Earth's History

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