ES Ch 12 Geologic Time

John Wesley Powell

studied the Grand Canyon & saw evidence of Earth's long history

Uniformitatiansim

states that the physical, chemical, & biological laws that operate today have also operated in the geologic past

relative dating

method that geologists used to place rocks in chronological order

Law of Superposition

in an undeformed sequence of sedimentary rocks, each layer is older than the one above it & younger than the one below it

Principle of Original Horizontality

states that layers of sediment are generally deposited in a horizontal position

Principle of Cross-Cutting Relationships

states that when a fault cuts through rock layers, or when magma intrudes other rocks & hardens, then the fault or intrusion is younger than the rocks around it

geologic column

model of the rock record that is made up of rocks arranged according to their relative ages

inclusions

pieces of one rock unit that are contained within another

Unconformity

surface that represents a break in the rock record

angular

layers of sedimentary rock form over older sedimentary rock layers that are tilted or folded

disconformity

two sedimentary rock layers are separated by erosional surface

nonconformity

erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks

correlation

matching rocks of similar age in different locations

extinct

organism is one that no longer exists on Earth

fossil

remains or traces of organism preserved from geologic past

Petrified fossils

form when organism's remains are petrified or "turned into stone"

mold

created when shell or other structure is buried in sediment & then dissolved by underground water, shows impression of organism

cast

created if hollow spaces of a mold are later filled with mineral matter

carbon film fossils

begins when organism is buried under fine sediment, & over time, pressure squeezes out liquids & gases & leaves behind thin film of carbon

preserved remains

remains of organisms in tar, amber, or ice

preserved remains

fossilization process preserves all or part of organism with relatively little change

trace fossil

indirect evidence of prehistoric life

trace fossil

a fossil of a footprint, trail, burrow, or other trace of an animal rather than of the animal itself.

rapid burial and possession of hard parts

What are two conditions needed for fossilization?

principle of fossil succession

states that fossil organisms succeed one another in a definite & determinable order

theory of evolution

states that life forms have changed over time, or evolved, from simpler to more complex forms

natural selection

individuals that are better adapted to their environment are more likely to survive & reproduce that others of the same type

adaptations

traits that affect an organism's ability to survive & reproduce

index fossil

fossil of organism that was geographically widespread & abundant in fossil record, but existed only a limited span of time

absolute age

approximate number of years before the time that the rock formed

radioactivity

process that involves the nucleus of atom breaking down & releasing energy

isotope

atoms of the same element with different numbers of neutrons

radioactive isotope

unstable isotope

half-life

time it takes for one half of nuclei in sample to decay to its stable isotope

Radiometric dating

way of calculating absolute ages of rocks & minerals that contain certain radioactive isotopes

radiocarbon dating

method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample

geologic time

timeline that divides Earth's history into units representing specific intervals of time

eon

largest time unit of geologic time scale

Precambrian

88% of geologic time is made up of first 3 eons, which we call?

Precambrian

Earth formed, atmosphere & oceans developed, & early life evolved

Phanerozoic

means "visible life"

era

step below eon & there are 3 eras within Phanerozoic eon: Paleozoic, Mesozoic & Cenozoic

Mesozoic

middle life, first mammals evolved

Cenozoic

("recent life") had many different types of mammals & birds evolving, flowering plants became abundant

period

step below era

Epochs

smallest unit of geologic time