Chapter 12 - Deep Time: How Old is Old?

geologic time

the span of time since Earth’s formation

fossils

remnants of ancient life preserved in rocks

uniformitarianism

physical processes we observe today also operated in the past at roughly the same rates, and these processes were responsible for the formation of geologic features that we now see in outcrops

relative age

the age of one feature with respect to another in a sequence

numerical (absolute) age

the age of a feature given in years

principle of uniformitarianism

physical processes we observe operating today also operated in the past, at roughly comparable rates

principle of original horizontality

layers of sediment when originally deposited are fairly horizontal

principle of superposition

the layer at the bottom of a sequence of strata is the oldest, and the layer at the top is the youngest

principle of lateral continuity

sediments usually accumulate in continuous sheets within a given region

principle of cross-cutting relations

if one geologic feature cuts across another, the feature that has been cut must be older

principle of inclusions

an inclusion must be older than the rock that contains it

principle of baked contacts

rock that has been baked by an intrusion must be older than the intrusion

fossil assemblage

the group of fossil species

principle of fossil succession

once a fossil species disappears at the horizon in a sequence of strata, it never reappears higher in the sequence

index fossils

species found over a broad region but only existed for a short period of time

unconformity

a boundary surface between two rock units, which represents a period of nondeposition and possibly erosion

angular unconformity

represents an erosional surface that cuts across previously tilted or folded underlying layers

nonconformity

sedimentary rocks overlie older intrusive igneous rocks and/or metamorphic rocks

disconformity

the boundary between two sequences

stratigraphic formation

an interval of strata composed of a specific rock type or group of rock types that together can be traced over a fairly broad region

geologic contact

the boundary surface between two formations

stratigraphic column

summarizes information about the sequence of sedimentary strata at a location

correlation

how geologists determine the age of strata at one location with respect to strata at another

marker bed

a key bed that has distinctive characteristics that its presence provides a definitive basis for correlation

fossil correlation

correlation of strata by comparing their fossil assemblages

geologic map

portrays the spatial distribution of rock units at the Earth’s surface

geologic column

a composite stratigraphic column

eons, eras, periods, epochs

ways of dividing/organizing geologic time

Cambrian explosion

a sudden diversification of life over a relatively short interval

radioactive element

one that spontaneously decays so its atoms transform into atoms of another element

radioisotopic dating

using measurements of radioactive elements to calculate the ages of rocks

geochronology

the overall determination and interpretation of numerical ages of rocks

isotopes

different versions of an element that have the same atomic number but different atomic weight

half-life

how long it takes for half of a group of atoms of a parent isotope to decay

closure temperature

the temperature below which these atoms can no longer move freely

growth bands

develops in trees

rhythmic layering

develops in sedimentary accumulations and glacier ice

fission track

the line that results from the ejection of an atomic particle during the decay of a radioactive isotope damages a nearby crystal