Transcript Fossil Dating - Catawba County Schools

Dating rocks
• Relative dating
• Using a set of principles to put rocks in
their proper sequences of formation
• Absolute dating
• Using radioactive decay to determine the
exact age of rocks
Absolute Dating:
any method of measuring the age of an event
or object in years
Most common:based on Radioactive Decay
Parent
daughter
Why does it work?
1. The decay rate is CONSTANT,
independent of external conditions
in the earth.
2. The daughter/Parent ratio can
be precisely measured.
Radioactive Decay
• The process in which a
radioactive isotope tends to
break down into a stable
isotope of the same element or
another element.
• Sounds great but what is an
isotope?
• An isotope is an atom that has
the same number of protons
(atomic #) as other atoms of
the same element do but that
has a different number of
neutrons (and thus a different
atomic mass)
Isotopes
• Most isotopes are stable,
meaning they stay in their
original form
• But some are unstable
• Unstable isotopes are
radioactive
• Radioactive decay is the
process in which a
radioactive isotope tends
to bread down into a stable
isotope of the same or
another element
How does it work?
What does this have to do with the age of rocks?
• An Unstable isotope is
called a parent isotope
• The stable isotope
produced by
radioactive decay is
the daughter isotope.
• Decay is constant
• The more daughter
isotope- the older the
rock!
Radiometric dating
• A method of
determining the age
of an object by
estimating the
relative percentages
of a radioactive
(parent) isotope and a
stable (daughter)
isotope
• Ratio or parent
material to daughter
material
– Absolute dating
• Helps scientists determine actual age of fossils
• Rocks near fossils contain radioactive elements –
unstable elements that break down into different elements
• Half-life of a radioactive element is the time it takes for
half of the atoms in a sample to decay
• Scientists compare the amount of radioactive element in a
sample to the amount of the element into which it breaks
down
• Scientists use this info to calculate the age of the rock,
which then tells the age of the fossil
Half-life – the time required for onehalf of the radioactive nuclei in a sample
to decay
Dating with carbon-14
(radiocarbon dating)
– Carbon is normally found in three forms:
stable C-12, stable C-14 & radioactive
C-14
– All combine with oxygen to form CO2
– Half-life of only 5730 years
– Used to date very recent events
– Carbon-14 is produced in the upper
atmosphere
– Useful tool for anthropologists,
archeologists, and geologists who study
very recent Earth history
Carbon-14
• Carbon-14 is continuously created in the
atmosphere by cosmic radiation.
• There is one atom of radioactive C-14 for
every trillion atoms of C-12 in the atmosphere
• Plants absorb C-14 directly through their
leaves in the form of carbon dioxide
• Animals take in C-14 indirectly when they eat
plants
• Although C-14 disintegrates at a constant rate,
it is continuously renewed as long as an
organism remains alive.
• When an organism dies, it stops absorbing new
C-14 and its radiocarbon “clock” is set.
Types of radiometric dating
• Potassium-argon method
– K-40 half-life 1.3 billion years
– Decays into argon and calcium
– Used to date rocks older than 100,00 years old
• Uranium-lead method
– U-238 half-life 4.5 billion years
– Decays into lead-206
– Used for rocks more than 10 million years old
• Rubidium-strontium method
– Rb-87 half-life 49 billion years
– Decays into strontium-87
– Used for rocks more than 10 million years old
• Carbon-14 method
Relative-Dating Principles
• Lateral continuity
– sediment extends laterally in all directions until
it thins and pinches out or terminates against
the edges of the depositional basin
• Cross-cutting relationships
– an igneous intrusion or a fault must be
younger than the rocks it intrudes or displaces
Cross-cutting
Relationships
• A dark-colored
dike has intruded
into older light
colored granite:
the dike is
younger than the
granite
North shore of Lake Superior, Ontario Canada
Cross-cutting Relationships
• A small fault
displaces
tilted beds:
the fault is
younger than
the beds
Templin Highway, Castaic, California
Back to Steno
www.gly.uga.edu/railsback/1121Steno.jpg
Why are layers tilted?
• Deformation of rocks
– Occurs after they are deposited
– Important factor in relative dating
• Folding
– Anticlines, synclines
– Rock bends, but does not break
• Faulting
– Normal, reverse, transform
– Rock breaks
Folding
www.hill.anorak.org.uk/dhtml/glgchap5.html
Faulting
www.stmarys.ca/academic/science/geology/structural/faults.html
The Map That Changed the World
Relative Geologic Time Scale
• The relative geologic time
scale has a sequence of
– eons
– eras
– periods
– epochs
E-or played at Epcot! (Eon,
Era, Period, and Epoch!)
– but no numbers
indicating how long ago
each of these times
occurred
Geologic Time Scale
• Large divisions based on characteristics of
fossils
• Paleozoic Era – early life dominated by
invertebrate animals
• Mesozoic Era – middle life
• Cenozoic Era – recent life
How was the timescale created?
