Geologic Time

atomic number

number of protons, identifer in Periodic table

mass number

number of protons + number of neutrons

isotopes

atoms of an element with different atomic masses

radioactive decay

process of forming a different nucleus and producing one or more particles → includes beta decay, beta capture, or alpha emission

beta decay (beta emission)

type of radioactive decay → neutron transforms to a proton and an electron, increases atomic number of product by 1, but leaving atomic mass unchanged → useful for determining age of geologically young organic matter (N→PE)

beta capture

type of radioactive decay → proton and electron transform to a neutron, decreasing the atomic number by 1, but leaving the atomic mass unchanged → can be positron emission (emits one electron) or beta capture (gains one electron) → useful for determining ages of rocks containing mica, feldspar, hornblende (PE→N)

alpha emission

type of radioactive decay → release of heavy atomic particle consisting of two neutrons and two protons (electrons must be lost to regain charge balance), dropping atomic number by 2 and atomic mass by 4 → happens in basements of areas rich in granitic rocks, need for radon detectors

half-life

time it takes for half of a given sample of radioactive isotope (parent isotope) to decay into a more stable form (daughter isotope) → number of half-lives increases, the amount of parent isotope decreases/decays exponentially and the amount of daughter isotopes increases

blocking temperature

the specific temperature at which a mineral crystal structure becomes closed enough to not lose daughter isotopes through diffusion to retain decay products of isotopes, marking the point in time when the “radiometric” clock records the age of the material

Radiometric Dating Techniques

collect rock → separate minerals → extract parent/daughter isotopes → analyze ratio with mass spectrometer → calculate age of mineral

Fundamental Radioactive Decay Laws

uniformitarianism, original horizontality, superposition, lateral continuity, cross-cutting relationships, baked contacts, inclusion, fossil succession

principles of basic stratigraphy (first four = sedimentary, last four = all rocks)

uniformitarianism

physical processes we see today operated in the past and produced the geologic features we see today

original horizontality

sediments produced from water produce nearly horizontal strata (layers). if they are inclined or folded, there must have been deformation since deposition.

superposition

sediment accumulate with the youngest (last deposited) on top

lateral continuity

similar formations extend over an area, rocks that are separated by a feature but otherwise similar were probably deposited together

cross-cutting relationships

if geologic feature cuts another, the feature that has been cut is older

baked contact

when igneous intrusion occurs, the rock that was intruded is often heated around the intrusion so it becomes contact metamorphosed

inclusions

any rock fragment that are included in rock must be older than the rock in which they are included

fossil succession

ongoing evolution of fossil (remains of ancient orgs. found in sedimentary rock record) lifeforms provides a relative geologic clock. fossils only occur in limited range of strata

index fossils

remains of widespread, recognizable organisms that did not hang around long which are most useful

key beds

absolute or relative → distinctive, widely distributed time stratigraphic markers that are produced geologically instantaneously like Bishop Tuff (ash/pyroclastic deposit) and Tektite (ejecta from meteorites)

need the precise rate of deposition, gaps in stratigraphic record, deposits are not global/continental (cannot date widely separated deposits)

Why can’t relative processes be used to establish absolute ages?

angular unconformity, nonconformity, disconformity

three types of unconformities in rock record

angular unconformity

layers are deposited → layers undergo folding → erosion produces a flat surface → new layers of sediment accumulate

nonconformity

pluton (intrusive igneous rock) intrudes (preexisting sedimentary layers) → erosion cuts down into crystalline rock → new sedimentary layers accumulate above the erosional surface

disconformity

layers of sediment accumulate → sea level drops and an erosional surface forms → sea level rises and new sedimentary layers accumulate