• The two types of dates used by geologists to interpret Earth history are (1) relative dates, which put events in their proper sequence of formation, and (2) numerical dates, which pinpoint the time in years when an event occurred.

• Relative dates can be established using the law of superposition (in an underformed sequence of sedimentary rocks or surface-deposited igneous rocks, each bed is older than the one above, and younger than the one below), principle of original horizontality (most layers are deposited in a horizontal position), principle of cross-cutting relationships (when a fault or intrusion cuts through another rock, the fault or intrusion is younger than the rocks cut through), and inclusions (the rock mass containing the inclusion is younger than the rock that provided the inclusion).

• Unconformities are gaps in the rock record. Each represents a long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed. The three basic types of unconformities are angular unconformities (tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata), disconformities (the strata on either side of the unconformity are essentially parallel), and nonconformities (where a break separates older metamorphic or intrusive igneous rocks from younger sedimentary strata).

• Correlation, the matching up of two or more geologic phenomena in different areas, is used to develop a geologic time scale that applies to the whole Earth.

  

• Fossils are used to correlate sedimentary rocks that are from different regions by using the rocks’ distinctive fossil content and applying the principle of fossil succession. It is based on the work of William Smith in the late 1700s, and states that fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. The use of index fossils, those that are wide-spread geographically and are limited to a short span of geologic time, provides an important method for matching rocks of the same age.

• Each atom has a nucleus containing protons (positively charged particles) and neutrons (neutral particles). Orbiting the nucleus are negatively charged electrons. The atomic number of an atom is the number of protons in the nucleus. The mass number is the number of protons plus the number of neutrons in an atom’s nucleus. Isotopes are variants of the same atom, but with a different number of neutrons, and hence a different mass number.

• Radioactivity is the spontaneous breaking apart (decay) of certain unstable atomic nuclei. Three common types of radioactive decay are (1) emission of alpha particles from the nucleus, (2) emission of beta particles from the nucleus, and (3) capture of electrons by the nucleus.

• An unstable radioactive isotope, called the parent, will decay and form stable daughter products. The length of time for one-half of the nuclei of a radioactive isotope to decay is called the half-life of the isotope. If the half-life of the isotope is known, and the parent/daughter ratio can be measured, the age of a sample can be calculated. An accurate radiometric date can only be obtained if the mineral containing the radioactive isotope remained in a closed system during the entire period since its formation.

  

• The geologic time scale divides Earth’s history into units of varying magnitude. It is commonly presented in chart form, with the oldest time and event at the bottom and the youngest at the top. The principle subdivisions of the geologic time scale, called eons, include the Hadean, Archean, Proterozoic (together, these three eons are commonly referred to as the Precambrian), and, beginning about 540 million years ago, the Phanerozoic. The Phanerozoic (meaning “visible life”) eon is divided into the following eras: Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”).

• A significant problem in assigning numerical dates is that not all rocks can be radiometrically dated. A sedimentary rock may contain particles of many ages that have been weathered from different rocks that formed at various times. One way geologists assign numerical dates to sedimentary rocks is to relate them to datable igneous masses, such as volcanic ash beds.

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