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Ages of Rocks

Geologist in ACTION
Age Stamp: 1902 on building block

Age Stamp: 1962 on building block

Location: Near Emerson Middle School, Berkeley, CA

When workers completed construction of these sections of sidewalk, they imprinted their company name and the year of completion. How many years ago did this sidewalk get built?

Radioactive Decay image

Radioactive Decay

Location: Radioactive material naturally occurs in very tiny amounts in most common rocks. (These amounts are way too low to hurt you, so don't worry!).

Rocks also have markers that help geologists determine their age. These markers involve radioactive decay of atoms. The exact process is complex, but all you need to know is that some atoms can change into other types of atoms (for example, Carbon can change into Nitrogen) by radioactive decay. Some radioactive materials decay more quickly than others (some take less than a second while others take billions of years!). If we know how fast the element decays, we can determine its age almost like reading it off the sidewalk.

Age Stamp: 1922

Image source: University of Missouri

Location: Memorial Union, University of Missouri

Determining the when this building was built is very easy -- you just read it off of the cornerstone. Many important buildings for government, schools, and churches have a cornerstone showing the year of construction. Determining the age of a real rock is a bit more complicated.

Cross-section of Grand Canyon geology

image source: University of Michigan

Location: Grand Canyon National Park, Arizona

This cartoon of the Grand Canyon shows the names of individual layers. Along the left side, it also gives the approximate age of each layer in millions of years. In order to determine the age of an individual layer, a geologist must collect a sample of that layer and take it back to the lab. The geologist uses precise laboratory machines to analyze the relative abundance of radioactive atoms and atoms that form as a result of radioactive decay. Finally, the geologist can calculate the rock's age. This calculation requires knowledge of math, physics, and chemistry! Note how the oldest rocks in the Grand Canyon are on the bottom and the youngest rocks are on the top (more about layers).

Key Concepts:
  • Most rocks have tiny amounts of radioactive material in them.
  • Radioactive elements decay (they change into a completely different element -- for example, Uranium which is used in nuclear reactors changes into Lead, a very dense metal).
  • Scientists use precise laboratory equipment to measure the amount of the new material that was created by radioactive decay.
  • By knowing how fast certain elements decay, we can calculate the age of rocks (the number of years since the rock formed).
Links for further Exploration:
Classroom Activities:
Geologic Age (USGS Activity, Grades 7-12)

Have your students look for evidence of date stamps and cornerstones on their walk home. Depending upon your area, there may be quite a few.

Common Misconceptions:

Misconception: Carbon-14 is the only dating technique

Fact: Carbon-14 is the best known radiometric dating technique, but it can only be used to date objects younger than about 40,000 years. It also only works for objects that contain carbon. Many other radiometric dating techniques exist and are more commonly used for dating rocks many millions of years old. Many of these techniques are based on the decay of Uranium. For example, the first physical evidence of the age of the earth came from a technique analyzing the decay of Uranium into Lead.

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