Geologic time. time is critical for geologic processes Rockies and Alps are ~3000 m tall Atlantic Ocean is ~5000 km across for comparison: fingernail.

geologic time

time is critical for geologic processes

Rockies and Alps are ~3000 m tall

Atlantic Ocean is ~5000 km across

for comparison: fingernail grows at 1 cm/yr

-- mountains grow at ~1 meter per 5000 yrs (0.2 mm/yr)

-- 3000 m x 5000 yr/m = 15,000,000 (yrs necessary)

-- today, seafloor spreading in Atlantic is ~4 cm/yr

-- 6000 km = 6000 km x 1000 m/km x 100 cm/m = 600,000,000 cm

-- 600,000,000 cm / 4 cm/yr = 150,000,000 years

age of the Earthprior to 19th century, accepted age from religious beliefs

-- 6,000 years for Western culture (Christian)…Bishop Usher from geneology in the Bible

-- old beyond comprehension (Hindu/Buddhist/Chinese)

during 19th century, length of time required forgeologic processes to occur was

recognized

-- age not certain (Islam)

-- fundamental contribution of geology to scientific knowledge

James Hutton (1726-1797) “Father of Modern Geology” • native of Edinburgh, Scotland • educated as a medical doctor in Leiden (1749) • passionate about scientific inquiry

historical developments

“Theory of the Earth” -- processes are slow; take a long time

Charles Lyell (1795-1875)• Scotsman who attended Oxford University • father was an avid naturalist • rebelled against prevailing thought of “catastrophism”.

“Principles of Geology” -- popularized Hutton’s views

idea of “uniformitarianism” -- same processes operating today occurred in the past

….the present is the key to the past….

the key to the past

study of timing of geologic events and processes is geochronology

relative time vs. absolute time

relative timeorder of events or objects from first (oldest) to last (youngest)she is older than he is; she was born first and he was born last

age of events or objects expressed numericallyshe is twenty-one and he is nineteen

absolute time

relative time and relative order

apply simple concepts to determine…

• original horizontality

• superposition

• lateral continuity

• cross-cutting relationships

• inclusions

• unconformities

relative age dating conceptsoriginal horizontality

all beds originally deposited in water formed in horizontal layers

sediments will settleto bottom

and blanketthe sea floor

relative age dating concepts

superpositionwithin a sequence of undisturbed sedimentary or volcanic rocks,oldest rocks are at the bottom and youngest at the top

….young upward…

oldest

youngest

lateral continuityoriginal sedimentary layers extend laterally until they thin at edges

continuecontinue

relative age dating conceptscross-cutting relationshipsa disrupted pattern is older thanthe cause of the disruptione.g. an intrusion is youngerthan the rocks it intrudes

relative age dating conceptsinclusionsfragments of other rocks contained in a body of rockmust be older than the host rock

e.g. 1) xenoliths in granite are olderthan granite and2) pieces of rock inconglomerate are older than conglomerate

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

relative age dating conceptsunconformities

a contact between sedimentary formations that represents a gap in the geologic record -- “gap” represented is variable (i.e. amount of time or the amount of missing section)

different types of unconformities

conformity• relatively continuous deposition

• deposition of a sequence of parallel layers

• contacts between formations do not represent significant amounts of time

from: http://www.elohi.com/photo/grandcanyon

conformity

relative age dating conceptsdifferent types of unconformities

angular unconformity• contact separates overlying younger layers from tilted older layers• sequence of layers is not parallel

• contacts between formations may represent significant amounts of time

angular unconformity

angular unconformityfrom: http://www.uakron.edu/envstudies/parks/rmgcan2.html




disconformity

• contact separates beds (formations) that are parallel

• sequence of layers is parallel

• contacts between formations may represent significant amounts of time

• missing time is difficult to recognize (may need otherinformation--paleosol?)



nonconformity• strata deposited on older crystalline (metamorphic/igneous) rock• erosion surface on igneous/metamorphic rock covered bysedimentary rocks

• large gap in geologic record

nonconformity

what events occur?


what events occur?

nonconformity

now that we know all this…what happened?

deposition

intrusion

tilting and erosion

subsidenceand

reneweddeposition

missing formation (time)?

