chapter 21 vocab words

mold

a mold forms when sediments cover the original hard part of an organism, such as a shell, and the hard part is later removed by erosion.

key bed

a rock or sediment layer used as a marker in this way is called a key bed.

periods

all eras are divided into periods. periods are generally tens of millions of years in duration, though some periods of the Precambrian are considered longer.

varves

bands of alternating light- and dark- colored sediments of sand, clay and silt are called varves.

unconformities

buried surfaces of erosion are called unconformities.

correlation

correlation is the matching of unique rock outcrops or fossils exposed in one geographic region to similar outcrops exposed in other regions.

Epochs

epochs are generally hundreds to thousands to millions of years in duration.

evolution

evolution is the change over time.

original preserves

fossils with original preservation are the remains of plants and animals that have been altered very little since organisms deaths.

altered hard plants

however, over time the remaining hard parts such as shells, bones, or cell walls can become fossils with altered hard plants.

uniformitarianism

huttons work lies at the foundation of uniformitaranism which states the geologic processes occurring today have been occurring since earth formed.

absolute-age dating

in contrast, absolute-age dating enables scientist to determine the numerical age of rocks and other objects.

mineral replacement

in the process of mineral replacement the pore spaces of an organism buried hard parts are filled in with minerals from groundwater.

index fossils

index fossils are fossils that are easily recognized, abundant and widely distributed geographically.

original horizontal

is the principle that sedimentary rocks are deposited in horizontal or nearly horizontal layers.

relative-age dating

one way to do this is by studying the order in geologic events occurred using a method called relative-age dating

half-life

scientist measure the length of time it takes for one half of the original isotope to decay, called its half-life.

radiocarbon dating

scientist use c-14 to determine the age of organic materials, which contain abundant carbon in a process called radiocarbon dating

radioactive decay

the emission of radioactive particles and the resulting change into other isotopes over time is called radioactive decay.

mass extinction

the end of the Paleozoic is marked by the largest mass extinction event in earths history. a mass extinction many groups of organisms disappear from the rock record about the same time .

cross-cutting relationships

the principle of cross-cutting relationships states that an intrusion is younger than the rock it cuts across.

principle of inclusions

the principle of inclusions states that the fragments, called inclusions, in a rock layer must be older than rock layer that contains them.

dendrochology

the science tree rings to determine absolute age is called dendrochology and has helped geologists date relatively recently geologic events that toppled trees.

precambrian

the three earliest eons make up about 90 percent of geologic time, known together as the Precambrian.

Eons

the time scale is divided into units called eons, eras, periods and epochs. an eon is the largest of these time units and encompasses the others.

Geologic time scale

the time units are part of the geologic time scale, a record of earth's history from its origin 4.6 billion years ago to the present.

superposition

this is an application of superposition, the principle that an undisturbed rock sequence, the oldest rocks are at the bottom and each layer is younger beneath it.

trace fossils

trace fossils can provide information about how an organism lived, moved, and obtained food.

radiometric dating

when scientist date an object using radioactive isotopes, they are using a method called radiometric decay

cast

A mold might later become filled with material to create a cast of the shell.