Geo Science Unit 2 Chapter 21

Front 1

eon

Back 1

Longest time unit in the geological time scale, measured in billions of years.

Front 2

epoch

Back 2

Time unit in the geological time scale, smaller that a period, measured in millions of years to thens of millions of years.

Front 3

era

Back 3

Second-longest time unit in the geological time scale, measured in hundreds of millions of years, and defined by differences in life-forms that are preserved in rocks.

Front 4

geologic time scale

Back 4

Record of Earth's history from its origin to the present used to correlate geologic events, environmental changes, and development of life-forms that are preserved in rocks.

Front 5

period

Back 5

Third-longest time unit in the geological time scale, measured in tens of millions of years to hundreds of millions of years, and defined by life-forms that were abundant or became extinct.

Front 6

correlation

Back 6

Matching of rock outcrops of one geographic region to another.

Front 7

cross-cutting relationships

Back 7

Principle stating that a fault or intrusion is younger than the rock it cuts across.

Front 8

original horizontality

Back 8

Principle stating that sedimentary rocks are deposited in horizontal or nearly horizontal layers.

Front 9

superposition

Back 9

Principle stating that in an undisturbed rock sequence, the oldest rock layers are at the bottom and each successive layer is younger than the layer beneath.

Front 10

unconformity

Back 10

Gap in the rock record caused by erosion or weathering.

Front 11

altered hard parts

Back 11

Fossils whose organic material has been removed and whose hard parts have been changed by recrystallization or mineral replacement.

Front 12

cast

Back 12

Fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes filled with minerals or sediment.

Front 13

dendrochronology

Back 13

Science of comparing annual growth rings in trees to date events and environmental changes.

Front 14

evolution

Back 14

Adaptation of life-forms to changing environmental conditions.

Front 15

fossil

Back 15

Remains or evidence of a once-living plant or animal.

Front 16

half-life

Back 16

Period of time it takes for a radioactive isotope, such as carbon-14, to decay to one-half its original amount.

Front 17

index fossil

Back 17

Remains of plants or animals that were abundant, widely distributed, and existed briefly that can be used by geologists to correlate or date rock layers.

Front 18

key bed

Back 18

Sediment layer that serves as a time marker in the rock record and results from volcanic ash or meteorite-impact debris that spread out and covered large areas of Earth.

Front 19

mold

Back 19

Fossil that can form when a shelled organism decays in sedimentary rock and is weathered away, leaving a hollowed out impression.

Front 20

original preservation

Back 20

Describes a fossil with soft and hard parts that have not undergone any change since the organism's death.

Front 21

permineralization

Back 21

Process in which pore spaces in a fossil are filled in with mineral substances.

Front 22

radioactive decay

Back 22

Emission of atomic particles at a constant rate from a radioactive substance and its resulting change into other elements over time.

Front 23

radiometric dating

Back 23

Process used to determine the absolute age of a rock or fossil by determining the ratio parent nuclei to daughter nuclei within a given sample.

Front 24

uniformitarianism

Back 24

States that processes such as mountain building, erosion, and sea-level changes that are occurring today have been occurring since Earth formed.

Front 25

varve

Back 25

Alternating light-colored and dark-colored sedimentary layers of sand, clay, and silt deposited in a lake that can be used to date cyclic events and changes in the environment.