Chapter 12: Geologic Time Vocabulary

Academic Earth/Space Science

What is the definition of uniformitarianism?

Uniformitarianism is the idea that the physical, chemical, and biological laws that operate today also operated in the past.

What is the definition of relative dating?

The method that geologists use to place rocks in chronological order is called relative dating. Relative dating identifies which rock units formed first, second, third, and so on.

What is the definition of the law of superposition?

The law of superposition states that in an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it.

What is the definition of the principle of original horizontality?

It states that sediment is generally deposited in flat, horizontal layers.

What is that definition of the principle of cross-cutting relationships?

It states that a fault or intrusion must be younger than any geologic formation through which it cuts.

What is the definition of unconformity?

A surface that represents a break in the rock record.

What is the definition of an angular unconformity?

In an angular unconformity, layers of sedimentary rock form over older sedimentary rock layers that are tilted or folded.

What is the definition of a disconformity?

In a disconformity, two sedimentary rock layers are separated by an erosional surface. Because the rocks on both sides of the unconformity are of the same type, disconformities can be difficult to recognize.

What is the definition of a nonconformity?

In a nonconformity, and erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks.

What is the definition of an inclusion?

Are pieces of one rock unit that are contained within another rock unit.

What is the definition of correlation?

The process of matching rock layers at different locations that formed at the same time and by the same processes.

What is the definition of extinct?

They died out.

What is the definition of a fossil?

The preserved remains or traces of an organism.

What is the definition of the principle of fossil succession?

This states that fossil organisms tend to be found in the same general order at different locations.

What is the definition of an index fossil?

A fossil that is both geographically widespread and abundant in the fossil record, but that existed for only a limited span of time.

What is the definition of evolution?

This had, by the mid-nineteenth century, convinced most scientists that life on Earth had undergone evolution, or changed over time.

What is the definition of natural selection?

According to the theory of natural selection, traits that improve an individual’s chance for survival and reproduction will be passed on more frequently to future generations that traits that do not.

What is the definition of adaptations?

According to the theory of natural selection, traits that improve an individual’s chance for survival and reproduction will be passed on more frequently to future generations than traits that do not. These beneficial traits are called adaptations.

What is the definition of radioactivity?

The term for the process by which atoms decay is radioactivity, or radioactive decay.

What is the definition of half-life?

The amount of time necessary for half of the nuclei in a sample of radioactive isotope to decay to it’s stable isotope.

What is the definition of radiometric dating?

A method of calculating the absolute ages of rocks and minerals that contain certain radioactive isotopes.

What is the definition of radiocarbon dating?

To date organic materials, carbon-14 is used in the method called radiocarbon dating. When an organism dies, the amount of carbon-14 gradually decreases as the carbon-14 decays. By comparing the ratio of carbon-14 to carbon-12 in a same, radiocarbon dates can be determined.

What is the definition of geologic timescale?

A timeline of Earth’s history.

What is the definition of eons?

Geologists divide Earth’s history into four long units called eons.

What is the definition of the Precambrian time?

About 88 percent of geologic time is made up of the first three of these eons — the Hadean, Archaean, and Proterozoic. During these eons, Earth formed, the atmosphere and oceans developed, and early life evolved. Another term for this time span is Precambrian time.

What is the definition of eras?

There are three eras within the Phanerozoic eon: the Paleozoic, Mesozoic, and Cenozoic eras.

What is the definition of periods?

Each era is subdivided into periods.

What is the definition of epochs?

Period are divided into still smaller units called epochs.