Download Relative versus Absolute Age of Rocks

Relative versus Absolute
Age of Rocks
Jarðsaga 1
– Þróun Lífs og Lands –
Ólafur Ingólfsson
• Relative dating
• Estimating the age of the Earth
• Radiometric (absolute) dating
• The age of the Earth
Amount of Time
Required for
Some Geologic
Processes and
Events
Relative
dating of
geological
strata:
Builds primarily
on Steno´s
principles of
superposition and
cross-cutting
relationships, as
well as biostratigraphical
divisions of
strata
Stratigraphic nomenclature
Era = Öld
• Paleozoic Era = Fornlífsöld
• Mezozoic Era = Miðlífsöld
• Cenozoic Era = Nýlífsöld
Period = Tímabil
• Devonian Period = Devon tímabilið
• Tertiary Period = Tertíer tímabilið
Epoch = Tími
• Paleocene Epoch = Paleocentími
• Pleistocene Epoch = Ísöld, Ísaldartími
Lithologic (rock) correlation
Sandstone
10 km
correla
Sandstone
tion
Shale
correla
Limestone
tion
Shale
Limestone
Age correlation
Sandstone
Extinction of
trilobite species
Sandstone
Shale
Limestone
Origin of
urchin species
Shale
Limestone
Biostratigraphy
- The study of the spatial and temporal distribution
of fossil organisms, often interpolated with paleoenvironmental information, as a means of dating rock
strata
Biostratigraphic units
• Index fossils (aka “guide fossils”) are fossils
that are particularly useful in biostratigraphic
correlation. Characteristics of index fossils
are:
–
–
–
–
–
Abundant
Easily identified
Geographically widespread
Occurs in a variety of rock types
Short stratigraphic range (rapid rates of evolution)
Principles of Stratigraphy
Index fossils: Organisms
evolve, thrive and then
become extinct. If they can
be recognized in the rock,
these fossils can be used to
correlate rock in one area
with rocks in a distant area
•
Stratigraphic correlations
Worldwide
Correlation
International Geological Correlation Chart based on
lithostratigraphical and biostratigraphical correlations
http://www.palaeos.com/Timescale/timescale.html
What is Absolute/Numerical Dating?
The age of an Earth material or
event in years
Relative age
3rd
2nd
1st
youngest
Absolute
12 kya
95 mya
1.7 bya
8 kya
Attempts to estimate the age of the Earth
• In the pre-scientific world view the issue of the age
of the Earth was a theological question. In 1640,
Bishop Ussher produced his famous calculation that
the Earth was created in 4004 BC.
The Earth
was created
on the 23rd
of October
4004 BC
• There was no single estimate of the Earth's age
in the mid 1800's and no good way to arrive at one.
There were various attempts to estimate the
Earth's age, working back from sedimentation
rates and other geophysical phenomena. The
attempts produced estimates from about 100
million years up to several billion years.
• There were two major problems with such
efforts. The first is that the geological history
was still being reconstructed. The second is that
the rates of the physical processes in question are
variable and knowledge of them was incomplete.
Estimating Earth's Age
Bishop Usher – Biblical Interpretation
Having established the first day of creation as
Sunday 23 October 4004 BC (refinements by
others further pinpointed this to 9 in the
morning, London time, or midnight in the Garden
of Eden), Ussher calculated the dates of other
biblical events, concluding, for example, that
Adam and Eve were driven from Paradise on
Monday 10 November 4004 BC, and that the Ark
touched down on Mt Ararat on 5 May 2348 BC
`on a Wednesday'.
History of ideas on the age of the Earth
Estimating Earth's Age Early (failed) Attempt
Sedimentation rates - 3 my – 500 my
Halley/Joly - Ocean Salinity – 100 my
Lord Kelvin – Rate of Cooling – 100-20 my
Lord Kelvin´s attempts to date the
Earth
Kelvin attempted to calculate the amount of time the Earth has been
a solid body by estimating cooling rates by conduction. In 1862 Kelvin
estimated the age of the Earth to be 98 million years. Later in 1897
he revised his estimate downwards to 20-40 million years.
The Radioactive Clock
Kelvin did not know about
radioactivity and heating of the
Earth's crust by radioactive
decay; for this reason his
estimates were completely wrong.
The discovery of the natural radioactive decay of
uranium in 1896 by Henry Becquerel, opened new
vistas in science. In 1905, the British physicist
Lord Rutherford made the first clear suggestion
for using radioactivity as a tool for measuring
geologic time directly.
Radioactive Revolution
around 1900 AD
• Radioactive decay – spontaneous
transformation of an element to
another isotope of the same
element or another element.
Radioactive Decay
of Rubidium to
Strontium and
Uranium to Thorium
Isotopes
Different forms of the same
element containing the same
number of protons, but varying
numbers of neutrons, i.e.:
235U, 238U;
87Sr, 86Sr; 14C, 12C
Rate of radioactive decay
We must know three things to calculate an age:
1. The current parent/daughter isotope ratio.
2. The original parent/daughter isotope ratio.
3. The rate of decay, or the half-life
• The rate of decay is an intrinsic property of the
isotope, independent of temperature, pressure and
chemistry.
• We know on average what percentage will decay in a
given time. This is the decay rate.
The fundamental assumption:
The half-lives of radioactive
isotopes are the same as they
were billions of years ago.
