Geologic Time

Correlation and Dating of the Rock Record

Time

• Relative– Order of deposition of a body of rock based on

position

• Absolute– A number representing the time a body of rock

was deposited

Relative Time

• Determining of sequence of events

• Which came first?

In what order did these events occur in American History?

A B C

ED

Relative Time

• Tools– Smith

• Fossil Succession

– Steno• Superposition

• Original horizontality

– Lyell• Cross-cutting relationships

• Intrusions

• Inclusions

Principle of fossil succession

• Fossils occur in a consistent vertical order in sedimentary rocks all over the world. (William"Strata Bill" Smith, late 1700's, England).

• This principle is valid and does not depend on any pre-existing ideas of evolution. (In fact, Charles Darwin's ideas on evolution did not appear until 50 years later - 1858).

Principle of fossil succession

Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.

"Fossil species succeed one another in a definite and recognizable order"

Fossils at the base of a thick sequence of sediments (so older, by previous principles) are less like present-day species than those near the top

Principle of fossil succession

* Fossils unlike present-day species, but like each other, are found in widely separated sites

* A fossil species which is observed to occur above (and so younger than, by previous principles) a second fossil species in one locality will always occur above that second species, wherever found.

Principle of fossil succession

Unconformities

• Unconformities are buried surfaces of erosion or non-deposition

Unconformities1.Angular unconformities

Implies tectonic deformation and erosion of underlying strata.

2.Nonconformity Sedimentary strata overlying igneous or metamorphic rocks

(in an erosional - not intrusive- contact)

3.Disconformity An irregular surface of erosion between two units of parallel

strata

DisconformityBig Brook

Angular Unconformity

NonconformityBrahma Schist underlies Tapeats Sandstone

http://www.physci.mc.maricopa.edu/Geology/FieldTrips/ColoradoRiver/ColoradoRiver_2006_Summer/ColoradoRiver_2006_Summer_Images_1024/DSC00512a_1024c.jpg

NonconformityBrahma Schist underlies Tapeats Sandstone

http://www.physci.mc.maricopa.edu/Geology/FieldTrips/ColoradoRiver/ColoradoRiver_2006_Summer/ColoradoRiver_2006_Summer_Images_1024/DSC00512a_1024c.jpg

Siccar Point

Relative Dating

• Principle of Cross-cutting relationships

• Principle of Intrusions

• Principle of Inclusions

Principle of Cross-cutting relationships

Faulting

Principle of Inclusions

In which picture is the Granite older?

Principle of Intrusions

Intrusions vs Unconformities

• A xenolith is a fragment of country rocks which has been broken off during an intrusion, and has become surrounded by magma. The xenolith is older than the igneous rock which contains it.

• Through erosion and resedimentation, younger rocks will often have pieces of the older rock included (sedimentary).

Principle of Inclusions

Principle of Inclusions

Principle of Inclusions

Test Your Knowledge

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History of Geologic Time• Geologic Systems

– Body of rock that contains fossils of diverse animal life

– Corresponds to geologic period

• Sedgewick– Named Cambrian

• Murchison– Named Silurian

Stratigraphy• Study of stratified rocks, especially their geometric

relations, compositions, origins, and age relations• Stratigraphic units

– Strata• Distinguished by some physical, chemical, or paleontological

property• Units of time based on ages of strata

– Geologic Systems

• Correlation– Demonstrate correspondence between geographically

separated parts of a stratigraphic unit• Lithologic • Temporal

Units of Time

• Time-rock unit– Chronostratigraphic unit– All the strata in the world deposited during a particular interval

of time• Erathem, System, Series, Stage

• Time unit– Geochronologic unit– Interval during which a time-rock unit is formed

• Eras, Period, Epoch, Age

• Boundary stratotype– Boundary between two systems, series or stages, formally

defined at a single locality

Geologic Time Scale

• Chronologic units - Time/Age

– Eons (largest): – Era– Periods– Epochs– Ages

Geologic Time Scale

• Geochronologic Units = Place

– Eon (largest) = Eon– Era = Era– System = Period– Series = Epoch– Stage = Age

Biostratigraphy

• Biostratigraphic unit– Defined and characterized by their fossil content

• Stratigraphic range– Total vertical interval through which that species

occurs in strata, from lowermost to uppermost occurrence

Biostratigraphy• Index fossil

– Abundant enough in the stratigraphic record to be found easily

– Easily distinguished from other taxa– Geographically widespread and thus can be used to

correlate rocks over a large area– Occurs in many kinds of sedimentary rocks and

therefore can be found in many places– Has a narrow stratigraphic range, which allows for

precise correlation if its mere presence is used to define a zone

Magnetic Stratigraphy

• Use of magnetic properties of a rock to characterize and correlate rock units

• Magnetic field– Reversals in polarity of

field are recorded in rocks when they crystallize or settle from water

Magnetic Stratigraphy

• Chron– Polarity time-rock unit

– Period of normal or reversed polarity

• Normal interval– Same as today

– Black

• Reversed interval– Opposite to today

– White

Lithostratigraphy

• Subdivision of the stratigraphic record on the basis of physical or chemical characteristics of rock

