Understanding Geologic Time

Geologic Time

• Earth is approximately 4.566 billion years old, discovered through geoscience advancements.
• Understanding geologic time places human civilization into perspective as a recent and small portion of Earth's history.

Visualizing Deep Time

• Deep time is a concept that refers to the vast scale of geological time.
• Earth's age is 4,566,000,000 years, requiring a substantial amount of resources to represent each year with a single zero.
• The vastness of geological time is hard to visualize.

Abbreviations for Geological Time

• ka: kilo-annum, representing one thousand years.
• Ma: mega-annum, representing one million years.
• Ga: giga-annum, representing one billion years.
• kya and mya represent events that occurred thousands or millions of years ago.
• Numerical (absolute) dating assigns actual ages in years before the present.

Geology as a Historical Science

• Geology studies Earth and its history, reconstructing past events using rocks, minerals, and fossils.
• Geology is a historical science, alongside evolutionary biology, climatology, archaeology, and astronomy.
• Astronomy gives a historical perspective as it takes billions of years for light from distant galaxies to reach Earth.

Foundational Concepts of Historical Geology

• Uniformitarianism, a basic assumption, states that chemical and physical laws of nature are constant through Earth’s history.
• Uniformitarianism allows observations of the modern world to understand Earth’s history.

James Hutton and Uniformitarianism

• James Hutton (1726-1797) developed uniformitarianism through observing slow erosion and sediment formation rates.
• Geological time is vast, challenging the idea of Earth being created recently and unchanging.
• Hutton viewed Earth as constantly changing, and saw "no vestige of a beginning, and no prospect of an end."

Charles Lyell and the Popularization of Uniformitarianism

• Charles Lyell (1797-1875) popularized uniformitarianism in "Principles of Geology" (1830).
• Geological processes act slowly and continuously, though rates and intensities have changed over time.
• Earth's history includes catastrophic events, like the asteroid impact that led to dinosaur extinction around 66 Ma.
• "The present is the key to the past" is the uniformitarian view, helping us to understand Earth's past and predict its future.

Principles of Relative Dating

• Geologists determined Earth's history and life's evolution without knowing actual rock ages before numerical dating.
• Relative dating determines the sequence of events in Earth’s past.
• Understanding the relationships between geological formations.

Relative Dating and Sedimentary Rock

• Principles of relative dating are used to understand the geological history of sedimentary rocks covering continental land.
• This layer preserves information about life's development and evolution.
• Sedimentary rock thickness varies, averaging around 1800m.

Applying Relative Dating

• Relative dating determines if one geological or paleontological event happened before or after another.
• Questions include the order of rock layer formation and organism existence.
• Six principles guide relative dating, with superposition being the most important.

Principle of Superposition

• Nicholas Steno (1638-1686) linked rock layers with sediment, minerals, and fossils.
• Steno developed superposition, the basis for relative dating.
• Superposition states that lower rocks are older than rocks above them.
• Superposition is observed in rock layers where the bottom layers are older.

Principle of Faunal Succession

• William Smith (1769 – 1839) used fossils to correlate rock layers of the same age.
• Smith’s principle of faunal succession is the ordering of fossil species which helps correlating rock layers.
• Rock layers in different locations are the same age if they have the same fossils.

Principle of Original Horizontality

• Sediments are deposited in layers parallel to the Earth's surface.
• Volcanic rocks formed by lava flows, pyroclastic flows, and ashfall deposits also follow this principle.
• Understanding the geological history of deformed rock layers requires unfolding or reconstructing them to their original horizontal position.

Principle of Lateral Continuity

• Sediment layers extend outward in all directions
• Separated rocks can be assumed to be continuous as limits are controlled by sediment and basin size and shape.
• Thickest deposits are found closest to the source, thinning with distance.

Principle of Cross-Cutting Relationships

• A geological feature that cuts across another is younger than the disrupted feature.
• A fault cutting across layers is younger than the sedimentary layers.
• Igneous dikes intruding into pre-existing rock are younger.
• Overprinting structures like tectonic cleavage or folding are younger than the original feature

Principle of Inclusions

• Rock fragments included in another rock are older than the rock they are included in.
• A xenolith of granite in basalt indicates the basalt intruded into the granite.
• Contact metamorphism, like baking of granite by basalt, needs for the "baked" rock to have been there before the intrusion.

The Grand Canyon Example

• The Grand Canyon demonstrates stratigraphic principles.
• Rock layers are arranged from oldest at the bottom to youngest at the top (superposition).
• Rock layers are laterally continuous, can be found on both sides of the canyon.
• The canyon reveals 1.7 billion years of fossils, volcanic activity, and geologic history.
• Metamorphic schist is the oldest rock formation in the lower Grand Canyon, and the cross-cutting granite intrusion is younger.
• The principle of original horizontality is displayed by the horizontal strata of the Colorado Plateau.

