Chapter 7 Geologic Time

______ ages are used to set the beginning and end points of eras, epochs, and periods on the geologic time scale

radiometric

the geologic time scale is broken into a series of eras, epochs, and periods based mainly on the fossils found in each section, this is an example of ___ dating

relative age

which of the answers below accurately reflects the following statement? rock a is older than rock b but is younger than rock c

rock c is the oldest, followed by rock a, and then rock b

if the relative ages of two formations are known, what else about them can be inferred?

their relative positions in a sequence of rock layers

in an undisturbed sequence of sedimentary rocks, younger layers overlie older layers, according to the principle of

superposition

uniformitarianism is succintcly summarized by which phrase?

the present is key to the past

a tilted sequence of sedimentary rocks violates which principle?

original horizontality

within the world’s sedimentary rocks, fossils…

occur in an ordered sequence

if an igneous dike cuts across a sequence of sedimentary beds…

the beds must be older

an erosional surface between 2 sequences of flat lying sedimentary rocks is called a

disconformity

the surface below sedimentary rocks that overlie igneous or metamorphic rocks is termed a

nonconformity

unconformities represent ____ in the rock record

erosion or nondeposition

if horizontal sedimentary strata overlie tiled strata and no fault is present, the surface between the horizontal and tiled strata must be an

angular unconformity

a ______ is a geologic layer that is distinct enough to allow it to be found regionally and used to correlate rocks

marker bed

stratigraphic correlation is conducted by matching ____ and ____

lithologic layers; fossils

a layer of shale that contains a distinctive fossil could be used as a

marker bed

which of the following statements regarding the geologic column is FALSE?

the geologic column will never be complete because new formations and fossils are found almost daily that shift the time boundaries

2 atoms of a single element that differ in number of neutrons are said to represent 2 distinct _____ of that element

isotopes

which of the following is always the same for different isotopes of a single element?

atomic number

how is the half life of a radioactive parent isotope defined?

the time it takes for half of the parent isotope to decay to daughter isotope

the closure temperature represents the point when

isotopes are no longer free to move out of a crystal lattice

what is the parent to daughter isotope ratio of one half life has passed?

8:8

how much of a radioactive parent isotope will remain after 3 half lives have passed? (hint: always start with 0 half life, then divide by 2 for each half life)

one eighth

radiometric dates applied to sedimentary rocks produce ages that

are too old (predate sedimentary deposition)

dendrochronology involves dating of historic and geologic events through the study of

annual growth rings in trees

seasonal changes create layering in glaciers known as

rhythmic layering

sedimentary rocks cannot be used to determine dates on the geologic time scale because the numerical age

is the age of the rocks that make up the sediment

relative ages expressed on the geologic time scale primarily resulted from the study of

fossil content and spatial relationships among sedimentary rocks

if uranium 235 has 92 protons, how many neutrons does it have?

143

______ is a technique where the decay rate of radioactive isotopes is measured and used to determine the age of a geologic material

radiometric dating