• Lithostratigraphic units– Formation

• Local three-dimensional bodies of rock– Group– Member

• Stratigraphic section– Local outcrop of a formation that displays a continuous vertical

sequence

• Type section– Locality where the unit is well exposed, that defines the unit

Lithologic Correlation

• Cross-sections of strata– Establish geometric

relationships

– Interpret mode of origin

Lithologic Correlation

• Grand Canyon– McKee

– Used Trilobite biostratigraphy to determine age relationships

– Eastern portion of units is younger than western

Facies• Transgression

– Landward migration of shoreline

– Grand Canyon• Cambrian transgression

• Facies– Set of characteristics of a body

of rock that presents a particular environment

• Facies changes– Later changes in the

characteristics of ancient strata

Absolute Age

• 4.6 billion years old• Early estimates

– Salts in the ocean• 90 million years old

– Accumulation of sediment• 100 m.y. or less• Gaps in stratigraphic record• Unconformities represent large breaks in accumulation• Didn’t include metamorphosed sedimentary rocks

– Earth’s temperature• Kelvin• 20-40 million years old

Absolute Ages

• How old is the Earth?

• 4.6 billion years (4,600,000,000 years)

• Radiometric dating (Uranium, Thorium). Mass spectrometer.

Early Attempts

• 1654 Archbishop Usher (Ireland), genealogy in Bible Earth was created October 22, 4004 BC,

• 9:00 am was added later• Earth was 6000 years old.• Led to the Doctrine of Catastrophism:• Earth was shaped by series of giant disasters. • Many processes fit into a short time scale.

Early Attempts

• 1770's, 1780's "Revolution"

• James Hutton, Father of Geology (Scotland) 1726-1797.

• Published Theory of the Earth in 1785.

Hutton• Hadrian's Wall built by Romans,

after 1500 years no change. Suspected that Earth was much older.

• Slow processes shape earth.

• Mountains arise continuously as a balance against erosion and weathering

Hutton• Doctrine of Uniformitarianism: "Present is key to

the past".

• The physical and chemical laws that govern nature are uniform

• • Unconformity at Siccar Point, Scotland

• "No vestige of a beginning, no prospect of an end"

Charles Lyell

• Charles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene.

• Estimated 80 million years for the Cenozoic alone.

Various Geologists• Thickness of total sedimentary record

divided by average sedimentation rates (in mm/yr).

• In 1860, calculated to be about 3 million years old.

• In 1910, calculated to be about 1.6 billion years old.

Lord Kelvin

• In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation.

• Age of Earth was calculated to be about 24-40 million years.

Lord Kelvin

• Problem: Earth has an internal heat source (radioactive decay)

• Discovery of radioactivity by Henri Becquerel in 1896.

John Joly• In 1899 - 1901, John Joly (Irish) calculated the rate

of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea.

• Evaporative concentration of salts.

• Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)

• Age of Earth = 90-100 million years.

John Joly

• Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.

von Helmholtz and Newcomb

The German physicist Hermann von Helmholtz and the American astronomer Simon Newcomb joined in by independently calculating the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.

100 million years, consistent with Thomson's calculations. However, they assumed that the Sun was only glowing from the heat of its gravitational contraction. They knew of no other ways for it to produce its energy.

Rutherford and Boltwood

• In 1905, they used radioactive decay to measure the age of rocks and minerals.

• Uranium decay produces He, leading to a date of 500 million years.

Rutherford and Boltwood

In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating.

Date was 1.64 billion years.

Age of Earth• So far, oldest dated Earth rocks are 3.96 billion

years. • Canadian Shield. (NW Territories near Great

Slave Lake, 3.96 byr).• Detrital Zircons in sedimentary rocks are 4.1 - 4.4

byr• Older rocks include meteorites and moon rocks

with dates on the order of 4.6 billion years.

Geologic Time Scale. The age for the base of

each division is in accordance with

recommendations of the International

Commission on Stratigraphy for the

year 2000.

Geologic Time Scale

The standard geologic time scale for the

Paleozoic and other eras developed without

benefit of a grand plan. Instead, it developed by the compilation of “type sections” for each of the

systems.

