Deep Time and Geological Time Scales

• Earth is 4.566 billion years old.
• The discovery of Earth's antiquity is a crowning achievement of the geosciences.
• Understanding geological time is central to understanding our place in nature and history.
• Essayist John McPhee coined the phrase "deep time."

Abbreviating Geological Time

• Geologists measure events in years before the present date.
• "ka" represents one thousand years (kilo-annum).
• "Ma" represents one million years (mega annum).
• "Ga" represents one billion years (giga annum).
• "kya" and "mya" characterize events thousands or millions of years ago.
• Numerical (absolute) dating assigns actual ages in years before the present date.

Geology as a Historical Science

• Geology is the study of Earth and its history.
• Geology is a historical science because it deals with past events.
• Geologists reconstruct the past using evidence from rocks, minerals, and fossils.
• Historical sciences include evolutionary biology, climatology, archaeology, and astronomy.

Uniformitarianism

• Uniformitarianism assumes that the laws of nature have not changed over Earth's history.
• James Hutton developed this assumption, observing slow erosion and sediment formation rates.
• Hutton concluded that geological time must be unimaginably vast.
• Charles Lyell popularized the idea that geological processes act slowly and continuously.
• The intensities and rates of some processes have changed, and Earth's history includes catastrophic events.
• "The present is the key to the past" is the uniformitarian view, allowing the use of the modern world to understand Earth's history and predict its future.

Principles of Relative Dating

• Principles of relative dating are applied to sedimentary rock layers.
• The average sedimentary rock layer is about 1800m thick.
• Relative dating determines whether one geological or paleontological event happened before or after another.
• Six simple principles are employed, with superposition being most important.

Principle of Superposition

• Nicholas Steno's principle of superposition states that rocks below others are older.
• Devonian-aged rocks (~380 Ma) at Taughannock Falls demonstrate this principle.

Principle of Faunal Succession

• William Smith formulated the principle of faunal succession.
• This is a basis for the development of the geologic time scale.
• The same ordering of fossil species occurs from place to place which allows to correlate rock layers.
• Rock layers in different places are the same age if they include the same fossils.

Principle of Original Horizontality

• Sediments are deposited in layers parallel to Earth's surface.
• The principle is useful when rock layers are deformed by tectonic activity.

Principle of Lateral Continuity

• Layers of sediment extend outward in all directions and are laterally continuous but controlled by the amount/type of sediment and the sedimentary basin's size/shape.
• Similar rocks separated can assumed to be connected
• Thickest deposits are proximal to the source.

Principle of Cross-Cutting Relationships

• A geological feature that cuts across another feature is younger.
• A fault disrupting sedimentary layers indicates the sedimentary layers pre-date the fault.
• Other examples include igneous intrusive features, like dikes.

Principle of Inclusions

• Rock fragments included in another rock are older than the rock in which they are included.
• A xenolith of granite in basalt indicates the basalt intruded into pre-existing granite.
• Contact metamorphism (baking) of a rock also indicates the baked rock was in place prior to the intrusion.

The Grand Canyon Example

• The Grand Canyon illustrates superposition, original horizontality, and cross-cutting relationships.
• The oldest formations are at the bottom, with igneous and metamorphic rocks at the very bottom (the "basement").
• The metamorphic schist is the oldest rock: granite intrusion is younger.
• The Colorado River exposes horizontal strata and has been barely disturbed from their original deposition except for regional uplift.

Unconformities - Missing Time

• An unconformity represents an interruption in the deposition process.

Angular Unconformity

• Angular unconformity: tilted Proterozoic rocks were eroded before Paleozoic rocks were deposited.
• The Great Unconformity involved tilting/erosion of Proterozoic rocks before Paleozoic deposition and has a time difference of close to 300 million years.

Nonconformity

• Nonconformity: erosional surface between igneous/metamorphic and sedimentary rock.
• There is a nonconformity at the base of the Grand Canyon, between 1.75 Ga Proterozoic rock and ~545 Ma Phanerozoic rock. Over 1 billion years separate the basement rock from the first overlying layer.

Disconformity

• Disconformity: occurs between parallel sedimentary layers.

