absolute age
the age of a rock given as the number of years since the rock formed
amphibian
an animal that spends its early life in water and its adult life on land
atom
the smallest particle of an element
continental drift
the very slow motion of the continents
element
any substance that cannot be broken down into simpler substances
era
one of the three long units of geologic time between the Precambrian and the present
erosion
the process by which water, ice, or wind, break down rocks and carry the pieces away
extrusion
an igneous rock layer formed when lava flows onto Earth's surface and hardens
fault
A break or crack in the Earth's crust along which the rocks move
geologic time scale
a record of the geologic events and life forms in Earth's history
geology
the study of the planet Earth and the forces that make and shape Earth
half-life
the time it takes for half of the atoms in a radioactive element to decay
igneous rock
a type of rock that forms from the cooling of molten rock at or below the surface
inclusion
a piece of rock that is contained in another rock; younger than the rock containing it
index fossil
fossils of widely distributed organisms that lived during only one short period
intrusion
an igneous rock layer formed when magma hardens beneath Earth's surface
invertebrate
an animal that does not have a backbone
lava
liquid magma that reaches the surface
law of superposition
the geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it
magma
molten material that hardens beneath Earth's surface
mammal
an endothermic vertebrate with a four-chambered heat, skin covered with fur or hair, and has young fed with milk from the mother's body
mass extinction
when many types of living things become extinct at the same time
metamorphic rock
a type of rock that forms from an existing rock that is changed by heat, pressure, or chemical reactions
period
one of the units of geologic time into which geologists divide eras
plate
one of the major pieces of Earth's rocky outer layer on which continents and oceans move
radioactive decay
the breakdown of a radioactive element, releasing particles and energy
relative age
the age of a rock compared to the ages of rock layers
reptile
an animal that lays eggs and has lungs and scaly skin
rock cycle
a series of processes on the surface and inside Earth that slowly change rocks from one kind to another
sedimentary rock
rock formed of hardened layers of sediments
theory of plate tectonics
theory that states that Earth's plates move slowly in different directions
unconformity
a place where an old, eroded rock surface is in contact with a newer rock layer
uniformitarianism
the geologic principle that the same geologic processes that operate today operated in the past to change Earth's surface
vertebrate
an animal that has a backbone