L8: dividing up deep time and clocks in rocks

radiometric dating suggests earth is 4.5 billion yrs old

absolute dating and relative dating

uniformitarianism

concept that geological features today formed in ways that we can observe and understand today

stratigraphy

a branch of geology which explores rock layers (strata) and layering (stratification)

principles of stratigraphy

principles of original horizontality

• layers of sediment are usually deposited horizontally under the action of gravity

principle of superposition

• in series of stratified sedimentary rocks, the lowest stratum is the oldest

principle of cross-cutting relationships

• if it is no longer horizontal the deformation occurred subsequently

• if a rock is cut by a feature, it must be older than the feature that cuts it

law of inclusions

• if one rock contains parts of another rock, those other rocks must be older

what if time goes missing?

unconformity

• is a stratigraphic gap in the geologic record

disconformity

• is when upper sedimentary sequences overlies an erosional surface

angular unconformity

• upper beds overlie lower beds that have been folded by tectonic processes then eroded to an even plane

what else can we see?

we know different organisms have come and gone

biostratigraphy

• correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them

geological division

• eon

  • us: phanerozoic (541-0 Ma)

• era

  • us: cenozoic (6-0 6Mya)

• period

  • us: quaternary (2.6Mya)

• epoch

  • holocene (0.01 Ma)

radiometric dating

method of dating geological specimens by determining the relative proportions of particular radioactive isotopes

isotopes are variants of a particular chemical element that have a different mass

radioactive isotopes

atoms that breakdown into another stable or radioactive atom by radioactive decay

half-life

• the time it takes for one-half of the atoms of a radioactive material to disintegrate

decay times (various carbon isotopes as an example)

decay curve for radioactive isotopes

radiocarbon dating

we have cosmic radiation that is firing neutrons into the atmosphere which is dominated by nitrogen. neutrons are slamming into the nitrogen and shooting off the proton. carbon is in the tissues of plants

fission track (another dating method)

where you have nuclear fission within some unstable radioactive elements. splitting atoms to create energy, when they split they can create light so photons or physical damage if it takes place in minerals (e.g. creates physical scars in zircon)

age = number of track + need to know how much uranium

dating window up to several billions of years

luminescence dating

can pick up radioactive elements