2 Laws Of Relative Dating Flashcards
Law of superposition
Because sediments are always lay down on some pre-existing surface the surface, it’s self must be older than the settlements been deposited on it.
Oldest on the bottom youngest at the top.
What is relative dating?
estimating the sequence in which rocks are laid down and geological events that occurred during deposition.
original horizontality
Active settlements will produce approximately flat layers of sediment. Therefore, if stratigraphy is folded or inclined, this must reflect some sort of disturbance or the formation after deposition.
Crosscutting relationships
Builds on the idea that if a rock intrusion is cutting through the boundaries of other rock layers There must be rock there to cut through in the first place, therefore the surrounding strata will be older.
Inclusions
If a solid object, e.g. a fossil or rock fragment is contained within a rock then the contain fragment must be older than the enclosing strata. Applies to clastic settlements and igneous rocks with inclusion such as xenoliths.
Unconformity’s
An unconformity is a break in time, allowing defamation, uplift and erosion to occur.
Unconformities represent dynamic changes in geological record from sedimentary deposition to non-deposition or active erosion, and then back to sedimentation again.
Layers of sediment deposited continually above each other are conformable.
Angular unconformity’s
tilted or folded sedimentary rocks that are cut by an erosion surface and overlain by younger horizontal layers
Disconformity
Where there is no angular discordance between the upper and lower package of sediment, but an erosion surface exists where the sand has filled a channel cut in sale
Paraconformity
Where no discreet erosion surface, separates the older and younger strata, but other evidence demonstrates a substantial missing section in stratigraphic package.
Nonconformities
Where sedimentary layers overlie intrusive igneous or metamorphic rocks.
Absolute dating
Absolute dating using radioactivity can identify a more exact age of strata for sedimentary rocks. This is done by measuring the amount of time it takes for a radioactive unstable, element to reach its half life.
Half life
The amount of time it takes for half a sample made of an unstable isotopes, to undergo radioactive decay.
Isotopic dating
1 find relative abundance of radioactive isotope and daughter product
2 determine the rate of radioactive decay
3 understand the difference, chemical and physical forces can have on daughter and parent isotopes
Radio carbon dating
Carbon 14 is in all organic things. Carbon-14 is an unstable element. This means when organisms die the carbon-14 levels in the body slowly decay. The half life of carbon 14 is 5730 years. By comparing the amount of carbon 12 which stays there and amount of carbon 14, and a fossil, you can tell how old the fossil is.
Classification of fossils
Uses morphological species concept; distinguishing by their physical features.
