Chapter 12: Geologic Time

Academic Earth/Space Science

Understand that the rock record tells us that the Earth is old, and the same process occurring today have occurred in the past. This helps geologists decode the past.

Fossils enable scientists to correlate rock layers and infer past environments, and they provide evidence for evolution.

Know what the principle of uniformitarianism states.

Uniformitarianism is the idea that the physical, chemical, and biological laws that operate today also operated in the past. Thus, to understand ancient rocks, we must firs understand present-day processes and their results. The principle of uniformitarianism is commonly expressed by saying, “The present is the keyto the past.”

Know what the principle of cross-cutting relationships states.

Later geologists developed another principle used in relative dating. This principle, called the principle of cross-cutting relationships, states that a fault or intrusion must be younger than any geologic formation through which it cuts.

Be able to compare and contrast relative and absolute age.

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.

Absolute Age: A rock or mineral’s absolute age is the approximate number of years before present that it formed.

Know what the law of superposition states.

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. This rule also applies to other surface deposited materials, such as lava flows and layers of ash from volcanic eruptions.

Be able to put rock layers in order using relative dating techniques.

Be able to compare and contrast a nonconformity, disconformity, and angular unconformity.

Unconformity: A surface that represents a break in the rock record is termed an unconformity. An unconformity indicates a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed. Unconformities help geologists identify what intervals of time are not represented in the rock record. There are three basic types of unconformities: angular unconformities, disconformities, and nonconformities.

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

2. 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.

3. Nonconformity: In a nonconformity, an erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks.

Fossils defined.

A fossil is the preserved remains or traces of an organism.

Know which rock types normally form fossils.

Sedimentary rocks normally form fossils.

Know examples of trace fossils.

Trace fossils are indirect evidence of prehistoric life. Tracks form when footprints are covered with sediment before they can be washed away. Other types of trace fossils include burrows, coprolites, and gastroliths. Burrows are holes made by an animal that were later filled with minerals and preserved. Coprolites and gastroliths provide useful information about the eating habits of organisms. Coprolites are fossils of dung and stomach contents and gastroliths are highly polished stomach stones used by some extinct reptiles to grind food.

Know the conditions that favor fossil preservation.

Two conditions that favor the preservation of an organism as a fossil are rapid burial and the possession of hard parts.

Principle of fossil succession defined.

The principle of fossil succession states that fossil organisms tend to be found in the same general order at different locations.

Correlation defined.

In geology, correlation is the process of matching rock layers at different locations that formed at the same time and by the same processes.

Index fossil defined.

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

Know how index fossils are used.

The presence of an index fossil in rock layers at different locations means that the layers are of roughly the same age. Rock layers, however, do not always contain a specific index fossil.

Radioactivity defined.

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

Half-life defined.

A half-life is a common way of expressing the rate of radioactive decay. A half-life is the amount of time necessary for one-half of the nuclei in a sample of radioactive isotope to decay to its stable isotope. If the half-life of the parent isotope is known, and the parent/daughter ratio can be measured, then the age of the sample can be calculated.

Know why organisms evolve.

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.

Know that radiometric decay rates are constant.

The radiometric decay rates are constant.

Be able to read and interpret the decay curve from page 348.

Know what radio-carbon dating is and how it is used.

To date organic materials, carbon-14 is used in a method called radiocarbon dating. When an organism is alive, the decaying radiocarbon is continually replaced. Thus, the ratio of carbon-14 to carbon-12 — the stable isotope of carbon — remains constant. 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 sample, radiocarbon dates can be determined.

Know the accepted age of the Earth.

Earth and all meteorites should be the same age, about 4.5 billion years old.

Geologic time scale defined.

The geologic timescale, is a timeline of Earth’s history. The geologic timescale is a record hat includes both geologic events and major developments in the evolution of life.

Be able to compare and contrast eons, eras, periods, and epochs.

The geologic time scale is divided into eons, eras, periods, and epochs. Eons represent the longest intervals of geologic time. Eons are divided into eras. Each era is subdivided into periods. Finally, periods are divided into eras. Each era is subdivided into periods. Finally, periods are divided into still smaller units called epochs.

Know the longest division (period) of the geologic timescale.

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 long time span is Precambrian time.

Know the eras of the Phanerozoic.

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

Know when abundant fossil evidence occurred on Earth.

About, 541-536 million years ago, which is during the Cambrian period.

Know which types of rocks are most useful in radiometric dating.

Igneous rocks are the most useful in radiometric dating.