earth time Flashcards
principle of uniformitarianism
processes seen today are the same as those of the past
relative ages
based upon order of formation
qualitative method
older vs younger relationships
numerical ages
actual number of years since event
quantitative method
original horizontality
sediments settle out of a fluid by gravity
sediments accumulate horizontally, not favoured on slope
principle of superpositio
younger strata on top, older on bottom
principle of lateral continuity
strata form laterally extensive horizontal sheets
erosion dissects one-continuous layers
flat rocks unlikely to have been disturbed
principle of cross-cutting relations
faults, dikes, erosion must be younger than the material that is faulted, intruded, or eroded
principle of baked contacts
iegnous intrusion cooks invaded country rock
baked rock must have been there first (older)
principle of inclusions
rock fragment within another
inclusions older than enclosing material
fragments older than igneous intrusion
principle of fossil succession
fossils preserved in sedimentary rocks
they are time markers for relative age-dating
specific fossils found within limited time span
correlate strata locally regionally and globally
unconformities
time gap in rock record
angular unconformity
huge gap in time
horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers
nonconformity
igenous/metamorphic rocks capped by sedimentary rock
I or M rocks exposed by erosion then sediment deposited on eroded surface
disconformity
parallel strata bounding non deposition
interruption/pause in sedimentation
correlating formations
formations can be traced over long distances
lithologic (rock type) and fossil correlation
geologic maps
William Strata Smith noted that strata could be matched across distances. similar rock types in similar order
geologic column
A geologic column is a record of the rock layers in a particular area across geologic time
stratigraphic correlation
formations can be traced over long distances
overlap is sequences of rock types
overlapping rock columns used to build composite
what is numerical age based on?
radioactive decay of atoms in minerals
decay proceeds at known fixed rate
radioactive elements act as internal clocks
isotopes
elements that have varying number of neutrons.
stable: isotopes don’t change
radioactice: isotopes that spontaneously decay
radioactive decay
progresses along decay chain. decay creates new unstable elements that also decay until reaching stable element endpoint
parent and daughter isotope
parent: isotope that undergoes decay
daughter: product of this decay
half life
time for half of unstable nuclei to decay
characteristic of each isotope
as parent disappears, daughter “grows in”
isotopic dating
age of mineral determined by measuring ratio of parent to daughter isotopes. calculating time with known half life
