Paleontologists use rock layers and radioactive clocks to figure out when dinosaurs lived and how fossils relate to one another. Relative dating places fossils and rock layers in order from older to younger, while absolute dating estimates numerical ages in years. Together, these methods turn a cliff face of sedimentary layers into a timeline of ancient environments, extinctions, and evolution.
This matters because dinosaur fossils are rarely found with a date stamped on them.
Key Facts
- Law of superposition: in undisturbed sedimentary rock, lower layers are older than layers above them.
- Relative dating gives order, such as Fossil A is older than Fossil B, but not an exact age in years.
- Absolute dating uses radioactive decay to estimate numerical ages, often from igneous ash layers near fossils.
- Half-life formula: remaining fraction = (1/2)^n, where n is the number of half-lives.
- Parent and daughter isotopes are compared to find age, such as 40K decaying to 40Ar.
- Index fossils help match rock layers across different locations when the fossil species lived for a short, known time span.
Vocabulary
- Relative dating
- A method of determining whether a rock layer or fossil is older or younger than another without finding its exact age.
- Absolute dating
- A method of estimating the numerical age of a rock or fossil using measurable evidence such as radioactive decay.
- Half-life
- The time it takes for half of the atoms of a radioactive parent isotope in a sample to decay into daughter products.
- Index fossil
- A fossil from a species that was widespread but lived during a short time interval, making it useful for matching rock layers.
- Stratum
- A single layer of sedimentary rock formed during a particular period of deposition.
Common Mistakes to Avoid
- Assuming every lower rock layer is always older, which is wrong because folding, faulting, or overturning can disturb the original order of strata.
- Using radiocarbon dating for dinosaur bones, which is wrong because carbon-14 is useful only for relatively recent organic remains and dinosaurs are tens of millions of years too old.
- Dating the fossil directly when only nearby volcanic ash was dated, which is wrong because the ash gives an age constraint for the layer, not necessarily the exact moment the organism died.
- Confusing relative age with absolute age, which is wrong because relative dating tells sequence while absolute dating gives an estimated numerical age.
Practice Questions
- 1 A rock sequence has Layer A at the bottom, Layer B in the middle, and Layer C at the top, and the layers are not disturbed. Which layer is oldest, and which is youngest?
- 2 A volcanic ash layer contains a radioactive isotope with a half-life of 10 million years. If 25% of the parent isotope remains, how old is the ash layer?
- 3 A dinosaur fossil is found between two ash beds. The lower ash bed is 150 million years old and the upper ash bed is 145 million years old. Explain what age range can be assigned to the fossil and why this is not the same as directly dating the fossil.