Ordovician

Silurian

Murchison

Silurian - Murchison

Jurassic - Jura Mountains

• Cretaceous

• Kreta = Chalk

• White Cliffs of Dover

Absolute Age

• Radioactive decay– Becquerel, 1895

• Uranium undergoes spontaneous decay

• Atoms release subatomic particles and energy

• Change to another element

– Parent isotope decays/daughter isotope produced

Principles of Radiometric Dating

• Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

• Radioactive parent elements decay to stable daughter elements.

What is an Isotope?

• Nuclide of an element with different masses

Absolute Age

• Three modes of decay– Loss of alpha particle

• Convert parent into element that has nucleus containing two fewer protons

– Loss of beta particle• Convert parent into

element whose nucleus contains one more proton by losing an electron

– Capture of beta particle• Convert parent into

element whose nucleus has one less proton

Absolute Age• Radiometric dating

– Radioactive isotopes decay at constant geometric rate

• After a certain amount of time, half of the parent present will survive and half will decay to daughter

• Half-life– Interval of time for half

of parent to decay

Absolute Age• Useful isotopes

– Uranium 238 and thorium 232• Zircon grains

– Uranium 238 and lead 206• Fission track dating

– Rubidium-Strontium– Potassium-Argon, Argon-Argon– Radiocarbon dating

• Produced in upper atmosphere• Half life = 5730 years• Maximum age for dating: 70,000 years• Bone, teeth, wood

Absolute Age

• Fission-Track Dating– Measure decay of

uranium 238 by counting number of tracks

– Tracks formed by subatomic particles that fly apart upon decay

Radioactive parent isotopes and their stable daughter products

Each radioactive isotope has its own unique half-life.

A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

Radioactive parent isotopes and their stable daughter products

Radioactive Parent Stable Daughter Half Life

Potassium 40 Argon 40 1.25 billion yrs

Rubidium 87 Strontium 87 48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 235 Lead 207 704 million years

Uranium 238 Lead 206 4.47 billion years

Carbon 14 Nitrogen 14 5730 years

Radioactive Decay

Radioactive Decay

• Radioactive decay occurs by releasing particles and energy.

• Alpha particles

• Beta particles

• Neutrons

• Gamma rays (high energy X-rays) are also produced.

Radioactive Decay

• Alpha particles (He)

• large, easily stopped by paper

• charge = +2

• mass = 4

Radioactive Decay

• Beta particles

• penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.

• charge = -1

• mass = negligible

Radioactive decay series of uranium-238 (238U) to lead-

206 (206Pb).

Datable Minerals

• Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time the magma cooled.

• Potassium 40 is found in: • potassium feldspar (orthoclase) • muscovite • amphibole • glauconite (greensand; found in some

sedimentary rocks; rare) •

Datable Rocks • Radioactive elements tend to become

concentrated in the residual melt that forms during the crystallization of igneous rocks. More common in SIALIC rocks (granite, granite pegmatite) and continental crust.

Datable Rocks • Radioactive isotopes don't tell much about

the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.

Carbon 14

How does 14Carbon dating work?

• Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive 14C, which combines with oxygen to form radioactive CO2.

How does 14Carbon dating work?

• Living things are in equilibrium with the atmosphere, and the radioactive CO2 is absorbed and used by plants. The radioactive CO2 gets into the food chain and the carbon cycle.

How does 14Carbon dating work?• All living things contain a constant ratio of 14C to 12C

(1 in a trillion).

• At death, 14C exchange ceases and any 14C in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new 14C.

How does 14Carbon dating work?• The change in the 14C to 12C ratio is the basis for dating.

• The half-life is so short (5730 years) that this method can only be used on materials less than 50,000 years old.

• Assumes that the rate of 14C production (and hence the amount of cosmic rays striking the Earth) has been constant.

Deviation of carbon-14 ages to true ages from the present back to about 5000 B.C. Data are

obtained from analysis of bristle cone pines from the western United States. Calculations of carbon-

14 are based on half-life of 5730 years.(Adapted from Ralph, E. K., Michael, H. N., and Han, M. C. 1973. Radiocarbon

dates and reality. MASCA Newsletter 9:1.)

Absolute Age

• Best candidates for most radiometric dating are igneous– Not necessarily useful

for sediments

• Error in age estimate can be sizable

Absolute Age

• Absolute ages change– Error increases in older

rocks– Techniques change

• Biostratigraphic correlations are usually more accurate – Radiometric dates used

when fossils not present

How old is the Old Red Sandstone?a. Older than 425 myrb. Younger than 370 myrc. Between 425 and 370 myrd. Have no idea