absolute age
the numeric age of an object or event as established by an absolute-dating process, such as radiometric dating
bedding plane
the boundary between two sedimentary rock layers
beds
sedimentary rock layers that are compressed and harden
fault
a break or crack in Earth's crust along which rock shifts their position
fossil
the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock
half-life
the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope
index fossil
a fossil that is used to establish the age of rock layers because it is distinct, abundant, and widespread and existed for only a short span of geologic time
isotope
atoms of the same element that have different numbers of neutrons
intrusion
a mass of igneous rock that forms when magma is injected into rock then cools and solidifies
law of superposition
the law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed
James Hutton
18th century Scottish physician; he thought that by studying the present, people could learn about the past (uniformitarianism) and that the Earth was not thousands, but millions of years old
paleontology
the scientific study of fossils
radiometric dating
using half-lives or isotopes to calculate age
relative age
the age of an object in relation to the ages of other objects
strata
layers of rock
trace fossil
a fossilized mark that formed in sedimentary rock by the movement of an animal on or within soft sediment
unconformity
a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time
law of crosscutting relationships
the principle that a fault or body of rock is younger than any other body of rock that it cuts through
uniformitarianism
a principle that geologic processes that occurred in the past can be explained by current geologic processes
varve
a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers, and that can be used to determine absolute age
erosion
the moving away of rock and sediment
deposition
the process of laying down sediment carried by a river, glacier, or sea
weathering
the breaking down of rock