The Sedimentary Archive

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Transcript The Sedimentary Archive

Time and Geology
Quantitative Geologic Time
Early Attempts
Tried to place events in chronology of actual age
James Ussher, Archbishop of Armagh and Primate of All Ireland
1650
The Annals of the World
-The Annals of the World
Ussher calibrated the Julian calendar with the dates and events
from the Old Testament.
Determined Earth was created on Sunday, 23 October, 4004 BC
Time and Geology
Quantitative Geologic Time
Sir John Lightfoot 1642
Determined that the Earth was created at 9 am
Geologic observations did not support this timeframe.
Later disproved by the theory of evolution
Time and Geology
Quantitative Geologic Time
Other attempts used depositional rates of sediment
If the depositional rate was known one could
estimate the time required to deposit a given
thickness of sediments
The total geologic time elapsed could be estimated
by dividing the annual sediment influx into the
total thickness of sedimentary rock.
Time and Geology
Quantitative Geologic Time
Sir Edmund Halley 1715
Speculated that the ocean must be
the same age as the Earth
Ocean started out fresh
Weathering of rocks on land
produced salt
If the amount of salt in the ocean was
known it could be divided by the amount added
each year
John Joly tried this and came up with an age of ~90 my in 1899
Time and Geology
Quantitative Geologic Time
Lord Kelvin (William Thomson) 1862
Thomson challenged the views on
geologic and biological
change of the early
uniformitarians
Argued Earth could not be that old
Assuming that the Earth started out as
a liquid at 7000°F and has been
cooling since formation
Using the cooling rate he determined
the Earth was 24-40 my old
Time and Geology
Quantitative Geologic Time
Geologists felt the Earth had to be older and argued
No PROOF!
Discovery of radioactivity
provided the answer.
X-ray of Kelvin’s hand
Radioisotopic Methods for Dating Rocks
A Quick Review
The solid Earth is composed of rocks and minerals
A MINERAL is a
naturally occurring,
inorganic solid,
consisting of either a single element or compound,
with a definite chemical composition (or varies within
fixed limits),
and a systematic internal arrangement of atoms.
ROCKS - aggregates of minerals
Radioisotopic Methods for Dating Rocks
MINERAL CHEMISTRY
Minerals are composed of elements in specific proportions.
Elements cannot be broken down into any smaller units and
still be recognizable.
Atoms of one or more elements combine in specific proportions
to produce compounds.
Specific proportions are expressed by the compound’s
chemical formula.
HALITE
QUARTZ
NaCl
SiO2
Radioisotopic Methods for Dating Rocks
MINERAL CHEMISTRY
Atoms are extremely small.
Atoms are composed of three particles:
Protons and Neutrons in the nucleus of the atom.
Electrons in energy levels around the nucleus.
Protons have a positive charge.
Electrons have a negative charge.
Neutrons are neutral.
Radioisotopic Methods
ATOMIC CHEMISTRY
ATOMIC MASS of an element is the total number of protons
and neutrons in the nucleus of the atom.
ATOMIC NUMBER is the total number of protons in the
nucleus of the atom.
Always the same number of protons in every atom of the same
element.
Number of neutrons in the nucleus can vary.
Variation in the number of neutrons in the same element
produces ISOTOPES of that element.
Radioisotopic Methods for Dating Rocks
ATOMIC CHEMISTRY
Isotopes of Oxygen
Always 8 protons in the nucleus.
Oxygen has atomic number of 8 on the Periodic Table.
16O has 8 protons and 8 neutrons.
17O has 8 protons and 9 neutrons.
18O has 8 protons and 10 neutrons.
Radioisotopic Methods for Dating Rocks
ATOMIC CHEMISTRY
Some isotopes have very
unstable nuclei and
those break down
spontaneously.
This reaction is termed
RADIOACTIVITY.
Generally a large amount
of heat is produced
in the reaction.
Radioisotopic Methods for Dating Rocks
Depends on the decay of radioactive isotopes.
Isotopes are varieties of elements that differ by the
number of neutrons in the nucleus.
