Fossils – Dating Lecture 2

Relative Dating As the name implies, this is to find the relative dates of rocks We look at different layers of rocks called, strata While you can use this to find very rough estimates of absolute time, you should mostly focus on looking at the comparison between rocks

Relative Dating – Principle of Superposition If the sedimentary rocks are layered nicely one on top of the other, oldest on the bottom, youngest on the top. Rocks will naturally be layered horizontally (Principle of Original Horizontally) However, rocks will often be tilted due to some geologic event

Relative Dating – Principle of Cross-Cutting Relationships If a rock (often an igneous intrusion) cuts or fractures into another layer of rock, the cutting rock is always younger than the original layer

Relative Dating – Principle of Inclusions Also known as the Law of Included Fragments Fragments in one rock are older than the rock that contains them These intrusive rocks are often igneous

Relative Dating – Law of Cross-Cutting Relationships Faults are younger than the rocks it cuts through Conversely, it a rock is not fracturedwhile surrounding rocks are, this indicates that the unfractured rock is younger than the surrounding layers

Relative Dating – Problems

Relative Dating – Problems (cont.)

Absolute Dating This is math based! We use half-lifes and radiometric dating to determine a very close estimate of the age of a rock One half-life is the time it takes for half of the parent atoms of a certain isotope decays We measure the amount of parent isotope still present in the material We are primarily going to look at Carbon-14 (half-life of 5730 years), but Potassium-40, Uranium-235, 238, Thorium-232, Rubidium-87 and Samarium-147 are used in science

Absolute Dating – Calculations