Applications of All Dating Methods

• Dating methods provide insight to geological history.
• Relative dating analysis includes: rock types, an erosional surface between the gneiss and the sandstone, and baking of the gneiss in contact with the igneous rock.
• The principle of superposition tells you that gneiss underlies sandstone.
• Cross-cutting and baked contacts indicate that the black igneous rock is a dike.
• The erosional surface at the contact between the gneiss and sandstone represents a nonconformity

Dating Rocks Using Fossils

• Paleontology dates to the early 19th century.
• The oldest fossils are from rocks around 3.5 Ga, while well-understood fossils are from around 600 Ma.
• Fossil records from that time forward provide a record of life's history/evolution although fossils alone cannot provide absolute ages.
• Based on the understanding of the sequence of evolution on Earth by applying knowledge to determining the relative ages of rocks
• William Smith's principle doesn't just apply to fauna", it also applies to fossils of plants and other organisms.

Geologic Range

• Identifying a fossil to the species/genus level can assign a time range to the rock, especially if several fossils with known time ranges are present.
• Index fossils lived for relatively short periods and were distributed over a wide geographic area.

Fossil Assemblage

• Fossil assemblage is a group of organisms that found fossilized together in a single sedimentary layer used to provide insight to the age.
• Bracketing of overlapping ranges constrains a rock's age based on multiple fossils resulting in a narrowed down time range.

Biozones

• Some organisms are biozone fossils, with each species living for a short time and easily distinguished based on features.
• Ammonoids' suture lines vary sufficiently to identify species and estimate the age of rocks.
• Foraminifera are small, marine shell organisms that originated during the Triassic. The number of different foraminifera that exists and the short duration of their existence, results in high accuracy for dating specimens.

Numerical (or Absolute) Dating

• It assigns actual dates (in years before the present) to geological events.
• The science of absolute age dating is known as geochronology using radiometric dating (also known as isotopic dating).
• Archbishop James Ussher (1581-1686) famously estimated that Earth formed in 4004 which made it around 6,000 years old
• Practitioners from the science of geology applied the uniformitarian views of Hutton and Lyell leading some to suggest that the Earth is many millions of years old.
• Radioactivity allowed the process of assigning absolute age dates to rocks.

Radiometric Dating

• It's determined from the rates of radioactive decay of some isotopes of elements that occur naturally in rocks
• In chemistry, an element is a kind of atom defined by the number of protons that it has in its nucleus
• An element's atomic number equals its number of protons.
• Isotopes of individual elements are defined by their mass number (protons + neutrons).
• Example: C-12 (6 protons, 6 neutrons), C-13 (6 protons, 7 neutrons), and C-14 (six protons, 8 neutrons).

Radioactive Decay

• Most isotopes are stable.
• Unstable isotopes undergo radioactive decay over time.
• Unstable isotopes lose energy and subatomic particles.
• This involves the isotopes shedding energy in the form of radiation and subatomic particles, causing their numbers of protons and neutrons to change.
• As a result the radioactive element will decay to form a different, stable element
• An atomic nucleus that undergoes radioactive decay is known at the parent product
• The resulting stable element is know as the daughter product.
• 238U (Uranium-238) will radioactively decay to 206Pb (Lead-206) through as a stable isotope.
• Key to understanding the process is realizing that the physical chemistry of a growing mineral crystal can be 'picky', allowing only a select amount of the specific element to become part of the mineral chemistry.
• When "picky" minerals are forming, their growing mineral crystal lattic can be very "exclusive" about what atoms it will let into its crystal structure, not including "sloppy" products
• The ideal mineral accepts radioactive parent isotopes but never includes daughters.

Example

• 40K in potassium feldspar (KAlSi3O8) decays to 40Ar: both elements remain trapped in the mineral as well, the 4ºAr
• Picky minerals let certain radioactive atoms into their crystals and actively exclude the stable daughter atoms that result when the radioactive parents break down
• It takes both valence state and atomic weight to determine whether the mineral can be used for dating.
• Zr dominate in the crystal lattice however radioactive 238U atoms can substitute for Zr, decay of 238U→ 206Pb occurs over time. The 238U being the parent, and the 206Pb being the daughter isotope.
• Parent-daughter isotopes start with 0% daughter isotopes, 100% radioactive parent isotopes, after some formation.
• A count of all parent/daughter isotopes will sum to 100%.

Half-Life Decay Constant Chart.

