Unlike people, you can’t really guess the age of a rock from looking at it. Yet, you’ve heard the news: Earth is 4.6 billion years old. Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1,000 years old.
How do scientists actually know these ages? Geologic age dating—assigning an age to materials—is an entire discipline of its own. In a way this field, called geochronology, is some of the purest detective work earth scientists do.
There are two basic approaches: relative age dating, and absolute age dating. Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you. Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is called the Rule of Superposition.
This rule is common sense, but it serves as a powerful reference point. Geologists draw on it and other basic principles (http://imnh.isu.edu/exhibits/online/geo_time/geo_principles.htm) to determine the relative ages of rocks or features such as faults.
Relative age dating also means paying attention to crosscutting relationships. Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Pretty obvious that the dike came after the rocks it cuts through, right?
With absolute age dating, you get a real age in actual years. It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes.
First, the fossils. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times. After all, a dinosaur wouldn’t be caught dead next to a trilobite. The narrower a range of time that an animal lived, the better it is as an index of a specific time. No bones about it, fossils are important age markers. But the most accurate forms of absolute age dating are radiometric methods.
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. This rate of decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. It’s sort of like a ticking clock. Good discussion from the US Geological Survey: http://geomaps.wr.usgs.gov/parks/gtime/radiom.html
So geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying.
There are a couple catches, of course. Not all rocks have radioactive elements. Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes. You might have noticed that many of the oldest age dates come from a mineral called zircon. That’s because zircon is super tough – it resists weathering. And it’s relatively common, too.
Each radioactive isotope works best for particular applications. The half-life of carbon 14, for example, is 5,730 years. On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years. So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range. Chart of a few different isotope half lifes: http://geomaps.wr.usgs.gov/parks/gtime/ageofearth.html#date
In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history. If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well. Like the other kind of dating, geologic dating isn’t always simple.
Further discussion: Good overview as relates to the Grand Canyon:
Relative age dating:
Use with this cross section of the Grand Canyon from the USGS’s teaching page: http://education.usgs.gov/images/schoolyard/GrandCanyonAge.jpg
Have students reconstruct a simple geologic history — which are the oldest rocks shown? Which are the youngest? Are there any that you can’t tell using the Rule of Superposition?
I also like this simple exercise, a spin-off from an activity described on the USGS site above. Take students on a neighborhood walk and see what you can observe about age dates around you. For example, which is older, the bricks in a building or the building itself? Are there repairs or cracks in the sidewalk that came after the sidewalk was built? Look for “absolute” ages such as cornerstones, dates carved into fresh concrete, or dates stamped on manhole covers.
Absolute age dating:
Have students work alone or in pairs to find an article or paper that uses radiometric age dating. (example search terms: “oldest rocks” “Cretaceous-Tertiary (KT) boundary” “Native American fire ring” )
Then as a class compile a chart to show:
- What materials were dated?
- Which method was used (e.g. Carbon 14, potassium-argon, etc)
- What was the result (what was the material?)
From the chart, which methods are best for older materials? Which for youngest? Can you tell why?