Chapter 18: Geologic Time

Relative Dating

Placing rocks and events in a sequence of formation (e.g., Rock A is older than Rock B) without assigning an exact age.

Absolute (Numerical) Dating

Assigning a specific date or number of years to an event or rock (e.g., 250 million years old).

Principle of Superposition

In an undeformed sequence of sedimentary rocks, the bottom bed is the oldest and the top bed is the youngest.

Principle of Original Horizontality

The rule that layers of sediment are typically deposited in flat, horizontal layers.

Principle of Lateral Continuity

Sedimentary beds originate as continuous layers that extend outward in all directions until they pinch out.

Principle of Cross Cutting Relationships

Any geological feature that cuts across another body of rock must be younger than the rock it cuts.

Principle of Inclusions

Fragments of one rock incorporated into another are always older than the rock mass containing them.

Unconformity

A missing gap in the rock record caused by a period where deposition stopped, erosion occurred, and then deposition resumed.

Angular Unconformity

An unconformity where tilted or folded older sedimentary rocks are overlain by younger, flat-lying horizontal beds.

Disconformity

A gap in the rock record between parallel sedimentary strata, representing a period of erosion or non-deposition.

Nonconformity

An unconformity where younger, flat-lying sedimentary rocks overlie older metamorphic or intrusive igneous rocks.

Paleontology

The scientific study of fossils.

Fossil

The traces or remains of prehistoric life preserved in rock.

Permineralization

A fossilization process where mineral-rich groundwater flows through buried organic matter and precipitates minerals, turning it to stone.

Mold and Cast

A fossil created when a buried shell dissolves, leaving a void, which is later filled with new minerals.

Carbonization

A process where pressure squeezes out liquids and gases from a buried organism, leaving behind a thin film of carbon.

Trace Fossil

Indirect evidence of prehistoric life, such as tracks, burrows, footprints, and coprolites (fossilized dung).

Correlation

The process of matching rocks of similar age from two different geographic areas.

Principle of Fossil Succession

The concept that fossils are arranged by age and succeed one another in a definite, determinable order.

Index Fossil

Organisms that were widespread geographically but only existed for a short, specific span of geologic time; used to date rock layers.

Fossil Assemblage

A group of different fossils found together in a rock layer, often used as environmental indicators.

Radioactive Decay

The natural, spontaneous decay of an unstable parent isotope into a stable daughter isotope.

Half-Life

The time required for exactly one-half (50%) of the radioactive parent isotopes in a sample to decay into daughter isotopes.

Precambrian

An informal name for the earliest 4 billion years of Earth's history, dominated by simple life-forms lacking hard parts.

Geologic Time Scale

A dynamic timeline that divides Earth's history into specific units of time (Eons, Eras, Periods, and Epochs).