Relative Dating: Principles of Geology

• The geologic time scale and the basic understanding of Earth's history were established long before the invention of scientific methods for assigning numerical ages.
• The understanding of Earth's history relied on key principles of relative time.
• Nicolas Steno (1638-1686) introduced stratigraphy in 1669, which is the study of layered rocks.
• William Smith (1769-1839) created the first national geologic map of Britain and is known as "the Father of English Geology".
• 19th-century scientists developed a relative time scale, allowing geologists to order Earth's history without knowing numerical ages.

Relative Dating

• Determines if a rock or geologic event is older or younger than another, without knowing their specific ages.
• James Hutton's uniformitarianism ("the present is the key to the past") provided a foundation for interpreting Earth's rocks using scientific principles and relative age dating.

Relative Dating Principles

• Stratigraphy involves the study of layered sedimentary rocks.
• The principles of relative time are useful in stratigraphy.
• Principle of Superposition: In undisturbed sedimentary strata, the bottom layers are oldest, and the layers above them are younger.
• Principle of Original Horizontality: Rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally.
• Principle of Lateral Continuity: Strata are continuous in all directions within a depositional basin until they thin out at the edge of that basin.
• Principle of Cross-Cutting Relationships: Deformation events like folds, faults, and igneous intrusions that cut across rocks are younger than the rocks they deform or cut across.
• Principle of Inclusions: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock.
• Principle of Fossil Succession: Evolution has produced unique fossils that correlate to the units of the geologic time scale.
• Assemblages of fossils are unique to their time and are used to correlate rocks of the same age across a wide geographic distribution.

Grand Canyon Example

• Illustrates stratigraphic principles.
• Layers are arranged from oldest at the bottom to youngest at the top, based on superposition.
• The Coconino Sandstone is a predominant white, laterally continuous layer below the canyon rim.
• The rock layers demonstrate lateral continuity, found on both sides of the Grand Canyon.
• The diagram "Grand Canyon's Three Sets of Rocks" shows cross-cutting relationships, superposition, and original horizontality, with oldest sedimentary, igneous, and metamorphic rocks at the bottom.
• Metamorphic schist is the oldest rock, and the cross-cutting granite intrusion is younger.
• The layers on the canyon walls are numbered in reverse order, illustrating superposition.
• The Grand Canyon region lies in the Colorado Plateau, characterized by horizontal strata, following original horizontality.

Unconformities

• Strata were originally deposited flat on top of older igneous and metamorphic "basement" rocks, following the horizontality principle
• The contact between strata and older basement rocks, broken by metamorphism, intrusion, and erosion, is an unconformity.
• An unconformity represents a period of no deposition or erosion, where rocks representing Earth's history are missing, shown as wavy lines in cross sections.
• A nonconformity occurs when sedimentary rock is deposited on top of igneous and metamorphic rocks.
• Alternating marine transgressions and regressions, where sea level rose and fell, formed the Grand Canyon strata: Marine strata during transgressions and unconformities during regressions.
• Disconformities are unconformities between parallel strata layers, indicating non-deposition or erosion, with absent rock layers.
• The Great Unconformity is an angular unconformity where horizontal strata overlie tilted strata.
• Lower strata were tilted by tectonic processes, eroded, and then covered by horizontal strata, creating the angular unconformity.

Applying Relative Dating Principles

• In the block diagram, the sequence of geological events can be determined by using the relative-dating principles and known properties of igneous, sedimentary, and metamorphic rock
• The sequence begins with the folded metamorphic gneiss on the bottom.
• Next, the gneiss is cut and displaced by the fault labeled A.
• Both the gneiss and fault A are cut by the igneous granitic intrusion called batholith B; its irregular outline suggests it is an igneous granitic intrusion emplaced as magma into the gneiss.
• Since batholith B cuts both the gneiss and fault A, batholith B is younger than the other two rock formations.
• Next, the gneiss, fault A, and batholith B were eroded forming a nonconformity.
• This unconformity was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression.
• Next, igneous basaltic dike D cut through all rocks except sedimentary rock E, showing a disconformity between sedimentary rocks C and E.
• The top of dike D is level with the top of layer C, establishing erosion flattened the landscape before the deposition of layer E.
• Fault F cuts across B, C, and E, producing a fault scarp creating the modern landscape.

