologic time. We use observations to make claims about how time might be measured and investigated.


We will experiment with developing our own “clock” model to measure time in terms of the rate of decay(melting) of an ice cube. We will learn that the earth is about 4.6 billion years old.

Materials and Preparation

Several ice cubes (ideally, all the same size); cups or saucers; a medicine syringe (10 ml size works best); a notebook or notepad to record observations; graph paper.


Focusing Questions: How old is this planet? How might scientists learn about the age of the earth?

Focus: How do scientists know how old the Earth is? They have learned by making observations about some of the rocks and minerals that make up our planet. The molecules that make up rocks and minerals change over time, in a process called decay or radioactive decay. The process of change by decay happens at a steady pace which scientists can measure. They can tell about how old a rock is by measuring how far the decay of molecules in that rock has progressed. (Click to see illustration.)

Explore: You can make your own version of this method of measuring time using the melting of ice cubes as your “clock”. Take two or three ice cubes out of the freezer and place them into separate cups or saucers on the table or countertop. Note the time. After ten minutes, measure how much of the ice has melted (suck the liquid up off the saucer using the syringe); after recording the amount, return the water to the saucer. Record the time and the amount for each ice cube. Average the measurements to get a single amount of melted water. Wait another ten minutes and repeat the process. Continue until the ice cubes have completely melted. You now have a record of the rate at which the block of ice melts. Now make a graph of your results on the horizontal axis label “time” in ten-minute increments, and on the vertical axis label “amount of liquid” in ml. Plot your average liquid measures against time on the graph. Next, ask a parent or sibling to take an ice cube out of the freezer (recording the time they do it, but not telling you) and place it into a cup. Sometime later (but before the ice is completely melted!), measure the amount of liquid that has melted off the ice cube. Plot this amount on your graph, and calculate backward from the time at which you took the measurement to determine when the ice cube was removed from the freezer.

Reflect: Your ice block clock is a model of a decay clock such as that used by scientists to find out the age of rocks in the earth. Can you think of factors that might make your ice block clock unreliable? What if we did this experiment on a warm summer day and then on a cold winter day? How might the results be different? What if we used different sizes of ice blocks? Do big blocks melt at the same rate as small blocks? These kinds of questions would have to be accounted for in our model. Scientists who study the age of earth using rates of decay of molecules have to know how fast those molecules decay, and also they need to know whether there are factors that can change those rates of decay. For example, different kinds of molecules decay at different rates. Based on radioactive decay measurements taken from meteors that crashed into Earth early in the planet’s formation, scientists calculate that the earth is about 4.6 billion years old! [I think we need a picture for this—one of those classic time line or clock illustrations that shows the relative timespans of the development of the planet]


Ask students to state how old the earth is. Ask them to explain the principles of estimating geologic time using molecular decay as a measurement.


To explore more ways to really understand numbers of magnitude like millions or billions, try the book “How Much is A Million”, by David M. Schwartz. Schwartz is a mathematician who gives concrete examples of these big numbers, using objects or concepts that are familiar, which help readers to picture and really understand them.

Science Grade Level Expectations Addressed (WA State EALRs):

Students know that: Students are expected to:
6-8 INQE
Models are used to represent objects, events, systems, and processes. Models can be used to test hypotheses and better understand phenomena, but they have limitations. Create a model or simulation to represent the behavior of objects, events, systems, or processes. Use the model to explore the relationship between two variables and point out how the model or simulation is similar to or different from the actual phenomenon.
6-8 LS3A The scientific theory of evolution underlies the study of biology and explains both the diversity of life on Earth and similarities of all organisms at the chemical, cellular, and molecular level. Evolution is supported by multiple forms of scientific evidence. Explain and provide evidence of how biological evolution accounts for the diversity of species on Earth today.
9-11 ES3B Geologic time can be estimated by several methods (e.g., counting tree rings, observing rock sequences, using fossils to correlate sequences at various locations, and using the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed). Explain how decay rates of radioactive materials in rock layers are used to establish the timing of geologic events. *a

Given a geologic event, explain multiple methods that could be used to establish the timing of that event.
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