Relative Dating Principles

• Relative dating determines if a rock or geologic event is older or younger than another without knowing their specific age.
• James Hutton's uniformitarianism ("the present is the key to the past") allowed interpreting rocks using scientific principles and relative age dating.

Relative Dating Principles

• Stratigraphy studies layered sedimentary rocks.
• Principles of relative time are useful in all of geology and especially stratigraphy.
• Principle of Superposition: In undisturbed sedimentary strata, bottom layers are oldest, and top layers are youngest.
• Principle of Original Horizontality: Sediments and lava flows are originally laid down horizontally, except at basin margins.
• Principle of Lateral Continuity: Strata are continuous in all directions within a depositional basin until they thin out or hit a barrier.
• Principle of Cross-Cutting Relationships: Deformation events (folds, faults, intrusions) are younger than the rocks they affect.
• Principle of Inclusions: Rock formations containing pieces of another rock are younger than the included rock.
• Principle of Fossil Succession: Unique fossils correlate to units of the geologic time scale, allowing correlation of same-age rocks across wide areas.

Grand Canyon Example

• The Grand Canyon illustrates stratigraphic principles.
• Superposition: Oldest rocks are at the bottom, youngest at the top.
• Lateral Continuity: Rock layers are found on both sides of the canyon.
• The Grand Canyon region is part of the Colorado Plateau, with horizontal strata following the principle of original horizontality.
• Metamorphic schist is the oldest rock formation, and the cross-cutting granite intrusion is younger, which show relationships.
• The contact between strata and older basement rocks is an unconformity, representing a period of non-deposition or erosion.

Unconformities

• An unconformity represents a period of non-deposition or erosion.
• Three types of unconformities: nonconformity, disconformity, and angular unconformity.
• Nonconformity: Sedimentary rock deposited on top of igneous and metamorphic rocks.
• Disconformity: Non-deposition or erosion between parallel layers of strata.
• Angular Unconformity: Horizontal strata overlie tilted strata.
• Marine transgressions form marine strata, and regressions expose land to erosion, creating unconformities.

Applying Relative Dating Principles

• Geological event sequence can be determined using relative-dating principles and rock properties.
• The sequence starts with folded metamorphic gneiss, cut by fault A, then batholith B.
• Erosion forms a nonconformity before sedimentary rock C is deposited.
• Dike D cuts through all rocks except sedimentary rock E, indicating a disconformity between C and E.
• Fault F cuts across older rocks, producing a fault scarp.
• Current erosion processes shape the modern landscape.

Absolute Dating

• Relative time does not provide specific numeric ages.
• Radioisotopic dating assigns specific time units to mineral grains.
• Radioisotopic dating is the most common absolute dating method.
• Canada’s Nuvvuagittuq Greenstone Belt may contain Earth’s oldest rocks as well as evidence of life on Earth.

Radioactive Decay

• Isotopes are atoms of an element with different numbers of neutrons.
• Radioactive isotopes are unstable and decay over time, releasing particles or energy.
• Radioactive decay is a random event for individual atoms, but predictable for large groups.
• Half-life is the time for half the atoms in a substance to decay.
• The half-life is constant and measurable for a given radioactive isotope.
• Key assumptions: Mineral grains formed with the rock, and the mineral crystals remain a closed system.
• Igneous rocks are best for dating, while metamorphic rocks may reset the clock.
• Alpha decay: An atom emits an alpha particle (2 protons, 2 neutrons), transforming into an element two atomic numbers lower.
• Beta decay: A neutron splits into an electron and a proton, increasing the atomic number by one.
• Electron capture: A proton captures an electron, decreasing the atomic number by one.
• Radioactive elements decay until a stable daughter isotope is formed.

Radioisotopic Dating

• Parent and daughter isotopes are separated from the mineral via chemical extraction and analyzed.
• Mass spectrometers measure the amounts of each isotope in the sample.
• The daughter-to-parent ratio is used for age calculation.
• Uranium/lead technique provides two clocks (238U and 235U) for cross-checking.
• Radiocarbon dating uses the ratio of carbon-14 (14C) to carbon-12 (12C) to date organic substances.
• Carbon-14 is created in the atmosphere and incorporated into living matter.
• Radiocarbon dating is useful for dating specimens up to 57,300 years old.
• Radiocarbon dating relies on daughter-to-parent ratios derived from a known quantity of parent 14C.
• Carbon-14 baseline levels have been calibrated against other dating methods.