Unconformities – Missing Time

• An unconformity is an interruption in the deposition of sedimentary layers.
• Types of unconformities can be seen in rock exposed in the Grand Canyon.
• The Great Unconformity is where Proterozoic rocks were tilted and eroded before Paleozoic rocks were deposited.
• The difference in time can be close to 300 million years.

Types of Unconformities

• A nonconformity is an erosional surface separating different rock types, such as igneous or metamorphic rock from sedimentary rock.
• A disconformity occurs between parallel sedimentary layers.

Applications of Relative Dating: Example 1

• An exposure in Albersweiler, Germany, has three rock types: gneiss, black igneous rock, and red sandstone.
• There is an erosional surface between the gneiss and the sandstone.
• The gneiss is baked adjacent to the igneous rock.
• Using superposition, the gneiss is older than the sandstone.
• The black igneous rock is a dike (magma intrusion).
• The nonconformity exists between metamorphic/igneous rock and sedimentary rock.
• The sequence of events is: Oldest: gneiss, igneous dike, nonconformity, sandstone Youngest.

Applications of Relative Dating: Example 2

• An imaginary sequence of rocks and geological events labeled A-J can be reconstructed using superposition and cross-cutting relationships.
• Rock layer F is older than E, E is older than D, D is older than C, and C is older than B, and B is older than J.
• H (igneous intrusion) cuts into rock layers B-F, but it is not known if it cuts into J.
• H is younger than B-F and assumed to be younger than J due to contact metamorphism.
• Fault A cuts across and displaces rock layers B-J and is younger than those layers.
• Fault A might be older or younger than rock unit H.
• G (another igneous intrusion) cuts across A-J and is younger than all of them, and not displaced by fault A.
• Erosional surface I at the top cuts both A and G and is younger than them.
• Oldest F, E, D, C, B, J*, (H or A), G, I Youngest.

Dating Rocks Using Fossils

• Paleontology dates the systematic cataloging and assignment of relative ages to different organisms.
• Oldest undisputed fossils are about 3.5 Ga old.
• Sedimentary record from 600 Ma forward is rich in fossil remains, providing a detailed record of the history and evolution of life on Earth.
• Fossils do not provide numerical ages of rocks directly.
• Geologists use isotopic dates from rocks associated with fossil-bearing rocks to put specific time limits on most fossils.

Faunal Succession

• Sequence of evolution can be used to determine the relative ages of rocks by William Smith’s principle of faunal succession.
• Principle can be applied to fossils of plants and other organisms.

Geologic Range

• Identifying a fossil to the species or genus level and knowing the time period when the organism lived helps assign a range of time to the rock.
• Index fossils are organisms that lived for relatively short time periods and were distributed over a wide geographic area.
• Index fossils can be used to compare rocks from different regions.
• Comparison of geologic ranges constrains the age of a rock based on several fossils.

Biozones

• Biozone fossils allow stratigraphic intervals to be defined on the basis of specific fossils.
• Individual species lived for a relatively short time and can be distinguished from others.
• Ammonoids' suture lines are variable enough to identify species and estimate the age of rocks.
• Foraminifera, small carbonate-shelled marine organisms, began in the Triassic, are used as biozone fossils and were diverse during the Cretaceous.

Numerical (or Absolute) Dating

• Numerical dating (absolute) assigns actual dates to geological events.
• Geochronology is the science of absolute age dating.
• Radiometric dating (isotopic dating) is the method of geochronology.

Early Attempts at Absolute Dating

• Archbishop James Ussher (1581-1686) estimated Earth's age using the Old Testament genealogy.
• He concluded Earth was formed in 4004 B.C., about 6,000 years ago.
• Geologists applied uniformitarian views in the 1800s, observing sediment accumulation rates which led them to estimate many millions of years old.
• These early estimates were too young due to unrecognized unconformities.
• The discovery of radioactivity allowed geologists to assign absolute age dates to rocks and discover the age of the Earth.

Radiometric Dating

• Hypotheses of absolute ages of rocks (as well as the events that they represent) are determined from rates of radioactive decay of some isotopes of elements that occur naturally in rocks
• Radioactivity is important for assigning absolute age dates to rocks.

Elements and Isotopes

• An element is defined by the number of protons in its nucleus (the atomic number).
• Periodic table shows elements, atomic numbers, and properties.
• Isotopes are variations of elements with different numbers of neutrons.
• Isotopes are defined by their mass number (protons + neutrons).
• Carbon has isotopes like Carbon-12 (12C), Carbon-13 (13C), and Carbon-14 (14C).

Radioactive Decay

• Most isotopes are stable, but some are unstable and undergo radioactive decay.
• Radioactive decay involves unstable isotopes shedding energy and subatomic particles.
• Atomic nucleus that undergoes radioactive decay is known as the parent and the resulting stable element, the daughter product (or, decay product)
• Radioactive element decay forms a different, stable element.
• The atomic nucleus undergoing radioactive decay is the parent, and the resulting stable element is the daughter product.

