Geology Final: Geologic Time Study

Geology Final Flashcards - Key Vocabulary and Definitions

Uniformitarianism

The principle that the processes operating today also operated in the past at similar rates.

Original Horizontality

Sediments are originally deposited in horizontal layers.

Principle of Superposition

In an undeformed sequence, younger rocks lie above older rocks.

Cross-cutting Relationships

A geologic feature that cuts across another is younger than the feature it cuts.

Unconformity

A surface representing a gap in the geologic record, where rock layers were lost by erosion or non-deposition.

Angular Unconformity

An unconformity where tilted or folded rocks are overlain by younger, more flat-lying strata.

Disconformity

An unconformity between parallel layers of sedimentary rock.

Nonconformity

An unconformity where sedimentary rocks lie above eroded metamorphic or igneous rocks.

Fossils

Form from the burial and preservation of organisms, often in sedimentary rock.

Principle of Faunal Succession

Fossil organisms succeed one another in a definite and recognizable order.

Radioactive Decay

The spontaneous breakdown of unstable isotopes into stable daughter products.

Half-life

The time it takes for half of a radioactive isotope to decay.

Decay Equation

Used to find the age of a rock by calculating the number of half-lives elapsed from the parent-to-daughter isotope ratio.

Carbon Dating

Used for recent organic materials up to ~50,000 years old.

U-Pb Dating

Used for much older rocks (millions to billions of years).

Brackets in Dating

Used to date sedimentary rocks by dating igneous rocks above and below sedimentary layers to constrain the age.

Four Eons of Geologic Time

Hadean, Archaean, Proterozoic, Phanerozoic.

Major Events of the Archaean

Formation of early continental crust and first evidence of life.

Significance of the Proterozoic

Development of oxygen in the atmosphere and buildup of stable continents.

Eras of the Phanerozoic

Paleozoic, Mesozoic, and Cenozoic.

Major Biological Events of the Paleozoic

Cambrian explosion and early development of complex marine life.

Defining the Mesozoic

Age of reptiles, dinosaurs, and early mammals.

Characterizing the Cenozoic

Rise of mammals and modern ecosystems.

Major Extinction Events

End-Permian (largest), End-Cretaceous (dinosaurs extinct), and others like Late Devonian.

Causes of Mass Extinctions

Volcanism, asteroid impacts, climate change, sea level changes, and ocean anoxia.

Unconformities

They indicate missing time in the rock record, often due to erosion or lack of deposition.

Angular Unconformities

They show that older layers were tilted or folded before new sediment was deposited.

Index Fossils

They are widespread, short-lived, and unique, helping to identify specific time intervals.

Relative Dating

They show order of events but don't provide exact numerical ages.

Isotopic Dating Techniques

Providing absolute ages for rocks and fossils, anchoring relative timelines.

U-Pb Dating

Uranium-238 and Uranium-235 decay to lead isotopes.

Radiometric Dating

Includes various isotope systems (U-Pb, K-Ar), while radiocarbon is specific to carbon-14 in organic matter.

Assumptions for Radiometric Dating

Closed system (no loss/gain of parent or daughter), known initial conditions, and constant decay rate.

Plate Tectonics

By recycling crust and enabling growth through accretion and magmatism.

Phanerozoic

The Cambrian Explosion of diverse life forms.

Proterozoic

Relatively little biological evolution compared to other times.

Asteroid Impact Hypothesis

A worldwide layer enriched in iridium and the Chicxulub crater.

End-Permian Extinction

Caused the loss of about 90% of marine species and 70% of terrestrial species.

Massive Volcanism

Releasing gases that cause climate change, acid rain, and ocean anoxia.

Extinction Events

They create opportunities for new groups to diversify and dominate.