Half-life
The fixed period of
time during which
half the parent
atoms present in a
closed system decay
to form daughter
atoms - the time for
one half of the
radioactive atoms to
decay
Half-Life
Example: a particular isotope has a half life of 1 year. If
we initially had 64 grams of the substance, then after:
•
•
•
•
•
•
1 year we have 32 g of the parent, 32 g of the
daughter;
2 years we have 16 g of the parent, 48 g of the
daughter;
3 years we have 8 g of the parent, 56 g of the
daughter;
4 years we have 4 g of the parent, 60 g of the
daughter;
5 years we have 2 g of the parent, 62 g of the
daughter;
6 years we have 1 g of the parent, 63 g of the
daughter;
Time change of isotope populations
• As the amount of the
parent nuclei decreases
exponentially…
… the amount of the
daughter nuclei
increases accordingly.
Some Radioactive Isotopes Commonly
Used in Absolute Dating
Parent
Isotope
Stable Daughter
Product
Currently Accepted Half-Life
Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
704 million years
Thorium-232
Lead-208
14.0 billion years
Rubidium-87
Strontium-87
48.8 billion years
Potassium-40
Argon-40
1.25 billion years
Samarium-147 Neodymium-143
Carbon-14
Nitrogen-14
Hydrogen-3
Helium-3
106 billion years
5.730 years
12.3
What rock types are most likely to be useful for
radiometric dating?
Which rocks can be dated?
Only certain types
of rocks, chiefly
igneous rocks, can
be dated directly
by radiometric
methods; but
these rocks do not
ordinarily contain
fossils...
Most sedimentary rocks such as sandstone, limestone, and shale are
related to the radiometric time scale by bracketing them within time
zones that are determined by dating igneous rocks
Uranium (U) - Thorium (Th) - Lead
(Pb)
238U decays to 206Pb
235U decays to 207Pb
232Th decays to 208Pb
(4.5 billion)
(713 million)
(14.1 billion)
• Rocks containing Uranium provide 3
possible techniques.
• Because all 3 occur together, it allows
a method to cross-check dates.
• Uses zircons, uraninite and uranium
ores
Dating rocks – theory and reality
The mathematical expression that relates radioactive
decay to geologic time is called the age equation and is:
Dating rocks by radioactive decay is simple in theory,
but the laboratory procedures are complex.
Magnetic stratigraphy
• Motion of magnetic
material (iron) in the
liquid outer core is
responsible for Earth’s
magnetic field
• The magnetic field is
polarized—north and
south poles
• Polarity has reversed
itself repeatedly over
the course of Earth’s
history
Magnetic “striping” of oceanic crust
The Magnetic Time Scale
Magnetic field reversals:
• Polarity: The N-S directionality of the Earth’s magnetic field
• normal polarity
• reverse polarity -- every 1/2 million years or so the Earth
reverses its polarity in what is called a magnetic reversal.
Dating techniques
that can be used
on this geologic
column:
• absolute dating
• paleomagnetism
• superposition
• fossil correlation
The Earth’s Oldest Crustal Rocks
•The Acasta gneiss in
northern Canada was
formed 4.0 billion years
ago and is the most
ancient body of rock
discovered thus far.
Rocks from
the Isua
sequence in
Greenland are
probably of
similar age
Dating oldest
rocks on Earth
This example comes from some of the oldest rocks on
Greenland. A suite of different samples from a group
of granites and granitic gneisses (sheared and
recrystallised granite) were analysed.
The ancient supracrustal rocks
of Isua and Akilia, West
Greenland, date between
3,800 and 3,700 Ma. These
metamorphosed rocks were
originally deposited as
sediments or volcanic flows in
shallow water.
The oldest material found on Earth
In Western Australia, single zircon
crystals found in younger sedimentary
rocks have radiometric ages of as much
as 4.3 billion years, making these tiny
crystals the oldest materials to be
found on Earth so far.
About 380 kg of rocks from the Moon
were returned to Earth by the Apollo
and Luna missions. The oldest dated
moon rocks have ages between 4.4
and 4.5 billion years and provide a
minimum age for the formation of our
nearest planetary neighbor.
The Solar system formed ~4.6 BY ago
History of thought as to the age of Earth
Constructing geological time scales
The Geologic time scale
• Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
• Built using a combination of
stratigraphic relationships,
cross-cutting relationships, and
absolute (isotopic) ages
Skoðið þessa vefsíðu:
http://www.palaeos.com/Timescale/timescale.html
References, further reading, useful web pages
Stanley, Earth System History, chapter 6
Lewis, C. 2000: The Dating Game – One Man´s Search for the Age of the
Earth. Cambridge University Press
http://www.ucmp.berkeley.edu/help/timeform.html
http://earth.leeds.ac.uk/dynamicearth/dating/index.htm
http://www.enchantedlearning.com/subjects/Geologictime.html
http://www.palaeos.com/Timescale/default.htm
http://www.geo.ilstu.edu/geology/POG/MrShields/LectureNotes/24
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/clkroc.html
http://www2.nature.nps.gov/geology/usgsnps/gtime/ageofearth.html#age
http://gpc.edu/~pgore/geology/geo102/age.htm
http://www.whfreeman.com/presssiever/content/instructor/content/ppl/1
http://www.gustavus.edu/oncampus/academics/geology/nobel_display/nobel_introd
uce.html
http://www.palaeos.com/Timescale/timescale.html
http://pubs.usgs.gov/gip/geotime/
http://www.earth.uni.edu/~groves/24