Applying Radioactive Decay to Dating Rocks

• Not dating the rock, dating a mineral in the rock.
• Radioactive elements become part of the mineral chemistry during crystallization.
• Ideal mineral accepts radioactive parent isotopes and excludes daughter isotopes.
• As the parent decays over time, their daughter products accumulate.
• During mineral growth, the crystal lattice is able to be “picky” or “sloppy" about what atoms it will let into its crystal structure.

Suitable Minerals for Numerical Dating

• Mineral chemistry involves radioactive isotopes of essential elements or elemental impurities.
• The parent-daughter decay pair within the mineral crystal starts off with 0% daughter isotopes, and 100% radioactive parent isotopes.
• Any daughter isotopes found in the mineral crystal got there due to radioactive decay of a parent isotope some time after the crystal originally formed, and they were NOT there from the very beginning.
• Minerals must actively exclude stable daughter atoms resulting from radioactive breakdown.
• Parent isotopes "fit" into the crystal by valence state and atomic size, while daughter isotopes do not.
• A comprehensive count of all the parent isotopes and all the daughter isotopes in that mineral system will sum to 100%.
• Zircon (ZrSiO4) allows 238U to substitute for Zr, but excludes 206Pb (daughter product) due to atomic characteristics.
• Suitable minerals are rare; only a handful are common and 'picky' enough for isotopic dating.
• Minerals that meet these criteria are very few.

Half-Life and Decay Constant

• Two pieces of information are needed to calculate how old a given mineral is: the ratio of parent to daughter isotopes, and the rate of decay, or half-life, of the radioactive parent.
• Ratio of parent to daughter isotopes, and the rate of decay of the radioactive parent determine mineral age.

Measuring Isotope Ratios

• The ratio of parent to daughter atoms is measured by using a mass spectrometer which is a very sensitive, high precision instrument that detects and separates atoms based on their mass.
• A mass spectrometer measures the ratio of parent to daughter isotopes
• The decay rate of a parent isotope into its daughter product is constant.
• A half-life is the amount of time needed for half of the parent atoms in a sample to decay into daughter products.
• During each half-life, half the parent atoms decay into daughter products.

Determining Age from Half-Lives

• Ratio of parent atoms relative to daughter products in a sample determines half-lives passed since formation.
• Example: Initial state has parent atoms.
• After one half-life, half the parent atoms become daughter products.
• After two half-lives, 75% of the original parent atoms transform into daughter products.

Calculating Radiometric Dates

• Convert half-lives passed into a numerical age.
• Multiply the number of half-lives by the half-life decay constant of the parent isotope.
• A granite sample reveals that 75% of its parent 40K remains while 25% of the daughter 40Ar is present.
• The half-life of 40K is 1.25 billion years, with 0.4 of one half life has passed since its original formation.
• Calculate the samples age by multiplying 1.25 by 0.4 to get 500 million years.

Carbon-14 (14C) Dating

• Carbon-14 (14C) is for determining archeological dates of wood, charcoal, seeds, pollen, pottery, paper, natural fabric and more recent bone and shell material.
• 14C has a very short half-life compared to the other elements, lasting about 5730 years. Meaning it is not suitable for dating rocks, as its main formation timeline is millions of years.
• 14C dating is useful in archeology for dating carbon-based materials up to around 50,000 years old.

Mineral Ages vs. Rock Ages

• The simplest case is with igneous rocks, wherein all the minerals form at the time molten rock (magma or lava) crystallizes into solid rock, it is more or less “instantaneous".
• The minerals are the same age as the rock.
• Xenoliths are an exception
• All the minerals are the same age, as a zircon crystal date may be representative of the age of the granite.

Clastic Sedimentary Rock

• Clastic sedimentary rocks are made of small pieces of other, older rocks.
• Minerals in clastic sedimentary rocks originate elsewhere and are not the same age as the rock formation.
• A zircon crystal provides a maximum age for the sedimentary deposit. It could be younger, though.
• Detrital zircons are survivors from older weathered-away igneous source rocks.
• The Earth's oldest dated mineral grain (4.404 billion years old) is a detrital zircon from Western Australia.

Chemical Sedimentary Rock

• Chemical Sedimentary rocks are impossible date because the crystals aren't “picky”.

Metamorphic Rock

• Mineral age can represent the rock’s protolith or its metamorphism date.
• In a metavolcanic rock that started off as a volcanic (igneous) rock before it got metamorphosed: zircon represents the age of the protolith.
• Elevated temperature and pressure may encourage the growth of new metamorphic minerals.
• We can use the mica to get the age of metamorphism.
• New layers can form through metamorphic recrystallization.

SHRIMP

• SHRIMP (Sensitive High-Resolution Ion MicroProbe) allows dating of different parts of a zircon crystal.
• The original crystal can add new layers with the new minerals when it gets metamorphosed.