Transcript relative dating lecture

Geologic Time
time is critical for geologic processes
Rockies and Alps are ~3000 m tall
-- mountains grow at ~1 meter per 5000 yrs (0.2 mm/yr)
-- 3000 m x 5000 yr/m = 15,000,000 (yrs necessary)
Atlantic Ocean is ~5000 km across
-- 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
for comparison: fingernail grows at 1 cm/yr
age of the Earth
prior 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)
-- age not certain (Islam)
during 19th century, length of time required for
geologic processes to occur was recognized
-- fundamental contribution of geology
to scientific knowledge
historical developments
James Hutton (1726-1797) “Father of Modern
Geology”
• native of Edinburgh, Scotland
• educated as a medical doctor in Leiden (1749)
• passionate about scientific inquiry
“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
relative time vs. absolute time
relative time
order 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
absolute time
age of events or objects expressed numerically
she is twenty-one and he is nineteen
study of timing of geologic events and processes is geochronology
relative time and relative order
apply simple concepts to determine…
• original horizontality
• superposition
• lateral continuity
• cross-cutting relationships
• inclusions
• unconformities
relative age dating concepts
original horizontality
all beds originally deposited in water formed in horizontal layers
sediments will settle
to bottom
and blanket
the sea floor
relative age dating concepts
superposition
youngest
within a sequence of undisturbed
sedimentary or volcanic rocks,
oldest rocks are at the bottom
and youngest at the top
….young upward…
lateral continuity
oldest
original sedimentary layers extend
laterally until they thin at edges
continue
continue
relative age dating concepts
cross-cutting
relationships
a disrupted pattern is older than
the cause of the disruption
e.g. an intrusion is younger
than the rocks it intrudes
relative age dating concepts
inclusions
fragments of other rocks contained
in a body of rock
must be older than the
host rock
e.g.
1) xenoliths in granite are older
than granite and
2) pieces of rock in
conglomerate are older
than conglomerate
relative age dating concepts
unconformities
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
conformity
from: http://www.elohi.com/photo/grandcanyon
relative age dating concepts
different 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
from: http://www.uakron.edu/envstudies/parks/rmgcan2.html
angular unconformity
angular unconformity
relative age dating concepts
different types of unconformities
disconformity
• contact separates beds (formations) that are parallel but not
continuous in age
• sequence of layers
is parallel
• contacts between
formations
may represent significant
amounts of time
• missing time is difficult to
recognize (may need other
information—paleosol
(remains of ancient soils)?)
relative age dating concepts
different types of unconformities
nonconformity
• strata deposited on older crystalline (metamorphic/igneous) rock
• erosion surface on igneous/metamorphic rock covered by
sedimentary rocks
• large gap in
geologic record
• Magma intruding and
destroying/altering part
of the rock record
nonconformity
what events occur?
angular unconformity
what events occur?
nonconformity
now that we know all this…what happened?
deposition
intrusion
tilting and
erosion
subsidence
and
renewed
deposition
missing formation (time)?
dike intrusion
erosion and exposure
subsidence and deposition
uplift/sea level fall and river deposition
relative ages of the formations
relative age: correlation
correlation -- determining time equivalency of rocks within
a region, between continents, etc.
how is this done?
physical continuity
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
lithologic similarity
assuming similar sequences of rocks formed at same time
-- inaccurate if common rocks are involved
e.g. the Grand Canyon and Zion National Parks
physical continuity -- Coconino Sandstone in Grand
Canyon
Coconino Sandstone
lithologic similarity -- Coconino and Navajo Sandstones
lithologic similarity -- Coconino and Navajo Sandstones
Navajo is much younger!
relative age: correlation
how is this done?
faunal succession (correlation by fossils)
fossil species succeed one another through the layers
in a predictable order
index fossil
short-lived organism;
points to narrow range
of geologic time
fossil assemblage
group of fossils
associated
together
use of index fossils/fossil assemblages
permits global correlation
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similar units found in India, Africa, S. America, Australia, Antarctica.
