geologic time. time is critical for geologic processes rockies and alps are ~3000 m tall atlantic...
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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
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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
angular unconformity
relative age dating concepts
different types of unconformities
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?)
relative age dating concepts
different types of unconformities
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?
angular unconformity
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
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established initially as a
relative scale using
sedimentary rocks
and fossils
absolute ages
were determined later
with
radiometric dating
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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
92 protons 90 protons
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
19 protons 20 protons
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…
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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
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modern equivalents in Shark’s Bay, Australia
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proportional time scale
combine relative and absolute time for geologic time scale