Age of the Earth

• Lord Kelvin estimated Earth's age based on cooling, but his estimate was inaccurate.
• Clair Patterson used uranium/lead dating on meteorites to estimate Earth's age as 4.55 billion years.
• The current estimate for the age of the Earth is 4.54 billion years.

Dating Geological Events

• The uranium/lead method is used with zircon crystals to date geological events in ancient rocks.
• Zircon crystals can record multiple metamorphic events.
• Jack Hills zircon crystals are the oldest known terrestrial mineral grains (4.4 billion years old).
• Igneous rocks provide dates of crystallization from magma.
• Scientists can use igneous events to date sedimentary sequences.

Other Absolute Dating Techniques

• Luminescence dating measures the time since silicate minerals were last exposed to light or heat at the surface of Earth.
• Luminescence dating is useful for dating young sediments less than 1 million years old.
• Fission track dating relies on damage to crystal structures from 238U decay.
• The number of tracks corresponds to the age of the grains.
• Fission track dating works from about 100,000 to 2 billion years ago.

Fossils and Evolution

• Fossils are evidence of past life preserved in rocks.
• Fossils are used for stratigraphic correlation.
• Fossils provide a method used for establishing the age of a formation on the Geologic Time Scale.
• Fossils may be remains of body parts, impressions, casts, or evidence of behavior.
• The fossil record is incomplete and represents a small percentage of life that existed.

Types of Preservation

• Remnants or impressions of hard parts are the most common fossils.
• Common types of fossil preservation: actual preservation, permineralization, molds and casts, carbonization, and trace fossils.
• Actual Preservation: Original materials are preserved, such as insects in amber and mammoth skin and hair.
• Permineralization: Groundwater elements impregnate all spaces within the body, preserving detailed structures such as petrified wood.
• Molds and Casts: Original material dissolves, leaving a cavity (mold), which can be filled with sediments (cast).
• Carbonization: Organic tissues are compressed, leaving a carbon silhouette.
• Trace Fossils: Indirect evidence of organisms, such as burrows and footprints.

Evolution

• Charles Darwin proposed natural selection drives evolution.
• Natural selection operates on organisms living under environmental conditions that posed challenges to survival.
• Species: Population of organisms with shared characteristics capable of reproducing fertile offspring.
• Variation occurs via mixing of genes and mutations.
• Advantageous characteristics increase an individual's survival chances.
• The average lifespan of a species in the fossil record is around a million years.
• Evolution allows life to meet challenges posed by a dynamic Earth.

Correlation

• Correlation matches sedimentary strata of the same age that are geographically separated.
• Correlation can be determined using magnetic polarity reversals, rock types, unique rock sequences, or index fossils.
• Types of correlation: stratigraphic, lithostratigraphic, chronostratigraphic, and biostratigraphic.

Stratigraphic Correlation

• Stratigraphic correlation establishes which sedimentary strata are the same age at distant geographical areas by means of their stratigraphic relationship.
• Geologists construct geologic histories of areas by mapping and making stratigraphic columns.

Lithostratigraphic Correlation

• Lithostratigraphic correlation establishes a similar age of strata based on the lithology (composition and physical properties) of that strata.

Chronostratigraphic Correlation

• Chronostratigraphic correlation matches rocks of the same age, even though they are made of different lithologies.

Biostratigraphic Correlation

• Biostratigraphic correlation uses index fossils to determine strata ages.
• Index fossils are individual fossils or assemblages that were uniquely present during specific intervals of geologic time.
• Index fossils come from geographically widespread lifeforms with a limited species lifespan.
• Microfossils, such as foraminifera and conodonts, are useful for biostratigraphic correlation.

Geologic Time Scale

• Geologic time is divided into eons, eras, periods, epochs, and ages.
• The partitions of the geologic time scale are the same everywhere on Earth.
• Rocks of all ages may not be present at a given location.
• The geologic time scale was developed using stratigraphy.
• Geologists used biostratigraphic correlation (fossils) to assign era and period names to sedimentary rocks on a worldwide scale.
• The Anthropocene is a proposed new geologic time period influenced by humanity.