absolute time
natural clock is necessary
1-- radiometric dating
(nuclear clock: decay of radioactive isotopes)
2-- dendrochrolonology
3-- astronomical methods
age of the Earth
early 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
radioactive isotopes
have nuclei that spontaneously decay
-- emit or capture subatomic particles
parent: decaying radioactive isotope
daughter: decay daughter
parent
daughter
loss 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
3 primary ways of decay
alpha decay (Z ≥ 58)
particle has 2 neutrons and 2 protons
U238
Th234
92 protons
90 protons
beta decay (n0 = p+ + e-)
breakdown of neutron into an
electron and a proton and loss
of the electron to leave a proton
(result is gain of one proton)
K40
Ca40
19 protons
20 protons
electron capture (e- + p+ = n0)
capture of an electron by a proton
and change of proton to neutron
(result is loss of proton)
K40
Ar40
19 protons
18 protons
radiometric dating
uses continuous decay to measure time since rock formed
only possible since late 1890’s -- radioactivity discovered in 1896
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
half-life
amount of time it takes for half the atoms of the
parent isotope to decay
different radioactive isotopes have different and
distinct half-lives
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 atoms
is predictable at each half-life
predictable ratios at each half-life
exponential 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
U-235 to Pb-207
Th-232 to Pb-208
half-life = 4.5 by
half-life = 713 my
half-life = 14.1 my
• potassium-argon dating
…argon is a gas--may escape
(ages too young--daughter missing)
K-40 to Ar-40
half-life = 1.3 by
• rubidium-strontium dating
Rb-87 to Sr-87
half-life = 47 by
basic geochronological assumptions
• decay constants constant through geological time
-- 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
• system closed to adding or subtracting of parent/daughter
-- isotopic system and type of mineral (rock) are important
-- careful procedure is essential to correct analysis
igneous rocks are most reliable for dating
…metamorphism may cause loss of daughter products…
…sedimentary rocks will give ages of source rocks…
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
Pt = Po exp
- (to –t)
1st order rate law
Whe re
Pt  quan tity of the parent isotope (i.e. 87Rb) at tim e t;
Po  quan tity of the parent isotope at some earlier time to, when the
isotopic system was closed to any additi ona l is otopic exch ange ;
  is the cha racteristic decay constant for the sys tem of interest, which
is related to the h alf-li fe, t1/2, by th e equa tion below:
ln 2 / t1/2
t1/2  is defined as the half-life, which is the amount of tim e requir ed for 1/2 of the
origin al parent to decay and is a constant.
Rb/Sr Age Dating Equation
87
Rb t = 87Rb o e -to – t)
(Assume that t = 0, for the p resent)
87
Rb o + 87Sro = 87Rb t + 87Srt
(Conserva tion of Mass, with 87Sro as the initi al
conc entration and 87Srt as the con centration today)
Srt - 87Sro = 87Rb t (e to – 1)
87
87Sr  87Sr  87 Rb t
86   86    86  (e  1)
 Sr t  Sr o  Sr t
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
Other dating methods: dendrochronology
annual growth of trees produces concentric rings
…dating back to 9000 years is possible…
- rings need to be calibrated
against C-14 dates to yield
“true” numerical age
- other information may also
be obtained from rings,
including rainfall and temperature
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- can develop composite
chronologies for specific regions
of interest for climate studies
photo © H.D. Grissino-Mayer
relative and absolute dates combined
same example
as in
relative age
geological time scale
eons, eras, periods, epochs
Oldest rocks: Greenland gneisses
Oldest rock fragments: W. Australia detrital zircons
earliest life
cyanobacteria: primitive single-celled organisms
found in Australia and dated at 3.7 billion years old
modern equivalents in
Shark’s Bay, Australia
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proportional time scale
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combine relative and absolute time for geologic time
scale