Absolute Dating

• Relative time reveals the sequence of Earth's events, absolute dating provides specific numeric ages.
• The discovery of radioactivity enabled radioisotopic dating, assigning numerical time units to mineral grains.
• Numerical values are independent of comparisons with other rocks, called absolute dating.

Radioactive Decay

• Most elements on the Periodic Table contain multiple isotopes.
• An isotope is an atom of an element with a different number of neutrons.
• Hydrogen (H) always has 1 proton but can have 0, 1, or 2 neutrons.
• Many elements have stable and unstable isotopes.
• Unstable isotopes, called radioactive isotopes, spontaneously decay, releasing subatomic particles or energy.
• The half-life is constant and measurable and it's the time it takes for half of the atoms in a substance to decay.
• Two assumptions: mineral grains formed with the rock (e.g., igneous crystallization) and remain a closed system (not altered by moving elements).
• Igneous rock is best for dating.
• Igneous pyroclastic layers and lava flows help date sedimentary sequences.
• Resistant mineral zircon establishes very old dates.

Radioactive Atoms Decay

• Alpha decay: an alpha particle (2 protons, 2 neutrons) emitted from the nucleus.
• Beta decay: a neutron splits into an electron (beta ray) and a proton, increasing the atomic number.
• Electron capture: a proton captures an electron, becoming a neutron, reducing atomic number.
• The radioactive decay product of an element is called its daughter isotope and the original element is called the parent isotope.
• Radioactive elements decay until they form a stable, non-radioactive daughter in a series called a decay chain.

Radioisotopic Dating

• Parent and daughter isotopes are separated from the mineral.
• The mass spectrometer measures the number of each isotope in the sample.
• Uranium-lead dating uses 238U and 235U isotopes and 206Pb and 207Pb isotopes.
• A simple example of calculating age uses the daughter-to-parent ratio of isotopes.
• At mineral formation, 0% daughter and 100% parent isotope, so the daughter-to-parent ratio (D/P) is 0.
• After one half-life, 50% daughter and 50% parent, D/P = 1.
• After two half-lives, 75% daughter and 25% parent (75/25 ratio) and D/P = 3.
• Applying the uranium/lead technique provides two separate clocks.
• Modern applications can have accuracies of plus or minus two million years in 2.5 billion years (±0.055%).
• Radiocarbon dating uses unstable isotope carbon-14 (14C) and stable isotope carbon-12 (12C).
• Carbon-14 is created in the atmosphere when cosmic particles interact with nitrogen-14 (14N).
• The ratio 14C/12C gives the age of the specimen after death since CO2 exchange with the atmosphere stops.
• 14C has a half-life of 5,730 years, making the radiocarbon dating technique useful for dating back about 57,300 years.
• Carbon dating calibrates baseline levels against other reliable dating methods and is reliable for dating archaeological specimens and recent geologic events.

Age of the Earth

• In the 1950s, Clair Patterson determined Earth's age using radioactive isotopes from meteorites, dating uranium/lead to 4.55 billion years.
• The current estimate for the age of the Earth is 4.54 billion years.

Dating Geological Events

• Radioactive isotopes common in mineral crystals are useful.
• The uranium/lead method is used with zircon (ZrSiO4) crystals with multiple layers recording metamorphic events.
• Zircon crystals from the Jack Hills of Western Australia formed 4.4 billion years ago and are the oldest known terrestrial mineral grains which indicate early Earth had continental rocks, oceans, and weathering.

Dating Sedimentary Sequences

• Scientists use igneous events to date sedimentary sequences: strata between lava flow and volcanic ash bed or volcanic dike cutting across sedimentary strata.
• Potassium in evaporite sediments and primary sedimentary minerals with 40K can also provide dates.

