states that the oldest layers of rock are found at the bottom and the youngest layers of rock are found at the top of a formation if the rock layers have not been disturbed.

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

record of Earth's history from its origin 4.6 bye to the present

time unit in the geological time scale, smaller than a period, measured in hundreds of thousands to millions of years.

third-largest time unit in the geologic time scale, measured in tens of millions of years.

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

longest time unit in the geologic time scale

sudden appearance of a diverse collection of organisms in the Cambrian fossil record

layer of iridium-rich material between rocks of the cretaceous period and rocks of the Paleogene period that provides evidence of a meteorite impact

geologic theory that Earth's surface is broken into several huge plates that move slowly on a partially molten rock layer.