Chapter 7: Geologic Time and Dating Techniques

PreCambrian Eon

The oldest eon in the geologic time scale.

Phanerozoic Eon

The eon following the Precambrian, characterized by abundant fossil evidence.

Paleozoic Era

The first era of the Phanerozoic Eon, known for the development of early life forms.

Mesozoic Era

The second era of the Phanerozoic Eon, known as the age of dinosaurs.

Cenozoic Era

The most recent era of the Phanerozoic Eon, known as the age of mammals.

Relative Dating

Determines sequences of events based on relative positions/relationships of features.

Absolute Dating

Calculates timing of events and rates of geologic processes, assigning a numeric age.

Principle of Superposition

States that in an undisturbed succession of strata, the oldest layers are at the bottom.

Principle of Original Horizontality

States that sedimentary layers were deposited nearly horizontally and parallel to the Earth's surface.

Principle of Lateral Continuity

Strata layers are continuous in all directions until they thin out at the edge of that basin.

Principle of Cross-Cutting Relationships

A rock unit or fault that cuts another geologic unit is younger than the unit that was cut.

Principle of Inclusions

Fragments of rock within a larger rock unit are older than the rock it's enclosed within.

Principle of Fossil Succession

Fossils occur in a definite determinable order due to evolution.

Radioactive Isotopes

Atoms that decay over time, releasing radiation, used to determine absolute age.

Common Isotopes in Dating

Uranium-238 decays to Lead-206 and Carbon-14 decays to Nitrogen-14.

Half-life of a Radioisotope

The time it takes for half of the radioactive atoms in a sample to decay into a more stable form.

Applications of Radioactive Isotopes

Used in medical imaging, cancer treatment, and nuclear power generation.

Law of Uniformitarianism

The principle that the same geological processes that operate today also operated in the past.

Quantitative Method

A method that involves assigning numeric ages to geologic events or materials.

Cyclical Features

Features such as tree rings, varves, and layers of ice that can be used in absolute dating.

Environmental Research

Radioactive isotopes aid in climate and environmental research.

Medical Imaging Isotopes

Iodine and Technetium are used in medical imaging and cancer treatment.

Radioisotope

An isotope that has a fixed half-life, meaning the decay rate is constant.

Half-life

The time required for half of the original isotope to decay; after one half-life, 50% remains, after two half-lives, 25% remains.

Radiocarbon dating

A method used to date organic materials such as wood, bones, shells, and charcoal, which can only date things up to 50,000 years.

U-Pb dating

A method used to date rocks and minerals that can date millions to billions of years.

Body Fossils

Remains of the actual organisms that have been altered, providing direct evidence of past life.

Trace Fossils

Evidence of ancient organisms' behaviors, representing activities of the organism while it was alive.

Indirect evidence

Evidence that does not include actual body parts but shows signs of past biological activity, such as trace fossils.

Direct evidence

Physical remains of organisms, providing clear proof of past life, such as body fossils.

Fossilization

The process by which fossils are preserved.

Cats & Molds

A fossilization process where original material dissolves, leaving a cavity.

Carbonization

A fossilization process where organisms get compressed and only carbon is perceived, resulting in a carbon silhouette.

Fossil preservation factors

Factors that increase the chance of fossil preservation: hard parts, low oxygen environment, and rapid burial.

Stratigraphic correlation

Matching or linking strata of rock layers of the same age from different locations.

Lithostratigraphic correlation

A correlation technique based on the physical and chemical characteristics of rock layers.

Chronostratigraphic correlation

A correlation technique that relates rock layers to specific time intervals.

Biostratigraphic correlation

A correlation technique that uses fossil content to correlate rock layers.

Magnetostratigraphy

A correlation technique that uses the magnetic properties of rock layers.

Chemostratigraphy

A correlation technique that uses the chemical composition of rock layers.

Unconformities

Surfaces between rock layers of greatly differing ages, representing 'missing time' in the rock record.

Angular Unconformity

An erosional surface with flat/horizontal rock layers above and tilted rock layers below.

Nonconformity

An erosional surface between sedimentary rocks and crystalline rocks.

Disconformity

A visible, uneven or irregular erosional surface between parallel sedimentary rock layers of different ages.