dike intrusion

erosion and exposure

subsidence and deposition

uplift/sea level fall and river deposition

relative ages of the formations

correlation -- determining time equivalency of rocks within a region, between continents, etc.

physical continuity

relative age: correlation

physically following a continuous exposure of a rock unit--most direct; easily done in some locations, not in others

e.g. within the Grand Canyon

how is this done?

lithologic similarityassuming similar sequences of rocks formed at same time

-- inaccurate if common rocks are involvede.g. the Grand Canyon and Zion National Parks

Coconino Sandstone

physical continuity -- Coconino Sandstone in Grand Canyon

lithologic similarity -- Coconino and Navajo Sandstones

lithologic similarity -- Coconino and Navajo Sandstones Navajo is much younger!

faunal succession (correlation by fossils)

relative age: correlationhow is this done?

index fossil short-lived organism;

points to narrow range of geologic time

fossil species succeed one another through the layers in a predictable order

fossil assemblage group of fossils

associatedtogether

similar units found in India, Africa, S. America, Australia, Antarctica.

use of index fossils/fossil assemblagespermits global correlation





established initially as a

relative scale using

sedimentary rocks

and fossils

absolute ages

were determined later

with

radiometric dating



natural clock is necessary

-- radiometric dating (nuclear clock: decay of radioactive isotopes)

-- dendrochrolonology

-- astronomical methods

absolute time

age of the Earthearly methods: long debated

• 1625: Archbishop Usher determined Earth was created in 4004 B.C. by counting generations in the Bible

• Hindus regarded Earth as old: 2000 A.D. is 1.97 million years according to Hindu calendar

• 1866: Lord Kelvin calculated age by assuming that Earth was molten and cooled to a solid; age between 20-40 million years old.

- did not know about radioactive decay (makes heat)- assumed all heat dissipated by conduction

early isotopic methods (radioactivity known in 1896)• 1905: first crude estimates yielded 2 billion year age

• meteorites gave dates of 4.5 to 4.6 billion years old

• modern uranium/lead methods yield values of 4.55 billion years

have nuclei that spontaneously decay

daughterparentloss or gain

loss or gain of neutron converts parent to daughter of same element

loss or gain of proton changes parent into entirely new daughter

radioactive isotopes

-- emit or capture subatomic particles

parent: decaying radioactive isotopedaughter: decay daughter

3 primary ways of decay

alpha decay (Z ≥ 58)

beta decay (n0 = p+ + e-)

electron capture (e- + p+ = n0)capture of an electron by a protonand change of proton to neutron

(result is loss of proton)

K40 Ar40

19 protons 18 protons

particle has 2 neutrons and 2 protonsU238 Th234


breakdown of neutron into anelectron and a proton and loss

of the electron to leave a proton(result is gain of one proton)

K40 Ca40


radiometric dating

as minerals crystallize in magma; they trap atoms of radioactive isotopes in their crystal structures

radioactive isotopes will decay immediately and continuously

as time passes, rock contains less parent and more daughter

uses continuous decay to measure time since rock formedonly possible since late 1890’s -- radioactivity discovered in 1896

amount of time it takes for half the atoms of the parent isotope to decay

if rock has 12 parents and 12 daughters--ratio of 1:1

…original rock had 24 parents and one half-life has elapsed…

…after another half life, rock will have 6 parents and 18 daughters……ratio of 1:3---note that total number (24) remains the same

regardless of isotope, the ratio of parent to daughter atomsis predictable at each half-life

half-life

different radioactive isotopes have different anddistinct half-lives

predictable ratios at each half-lifeexponential decay (half always remains)

exponential decay: never goes to zero

exponential linear

example: Uranium 238 decay to Lead 206 (stable)several steps

(each has its own half-life)

most common dating systems

• uranium-thorium-lead dating (previous example)U-238, U-235, Th-232

each of these decays through a series of steps to Pb

U-238 to Pb-206 half-life = 4.5 byU-235 to Pb-207 half-life = 713 myTh-232 to Pb-208 half-life = 14.1 my

• potassium-argon dating

K-40 to Ar-40 half-life = 1.3 by

…argon is a gas--may escape (ages too young--daughter missing)

• rubidium-strontium dating

Rb-87 to Sr-87 half-life = 47 by

basic geochronological assumptions• decay constants constant through geological time

igneous rocks are most reliable for dating…metamorphism may cause loss of daughter products…

…sedimentary rocks will give ages of source rocks…

• system closed to adding or subtracting of parent/daughter

-- good reasons to believe this is correct from nuclear physics

-- measurements of decay sequences in ancient supernovae yield the same values as modern lab measurements

-- isotopic system and type of mineral (rock) are important

-- careful procedure is essential to correct analysis

Instruments and Techniques• Mass Spectrometry: measure different abundances

of specific nuclides based solely on atomic mass.

– Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g. 87Sr and 86Sr).

Count individual particles using electronic detectors.

– TIMS: thermal ionization mass spectrometry– SIMS: secondary ionization mass spectrometry - bombard

target with heavy ions or use a laser

• Sample Preparation: TIMS requires doing chemical separation using chromatographic columns.

Clean Lab - Chemical Preparation

http://www.es.ucsc.edu/images/clean_lab_c.jpg

Thermal Ionization Mass Spectrometer

From: http://www.es.ucsc.edu/images/vgms_c.jpg

Schematic of Sector MS

Zircon Laser Ablation Pit

Rate Law for Radioactive Decay

WherePt ≡ quantity of the parent isotope (i.e. 87Rb) at tim et;Po ≡ quantity of the parent isotope a t some earlie r tim eto, whentheisotopic system was closed t o any additional isotopic exchange;λ ≡ i s the characteristic decay constant for the system of interes ,t whichis relat edto t he hal -f life, t1/2, by the equation below:

λ = l 2n / t1/2

t1/2 ≡ i sdefined as the half-life, whi chis the amount of time required fo r 1/2 of theoriginal parent to decay and i s aconstant.

Pt = Po exp -λ (to –t)

1st order rate law

Rb/Sr Age Dating Equation 87Rbt = 87Rbo e

-λ (to – )t

(Assum etha t t = 0, fo r t he present)

87Rbo + 87Sro =

87Rb t+ 87Srt

(Conservation of Mass, with 87Sro ast heinitialconcentration and87Srt ast he concentration today)

87Srt - 87Sro = 87Rbt (e

λ to – 1)

€

87Sr86Sr

⎛ ⎝ ⎜

⎞ ⎠ ⎟t

=87Sr86Sr

⎛ ⎝ ⎜

⎞ ⎠ ⎟o

+87Rb86Sr

⎛ ⎝ ⎜

⎞ ⎠ ⎟t

(eλt −1)

y= b+ x⋅m

Rb/Sr Isochron Systematics

M1 M2 M3

Independent Checks on Radiometric Ages

• Correlation of erosion with age on Hawaiian Island Chain: Dates increase in age to the NW as does erosion.

• Annual growth bands in Devonian corals: 400/yr yields date that is similar to radiometric date. Consistent with slowing of Earth rotation with time.

• Independent determination of Pacific plate motion yields age progression that is consistent with K/Ar dates of the island chains formed by “hotspots”.

• Agreement between magnetic “age” from deep marine sediments and radiometric ages of tuffs in East African Rift

annual growth of trees produces concentric rings…dating back to 9000 years is possible…



photo © H.D. Grissino-Mayer

Other dating methods: dendrochronology

- rings need to be calibrated against C-14 dates to yield “true” numerical age

- other information may alsobe obtained from rings, including rainfall and temperature

- can develop composite chronologies for specific regions of interest for climate studies

relative and absolute dates combined

same exampleas in

relative age

geological time scale

eons, eras, periods, epochs

Oldest rock fragments: W. Australia detrital zircons

Oldest rocks: Greenland gneisses

earliest lifecyanobacteria: primitive single-celled organisms found in Australia and dated at 3.7 billion years old









modern equivalents in Shark’s Bay, Australia



proportional time scale

combine relative and absolute time for geologic time scale

Geologic time. time is critical for geologic processes Rockies and Alps are ~3000 m tall Atlantic Ocean is ~5000 km across for comparison: fingernail.

Documents

geologic time slide

erosion slide

exposure slide

conglomerate slide

absolute time slide

river deposition slide

renewed deposition slide

dike intrusion slide