Geologic Time - Vocabulary Flashcards

Flashcards covering key vocabulary terms and definitions from the Geologic Time lecture notes.

Uniformitarianism

The idea that present geological processes operating today have worked in the same way throughout Earth’s history.

Original horizontality

Sediments are deposited in originally horizontal layers; deformation after deposition tilts or folds them.

Principle of superposition

In a sequence of undisturbed layers, the oldest rocks are at the bottom and the youngest at the top.

Cross-cutting relations

A feature (like a fault, dike, or intrusion) that cuts across existing rocks is younger than those rocks.

Inclusions (clasts)

Pieces of older rock contained within a younger rock indicate relative ages.

Breccia

Rock composed of angular fragments (clasts) cemented together.

Dike

A tabular igneous intrusion that cuts across preexisting rock layers.

Sill

An igneous intrusion that runs parallel to existing layers, intruding between them.

Unconformity

A surface representing a gap in the geological record due to erosion or non-deposition.

Angular unconformity

An unconformity where tilted rocks lie beneath younger, flat-lying rocks.

Great Unconformity

A major, widespread unconformity representing a huge gap in time (roughly 1.6–1.2 Ga in North America).

Terraces

Raised, levelled areas along river valleys formed by past erosion and deposition.

Desert varnish

Dark coating of iron-oxide and manganese-oxide on rock surfaces from long exposure.

Desert pavement

Surface covered with pebbles and larger clasts after finer material is removed.

Lithification

Process by which sediments are transformed into rock through compaction and cementation.

Isotopic dating

Determining actual ages of rocks using radioactive decay of isotopes.

Half-life

The time required for half the parent isotope to decay to daughter isotopes.

Parent isotope

The original unstable isotope that decays over time.

Daughter isotope

The product formed when a parent isotope decays.

Zircon

A timekeeper mineral used in radiometric dating; extremely resistant to weathering.

Jack Hills zircon

Zircon crystals from Jack Hills, among Earth’s oldest minerals.

Fossils

Remains, traces, or imprints of ancient life preserved in rocks.

Hard parts

Fossils formed from durable materials like shells, bones, or teeth.

Replacement fossilization

Original organic material is replaced by minerals (e.g., silica or pyrite).

Mold

A cavity left in sediment after the organism decays, which can later be filled.

Cast

Mineral-filled mold that forms a replica of the original organism’s shape.

Carbon film

A thin carbon layer preserving the silhouette of a fossil.

Impressions

Imprints left by organisms in sediment, often preserving external form.

Index fossil

A widespread, distinctive fossil used to date rock layers.

Faunal succession

Fossil assemblages successively change upward in time, aiding correlation.

Geologic time scale

A standard division of Earth’s history into Eons, Eras, Periods, Epochs, and Ages.

Phanerozoic Eon

Eon that includes the fossil record, comprising the Paleozoic, Mesozoic, and Cenozoic.

Paleozoic Era

Earliest of the three Phanerozoic Eras, dominated by marine life and early terrestrial life.

Mesozoic Era

Middle Phanerozoic Era, known as the “age of dinosaurs.”

Cenozoic Era

Youngest Phanerozoic Era, the era of mammals and birds.

Precambrian

Most of Earth’s early history before the Phanerozoic, including Archean, Proterozoic, and Hadean.

Relative dating

Determining the order of events without knowing exact numeric ages.

Absolute dating

Determining numeric ages of rocks using isotopic methods.

Isotopes

Variants of an element with the same number of protons but different numbers of neutrons.

Ma (Megaannum)

Unit equal to one million years ago, used in geologic time.