Geologic Time

Dating 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.

Relative Dating 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 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?

Law of Superposition In undisturbed strata, the layer on the bottom is • • In undisturbed strata, the layer on the bottom is oldest, those above are younger.

Original Horizontality • • Sediments are generally deposited as horizontal layers. Lateral Continuity • • Sediment layers extend laterally in all direction until they thin & pinch out as they meet the edge of the depositional basin.

Cross-cutting Relationships That which cuts through is younger than the Object that is cut dike cuts through granite is cut

Relative Ages of Lava Flows and Sills

Principle of Inclusions • Inclusions (one rock type contained in another rock type) are older than the rock they are embedded in. That is, the younger rock contains the inclusions

Principle of Inclusions

Faunal/Floral Succession • • Fossil assemblages (groupings of fossils) succeed one another through time.

relating rocks in one location to those in • Correlation- relating rocks in one location to those in another using relative age stratigraphic principles - - Faunal Succession - - Superposition Lateral Continuity - - - - Cross-cutting

Unconformities • • surfaces represent a long time. Hiatus a time when rocks were not deposited or a time when rocks were eroded Hiatus the gap in time represented in the rocks by an uncon- formity 3 kinds Angular Unconformity Nonconformity Disconformity

Absolute Dating 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.

Absolute Dating Methods Radioactive Decay sequences acts as an atomic clock we see the clock at the end of its cycle analogous to starting a stopwatch allows assignment of numerical dates to rocks. > > decay ) into Radioactive isotopes change ( daughter isotopes at known rates. rates vary with the isotope e.g., U , K , C, etc. + + 235 40 14