Geologic Time

Relative dating

Placing rocks in their proper sequence of formation. Supplemented numerical dating.

Law of superposition

The geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it.

Law of original horizontality

States that sedimentary layers are laid down in horizontal layers and so if they are titled something has disturbed them.

Principle of cross-cutting relationships

A principle of relative dating; a rock or fault is younger than any rock or fault through which it cuts.

Inclusions

Pieces of one rock unit that are contained within another.

False.

Rocks containing inclusions are older.

Unconformity

A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

Angular unconformity

An unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted or folded older rocks. Suggests a period of deformation occurred during deposition.

Disconformity

a type of unconformity in which the sedimentary layers above and below the unconformity are parallel. Most common but the most conspicuous.

Nonconformity

A type of unconformity in which layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks.

Correlation

Matching up rocks of different locations but of similar age. Gives comprehensive view of the geologic history of the region.

Fossils

Remains or traces of prehistoric life. Important inclusion in sediments and sedimentary rocks.

Paleontology

Scientific study of fossils. Blends biology and geology. Important in understanding past environmental conditions and in correlation.

Petrified

Type of fossil where sample turned into stone. Cavities and pores are filled with precipitated mineral matter.

Replacement

Cell walls and other solid materials replaced with mineral matter. Microscopic detail of organisms retained.

Mold

Reflects shape and surface marking but no information about internal structure.

Cast

Hallow spaces filled with mineral matter.

Carbonatization

Effective in preserving leaves and delicate animal forms. Fine sediment encases remains then pressure squeezes out liquid and gas components leaving behind a thin residue of carbon.

Impression

Occurs when carbon film is lost leaving a replica of the surface.

Amber

Hardened resin of ancient trees. Effective in preserving insects.

Tracks Burrows Coprolites Gastroliths

Types of trace fossils

Tracks

Animal footprints made in soft sediment later lithified.

Burrows

Tubes in sediment, wood, or rock made by a burrowing animal.

Coprolites

Fossilized dung and stomach contents. Provide information regarding food habits.

Gastroliths

Polished stomach stones used in grinding food.

Rapid burial Possession of hard parts

Two conditions needed for fossilization

Principle of faunal succession

A stratigraphic principle stating that the sedimentary rock strata in an outcrop contain distinct fossils in a definite sequence therefore any time period can be recognized by its fossil content.

Index fossils

Distinctive fossils used to establish and compare the relative ages of rock layers and the fossils they contain. Geographically widespread and limited in presence in the geologic time.

Group fossils

Used to establish age because rock formations does not always contain a specific index fossil.

Atom

Basic unit of matter. Composed of a nucleus and a cloud of electron.

Atomic number

The number of protons in the nucleus of an atom.

Mass number

The sum of the number of neutrons and protons in an atomic nucleus.

Isotope

Atoms of the same element that have different numbers of neutrons.

Radioactivity

The spontaneous emission of radiation by an unstable atomic nucleus.

Parent

Unstable radioactive isotope.

Daughter

Result of the decay. Stable isotope.

Radiometric dating

The process of measuring the absolute age of geologic material by measuring the concentrations of radioactive isotopes and their decay products. Provides reliable means of calculating the ages in rocks and minerals. Reliable.

Alpha particle

A cluster of 2 protons and 2 neutrons emitted from a nucleus in one type of radioactivity.

Beta particle

A high-speed electron or positron emitted in the decay of a radioactive isotope.

Alpha particle emission Beta particle emission Electron capture

Scenarios that can happen when a radioactive element becomes unstable.

Half-life

The time required for one half of the atoms of a radioisotope to emit radiation an decay products. Rate of radioactive disintegration.

Potassium-40

Among the 3 isotopes of potassium, which among them is radioactive.

Potassium-argon dating

Radiometric technique using the decay of K to Ar in potassium-bearing rocks; estimates the age of sediments in which fossils are found. Can be used to date rocks as young as 100,000 years old.

1.3 billion years

The half-life of potassium-40.

Potassium-39 Potassium-40 Potassium-41

The isotopes of potassium

Radiocarbon dating

A chemical analysis used to determine the age of organic materials based on their content of the radioisotope carbon-14. Used to date recent events and even as old as 700,000 years old. Useful in dating organic materials containing wood, charcoal, bones, flesh, and cotton fibers.

5730 years

The half-life of carbon 14.

Tree rings

Tree growth is influenced by climate conditions, patterns in tree rings and isotopic composition of material within each ring reflect variations in past climate.

Dendrochronology

The process of counting tree rings to determine the age of a tree.

Eons

Greatest expanses of time. Began during the formation of the Earth.

Era

Marked by worldwide changes in life forms.

Period

Marked by worldwide changes in life forms less profound than eras.

Epoch

Smaller units dividing periods.

Geologic time scale

The standard method used to divide the Earth's long natural history into manageable parts.

4.5 billion years

Half life of uranium-23