Other Absolute Dating Techniques

• Luminescence dating dates when silicate minerals were last exposed to light or heat, useful for sediments less than 1 million years old.
• During Thermoluminescence, buried sediments are exposed to radiation, trapping electrons in silicate minerals.
• Fission Track dating counts damage tracks in crystal atomic structures from 238U decay, useful from 100,000 to 2 billion years ago.

Fossils and Evolution

• Fossils are any evidence of past life preserved in rocks.
• Fossils may be body part remains, impressions, casts, mineral replacements, or evidence of behavior like footprints and burrows.
• Preservation is best in the ocean, but even then, the likelihood is limited.
• The fossil record is incomplete.
• Fossil records are used for stratigraphic correlation, using the Principle of Faunal Succession.

Types of Preservation

• Remnants/impressions of hard parts are the most common fossils.
• Actual preservation: original materials preserved (e.g., insects in amber, mammoth skin).
• Permineralization: groundwater elements impregnate spaces within the body (e.g., petrified wood).
• Molds and casts: original material dissolves, leaving a cavity, and the mold is filled subsequently.
• Carbonization: organic tissues compress, volatiles disappear, leaving a carbon silhouette (e.g., leaf and fern fossils).
• Trace fossils: indirect evidence like burrows and footprints (e.g., dinosaur tracks). Ichnology is the study of prehistoric animal tracks.

Evolution

• Charles Darwin recognized life forms evolve, driven by natural selection.
• Species: organisms with shared characteristics capable of reproducing fertile offspring.
• Variation occurs by gene mixing or mutations, which can introduce advantageous characteristics.
• Individuals with characteristics suited to environmental challenges are more likely to reproduce and pass on those traits.
• Species extinction occurs if they fail to overcome environmental challenges.
• Average lifespan of a fossil species: ~1 million years.
• Genetic isolation may be enhanced by geographic barriers, leading to species evolution.
• Darwin elaborated on evolution by natural selection in On the Origin of Species.
• Evolution is a well-established theory.

Correlation

• Matching sedimentary strata of the same age in different geographical areas.

Four Main Types of Correlation

• Stratigraphic: Establish the same age of sedimentary strata at distant geographical areas using their stratigraphic relationship.
• Lithostratigraphic: Similar age of strata based on lithology (composition and physical properties).
• Chronostratigraphic: Matching rocks of the same age, even with different lithologies.
• Biostratigraphic: Uses index fossils to determine strata ages.

Stratigraphic Correlation

• Relies on stratigraphic relationships to establish the same age of sedimentary strata at distant geographical areas.
• Geologic histories are constructed by mapping and creating detailed descriptions of the strata from bottom to top.
• Jurassic rocks correlate between Canyonlands and Zion National Parks in Utah.

Lithostratigraphic Correlation

• Establishes similar age of strata based on lithology (composition and physical properties).
• Navajo Sandstone is found in Utah (Zion and Canyonlands NPs.) and is the Aztec Sandstone in Nevada and Nugget Sandstone near Salt Lake City.

Chronostratigraphic Correlation

• Matches rocks of the same age, even with different lithologies.
• Different sedimentary rock lithologies can form simultaneously in different places.
• Shallow-water marine lagoon, reef, and deep-water marine siltstone formed at the same time in the Permian.

Biostratigraphic Correlation

• Employs index fossils to ascertain strata ages.
• Fossils assign an absolute date range to a formation.
• Index fossils: geographically widespread lifeforms with narrow time intervals.
• Microfossils: single-celled organisms are the best fossils for biostratigraphic correlation.
• Foraminifera are useful index fossils for Cretaceous Period and younger rocks.
• Conodonts date Cambrian through Triassic rocks in shallow marine environments.

Geologic Time Scale

• Geologic time is divided into eons, eras, periods, epochs, and ages.
• Partitions are the same everywhere.
• Rocks of all ages may not be present at a given location, leading to broken rock records.
• The geologic time scale was developed in the 19th century using stratigraphy.
• Geologists used biostratigraphic correlation to assign names to rocks worldwide