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Unlike people, you can’t really guess the age of a rock from looking at it.

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Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.The most obvious feature of sedimentary rock is its layering.This feature is produced by changes in deposition over time.This means that a quartz sandstone deposited 500 million years ago will look very similar to a quartz sandstone deposited 50 years ago.Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments.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.

It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. 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.

But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.

This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: 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. 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.

With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?