Since we have our first test coming up, I thought that I would write my blog on the one subject that I have the most trouble understanding, Geologic Time.
To understand the geologic history of a given area, you must first start by finding their relative age. There are 8 given law that help establish the relative age of sedimentary rocks.
1) The law of original horizontality-sediments deposited in water are laid down in strata that are horizontal or nearly horizontal. Therefore most rocks (igneous and sedimentary) for horizontally. If it had formed vertical that means that a massive earth process pushed it on its side.
2) Lao of superposition- any undisturbed sequence of sedimentary rocks, the layer at the bottom is older than the layer at the top of the sequence.
3) Law of original (lateral) continuity- sedimentary rock units extend laterally until they think or pinch out at their margins. So if an elevated rock has the same third and fourth layer of a first and second layer of a neighbor rock, we can assume that these similar layers are of the same age.
4) Law of faunal assemblages- similar assemblages of fossils indicate similar geologic ages for rocks that contain them.
5) Law of faunal succession- each geologic formation has a different aspect of life from that in the formation above it and below it.
Cenozoic: Today -65 mya (mammals)
Mesozoic: 251 mya (dinosaurs and flowering plants)
Paleozoic: 542 mya (fish, plants, insects)
Precambrian: 4.5 bya
6) Law of crosscutting relationships- any rock unit cutting another must be younger than the rock unit it cuts (Igneous rocks).
7) Law of inclusi0ns- fragments of rocks in another rock unit are older than the host rock containing them.
8) Law of unconformities- units above an unconformity are younger than those below an unconformity.
Unconformities- can be described as something that is visually unique or a change in uniform. This can also be called a “gap in time.” One example of this is when an old erosional surface that is buried beneath a younger layer is called an unconformity or a surface of erosion. There are three types of unconformities that geologist recognize. A disconformity is a surface that represents missing rock strata but the beds above and below are parallel to one another. A second type is nonconformity, which can best be described as an Igneous intrusion. The third type of unconformity is an angular unconformity, where there has been a period of erosion or folded or tilted rocks followed by deposition of younger, flat-lying sedimentary rocks.
Through the application of the 8 laws and looking back at the unconformities, humans can establish the relative age of sedimentary strata around the world.