Relative Dating of Rocks 6th Grade Earth Science Unit 2: Geologic Time

Law of Superposition! Intro to the How do geologists look at rock layers? How do they reconstruct its history? They have to look at the relative ages of the layers!

Rock Strata A stratum (plural: strata) is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers. The "stratum" is the fundamental unit in a stratigraphic column and forms the basis of the study of stratigraphy.

Superposition! The Law of Superposition states that: the older rocks are deposited first and each sequential layer is deposited on top of that oldest layer meaning that the rocks on the bottom are the oldest and the rocks on top are the youngest. Wait a second…is it that easy?…layers?…that’s it?….like a cake?

Not so fast! Geologists also have to take into account geologic events that happened and that can disrupt the nice neat layers such as erosion or uplift or tilting. Therefore the layers aren’t so nice and neat and to make matters worse, some time is missing! This is called an unconformity.

How do they make sense of the madness?? They are super geniuses of course! They use simple rules and logic as well as their knowledge of the geologic time scale. Cross cutting relationships: Anything that cuts across an already existing layer is younger ( intrusion of rock or a fault). How could an intrusion or fault cut across a layer that didn’t exist?? The layer that was cut across had to be there first! Principle of Original horizontality: Layers of sediment are originally deposited horizontally due to gravity. Tilting: Any layer that is now tilted or deformed must have been there before the tilting event occurred…again, basic logic.

Now its your turn… Watch this! : http://study.com/academy/lesson/what-is-relative-dating- law-of-superposition-principles-of-original-horizontality-cross-cutting- relationships.html 1. B 2. W 3. E 4. Q 5. K