1. Geologic Time

2. 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.

3. 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?

4. 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.

5. 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.

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

7. Relative Ages of Lava Flows and Sills

8. 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

9. Principle of Inclusions

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

11. 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

12. 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

13. 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.

14. 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. + +
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