• Most applicable are are listed below on in Half-life Decay Constant Chart.
• Examples include Zircon, Muscovite, biotite, hornblende, potassium feldspar.
• This chart includes Radioactive parent in Zircon like is allowed in the crystal, the daughter isotope that created, what sediment it it can be applied to and the age range of the specemin.
• It needs the ratio of parent to daughter isotopes for calculations
• It's measured by using a mass spectrometer high precision instrument that detects and separates atoms based on their mass
• Its decay rate is constant determined in labs
• A half-life is the amount of time that's needed for half of the parent atoms in a sample to decay into daughter products shown below.

Radioactive Decay Curve diagram

• It represents he Relationship between the amount of radioactive percent/daughter atoms/half-lives in the sample during the passage of time
• It measures through half-lives. Image by Jonathan R. Hendricks
• Time passing 50% of one element will become the other.. Three half-lives passed and as more half-lives pass that number approaches zero.
• The ratio of parent atoms relative to daughter products will determine half lives. shown by Magma hot- igneous rocks change over time.
• Magma cooling with different minerals will half 50%

Calculating Radiometric Dates

Step 1: It must convert the number of half-lives that have passed into a numerical (absolute) age. Step 2: It doone by multiplying the number of half-lives by the half-life decay constant of the parentotope. Lets say that Granite contains the mineral potassium feldspar which can be samples mass spectrometry and has both parent and daughter isotopes.

1. Determine the percent of Parent/Daughter isotopes that exist within the sample.
2. Determine the number of half-lives that have passed by using the percent parent (or daughter) isotope and the Radioactive Decay Curve diagram.
3. Determine the age of the sample by multiplying the number of half-lives by the decay rate constant for the parent/daughter isotope pair found on the Half-life Decay Constant chart.

Carbon-14 (14C) Dating

It has short half-time as it is found in the sample, so it can go from 50,000 years to close to nothing. Its mainly used for archaological dates of wood seeds natural fabric more recent bone and show material organic carbon and carbonate fossils are not ideal because they include lithium

Mineral Ages vs. Rock Ages

• Mineral age tells the rock. depends on the rock
• Igneous rocks mineral all form at the time of liqudis, so the mineral and igneous crystal ate both the same. the xenoliths can be differnet, but its not likely
• Clastic sendemtary : those small pieces must be older and then get depositied together when weatherd.
• The derital are survivirs form other sources by wethering (detrius)

Chemical and Metasedimentary Rock

• Its not plausible that a mineral date could be used to date the deposistional age (lacks "pickiness"
• Mineral age can be the protolith , or the date of metamorphism. It depends on which mineral, because we can have multiple levels of mineral ages

Shrimp (Sensitive High Resolution Ion Microbep)

• Can test the dating of different part of a single zircon
• 3d laser allow us to date layers for example, one for the ingeous , then another for the metamorphic, we can extract two + dates from the surface with a size of a sand grain

Mineral Changes

• Mins can complicate due to temperature of metamorhphis.
• So dating needs to be used by mass spectrometer with isotopic and ratios

ALL

• the summary for our analysys is that most cases F-I that is assumed and could be used in future analyzied or dating

The Geological Time Scale

• An achievement of of sceience , and paleontology, evoulutionary biology , and earth work. its related back with fossil
• The time sclae has eons, eras, pereidos , and epoes and now has a updated listing online.

Time line

• Smith's show primitive geloigcal time scales so know that all the time perodis are represented along the section
• and the recontstructure can hlep with many new snapshots
• all these events correlate and hve a great effect.

Learning Geological Timescale

• Committ to memories the eons, eras, prriouds , and epochs

Eons

• The broadest range like for Hadean , Archean, Proterozoic, and Phanerozoic (pre cambrain).
• Phanerizoa mean visible lifes
• The phanerozic eon represent 12% of earth story,and precambrian are 3

Eras

the second logest unit. for paleozioc mezozoic and ceonzic ( new middle and old lifes ). these all have been affected by mass event extcinction or the history of time

Periods of Times

they follow Eans the palezoic are devided in Six , carbinerous, and permian.

mesozoci : Triassic, Jurassic, and Cretaceous Cenozoic: Oaleogene Neogene , and Quaternary

Eposhcs ANDAGES

Paleogene period is divided into and olgocene the Neogen is divided into iceoeen the Quaternary divided new epoch the Anthropocene humans havng affect because a recent period. Also there are different charactersitics that should be define a beginning.

Time Scale

The interactive opens on the Phanerozoic Eon; you can select "Ancient Earth" to view the globe and events of the Proterozoic Eon. The geologic time scale appears at the left with a silver slider that you can grab and peruse through time past. .