unit 18

Absolute age is the numeric age of a rock. Relative age tells which rocks are older or younger than others.

The Principle of Uniformitarianism states that changes in the Earth’s crust result from continuous processes, such as storms, earthquakes, volcanism, and weathering.

Charles Lyell used his observations of erosion to conclude that the Earth was at least several million years old, which was radical because it contradicted a literal reading of the Christian Bible and influenced geologists as well as Darwin and Wallace.

The Principle of Original Horizontality states that sediments are deposited in flat, horizontal layers, or strata, parallel to the Earth’s surface. Tectonic forces can fold, tilt, and fault rock layers.

The Principle of Superposition states that younger layers of rock lie on top of older layers of rock.

Intrusions are igneous bodies that cut through layers of rock. They are younger than the surrounding rock.

The two different types of inclusions are xenoliths in igneous rock and clasts in sedimentary rock. They are older than the surrounding rock.

The Principle of Lateral Continuity states that rock layers extend horizontally in all directions over a very large area. Geologists use information from many locations to reconstruct the geologic history of an area and build the geologic column.

An unconformity can form when sediment is not deposited for a long time or when weathering and erosion removes a layer after it formed. An angular unconformity is a place where horizontal rock layers lie on top of tilted or folded rock layers.

A disconformity is an unconformity where part of a sequence of parallel sedimentary rock layers is missing.

In a nonconformity, the earlier layers are igneous or metamorphic rock, while the top layers are sedimentary.

The Principle of Faunal Succession states that different fossil species always appear and disappear in the same order, and once a fossil species goes extinct, it cannot reappear in younger rocks.

If you find a fossil in a rock, you can estimate the age of the rock because fossil species appear and disappear in a known order.

To be useful, a species must have existed for a limited interval of time, been common, found across a large area, and have distinctive features that make it easy to identify.

The more species there are in a layer of rock, the easier it is to date it because the rock must have formed when all those species coexisted.

Radiometric dating allowed scientists to determine the absolute age of rocks and fossils. It became possible only in the 20th century after the discovery of radioactive decay.

Atoms of the same element with different numbers of neutrons are isotopes. An unstable isotope is one that can spontaneously decay into another element.

The half-life of a radioactive isotope is how long it takes for half of a sample to decay.

Radiometric dating determines the absolute age of a sample based on the ratio of a radioactive parent isotope to daughter isotope.

You want isotopes with long half-lives so that some of the parent isotope is still left and you can determine the ratio.

Archaeologists use carbon-14 to date organic materials. It is not useful for most rocks because its half-life is too short, about 6000 years.

The three isotopes most often used to date rocks are potassium-40, uranium-238, and rubidium-87.

Potassium-40 is often used because most rocks contain some potassium and do not contain argon when they form, and it has a long 1.3 billion year half-life. Uranium-238 can be better because many rocks, such as zircon crystals, contain uranium and it has an even longer half-life of 4.5 billion years.

Sedimentary rocks cannot be reliably dated by isotopic methods because they are made up of parts of rocks that formed at some time in the past.

Electron spin resonance can be used to date sedimentary rocks by determining the age at which a rock was buried based on the abundance of radiation induced imperfections in mineral lattices.

Magnetostratigraphy uses the alignment of minerals with Earth’s magnetic field to determine rock age. Earth’s field periodically reverses polarity, and the reversal pattern can be matched to known strata.