Dating methods
A geologist looking at the rock layers of the Grand Canyon faces two different questions: in what order did everything happen? and how many years ago did it happen? The first question is answered by relative dating; the second, by absolute dating. Neither is enough on its own: the complete history of the Earth is reconstructed by combining both.
Relative dating: the stratigraphic principles
On the wall of a gorge we see three layers of rock stacked horizontally and crossed by a vertical crack: how can we know for certain what happened first without measuring any numerical date?
Explain the difference between relative dating and absolute dating, and give an example of a geological event that can be dated using each type of method.
Explain the difference between relative dating and absolute dating, and give an example of a geological event that can be dated using each type of method.
State the law of superposition of strata and apply it to the quarry sequence in the diagram (layers A, B, and C). Which is the oldest layer and what environmental change…
State the law of superposition of strata and apply it to the quarry sequence in the diagram (layers A, B, and C). Which is the oldest layer and what environmental change does the sequence suggest?
Fossils and how they form
A hiker in the badlands of South Dakota finds a curved, rock-hard shape in a cliff that looks like a shell. How did a living thing turn into stone, and what can it tell us?
Define fossil and name three different types of evidence that can become fossilized (hard remains, tracks or ichnites, molds, coprolites, inclusions in amber),…
Define fossil and name three different types of evidence that can become fossilized (hard remains, tracks or ichnites, molds, coprolites, inclusions in amber), explaining what information each type provides.
Explain what the fossilization process consists of (rapid burial, mineralization of the hard parts) and discuss why soft-bodied organisms (jellyfish, worms) rarely…
Explain what the fossilization process consists of (rapid burial, mineralization of the hard parts) and discuss why soft-bodied organisms (jellyfish, worms) rarely appear as fossils.
Index fossils and correlation between sections
Human beings cannot serve as index fossils for ancient rocks because our species appeared only about 300,000 years ago: we are absent from almost all of Earth's history, so we cannot mark the age of older strata.
What is an index fossil (or characteristic fossil)? State the three requirements that a good index fossil must meet and explain why human beings cannot serve as such to…
What is an index fossil (or characteristic fossil)? State the three requirements that a good index fossil must meet and explain why human beings cannot serve as such to date ancient rocks.
Explain the principle of faunal identity and how it allows strata from two quarries hundreds of kilometers apart to be correlated, indicating which type of fossils serve…
Explain the principle of faunal identity and how it allows strata from two quarries hundreds of kilometers apart to be correlated, indicating which type of fossils serve best for that task.
Absolute dating: radioactive decay
In the laboratory we find that a sample with $1{,}000$ radioactive atoms falls to $500$ after $10$ days, $250$ after $20$ days and $125$ after $30$ days, regardless of pressure or heat: how does this unchanging rate make it possible to calculate the exact age of a rock?