Radioactive isotopes have nuclei that spontaneously
decay by emitting or capturing a variety of
subatomic particles.
The decaying isotope is known as the parent isotope.
By decay, the parent isotope forms a daughter isotope.
Radioisotopic Methods for Dating Rocks
Loss or gain of neutrons converts a parent isotope into
a daughter isotope of the same element.
Loss or gain of protons changes the parent isotope into
a daughter isotope of a completely different
element.
Through this process, unstable radioactive isotopes
decay to form stable, non-radioactive daughter
isotopes.
Radioisotopic Methods for Dating Rocks
ALPHA () DECAY
Alpha () particles are composed of two protons and
two neutrons (He nucleus)
By expulsion of  particles, the atomic mass decreases
by 4 and the atomic number decreases by 2.
Produces a daughter isotope that is a completely new
element.
Radioisotopic Methods for Dating Rocks
ALPHA () DECAY
238U decays
92
by alpha () decay to form 234Th90
Radioisotopic Methods for Dating Rocks
BETA () DECAY
Beta () particles are essentially electrons.
These electrons are released from the nucleus of the
parent isotope.
Neutrons are composed of a proton and an electron.
Neutron decays, releasing an electron, while at the
same time produces a proton.
Beta () decay increases the atomic number by 1.
No change in the atomic mass.
Radioisotopic Methods for Dating Rocks
BETA () DECAY
40K decays
19
by beta () decay to form 40Ca20
Radioisotopic Methods for Dating Rocks
ELECTRON OR BETA () CAPTURE
Electron or Beta () capture involves capture of an
electron from the surrounding orbiting cloud
by the nucleus.
These electrons join with a proton and form a neutron.
Electron or Beta () capture decreases the atomic
number by 1.
No change in the atomic mass.
Radioisotopic Methods for Dating Rocks
ELECTRON OR BETA () CAPTURE
40K decays
19
by beta () capture to form 40Ar18
Radioisotopic Methods for Dating Rocks
Radioisotopic Methods for Dating Rocks
Radioactive isotopes are incorporated in minerals and
rocks in a variety of ways.
As minerals crystallize from magma, radioactive
isotopes are included in mineral crystal structure.
At the time of crystallization, only parent isotopes are
included in the mineral.
Radioactive parent isotopes then begin to decay
producing daughter isotopes.
Radioisotopic Methods for Dating Rocks
ISOTOPE DATING uses this process to measure the
amount of time elapsed since the mineral’s
formation.
With time, the amount of parent isotope will decrease
and the amount of daughter isotope will increase.
The DECAY RATE is constant and acts like a “clock”.
Decay rates are not affected by temperature, pressure,
or chemical reaction with the parent isotope.
By measuring the ratio of parent to daughter isotopes in
the mineral and comparing it with the rate of
radioactive decay, we can determine the numerical
age of a rock.
Radioisotopic Methods for Dating Rocks
The time it takes for HALF of the atoms of the parent
isotope to decay into daughter isotopes is known
as the isotope’s HALF-LIFE (t½).
1:1 parent to daughter
1:3
1:7
Radioisotopic Methods for Dating Rocks
To calculate the numerical age of a rock, mineral, bone,
etc., we determine the number of half-lives or
fraction thereof and multiply the number of
half-lives gone by by the known half-life (in years).
Simply put:
In a rock we find 23 atoms of 235U and 161 atoms of 207Pb
Half-life (t½) is 713 million years.
Age of the rock is 2.139 billion years.
Radioisotopic Methods for Dating Rocks
FACTORS AFFECTING ISOTOPIC DATING
Most useful in igneous rocks.
As minerals crystallize, radioactive isotopes become
incorporated in the minerals.
No daughter isotopes at that time.
Crystallization sets the isotopic “clock”.
Doesn’t work in sedimentary rocks. How come?
Radioisotopic Methods for Dating Rocks
FACTORS AFFECTING ISOTOPIC DATING
Works best when a rock or mineral represents a
“closed” system.
Parent and daughter isotopes cannot move in or out of
a mineral or rock.
Igneous rocks best fit this criteria.