• Mapping in 1800s using the principles of
– Superposition
– Original Horizontality
– Original Lateral Continuity
– Cross-cutting relationships
– Also Fossil Correlation
Absolute Dating
• Radiometric dating is the most common
method of obtaining absolute ages
– calculated from the natural rates of decay of
various natural radioactive elements present in
trace amounts in some rocks
• Other methods?
– Tree ring counting
– Varves
– Ice cores
Geologic Time Scale
• The discovery of radioactivity
near the end of the 1800s
allowed absolute ages to be
accurately applied to the
relative geologic time scale
• The geologic time scale is a
dual scale
– a relative scale
– and an absolute scale
Changes in the Concept of
Geologic Time
• The concept and measurement of geologic time has changed
through human history
• James Ussher (1581-1665) in Ireland
– calculated the age of Earth based on recorded history and
genealogies in Genesis
• he announced that Earth was created on October 22,
4004 B.C.
• a century later it was considered heresy to say Earth
was more than about 6000 years old
Changes in the Concept of
Geologic Time
• During the 1700s and 1800s Earth’s age
was estimated scientifically
– Georges Louis de Buffon (1707-1788)
calculated how long Earth took to cool
gradually from a molten beginning
• used melted iron balls of various diameters
• he estimated Earth was 75,000 years old
Changes in the Concept of
Geologic Time
– Others used rates of deposition of various
sediments and thickness of sedimentary rock
in the crust
• gave estimates of <1 million
• to more than 2 billion years
– Or the amount of salt carried by rivers to the
ocean and the salinity of seawater
• John Joly in 1899 obtained a minimum age of 90
million years
Crisis in Geology
• Lord Kelvin (1824-1907)
– knew about high temperatures inside of deep mines and
reasoned that Earth is losing heat from its interior
• Assuming Earth was once molten, he used
• the melting temperature of rocks
• the size of Earth
• and the rate of heat loss
– to calculate the age of Earth as between 400 and 20 million
years
Crisis in Geology
• This age was too young for the geologic
processes envisioned by other geologists at that
time
– leading to a crisis in geology
• Kelvin did not know about radioactivity as a heat
source within the Earth
Absolute-Dating Methods
• The discovery of radioactivity destroyed Kelvin’s argument for the
age of Earth
• Radioactivity is the spontaneous decay of an atom’s nucleus to a
more stable form
• The heat from radioactivity helps explain why the Earth is still warm
inside
• Radioactivity provides geologists with a powerful tool to measure
absolute ages of rocks and past geologic events
Absolute-Dating Methods
• Understanding absolute dating requires knowledge of atoms
and isotopes: we have it!
• Atomic mass number
= number of protons + number of neutrons
• Isotopes: different numbers of neutrons
• Different isotopes have different atomic mass numbers but
behave the same chemically
• Most isotopes are stable
– but some are unstable
• Geologists use decay rates of unstable isotopes to determine
absolute ages of rocks
Radioactive Decay
• Radioactive decay is the process whereby an unstable atomic
nucleus spontaneously changes into an atomic nucleus of a
different element
• Three types of radioactive decay:
– alpha decay, two protons and two neutrons (alpha particle)
are emitted from the nucleus
Half-Lives
• Half-life of a radioactive isotope is the time it
takes for one half of the atoms of the original
unstable parent isotope to decay to atoms of a
new more stable daughter isotope
• The half-life of a specific radioactive isotope is
constant and can be precisely measured
Half-Lives
• The length of half-lives for different isotopes of
different elements can vary from
– less than 1/billionth of a second
– to 49 billion years
• Radioactive decay
– is geometric not linear
– a curved graph
Geometric Radioactive Decay
• In radioactive
decay, during
each equal
time unit, one
half-life, the
proportion of
parent atoms
decreases by
1/2
Determining Age
• By measuring the parent/daughter ratio and
knowing the half-life of the parent which has
been determined in the laboratory geologists can
calculate the age of a sample containing the
radioactive element
• The parent/daughter ratio is usually determined
by a mass spectrometer
– an instrument that measures the proportions of atoms
with different masses
Determining Age
• For example:
– If a rock has a parent/daughter ratio of
1:3  a parent proportion of 25%
– and the half-live is 57 million years, how
old is the rock?
– 25% means it is 2 halflives old.
– the rock is 57 x 2 =114
million years old.
What Materials Can Be Dated?
• Most radiometric dates are obtained from
igneous rocks
• As magma cools and crystallizes, radioactive
parent atoms separate from previously formed
daughter atoms
– they fit differently into the crystal structure of certain
minerals
• Geologists can use the crystals containing the
parents atoms to date the time of crystallization
Long-Lived Radioactive
Isotope Pairs Used in Dating
• The isotopes used in radiometric dating need to be
sufficiently long-lived so the amount of parent material left is
measurable
– Such isotopes include:
Parents
Uranium 238
Uranium 234
Thorium 232
Rubidium 87
Potassium 40
Daughters Half-Life (years)
Lead 206
4.5 billion
Lead 207
704 million
Lead 208
14 billion
Strontium 87
48.8 billion
Argon 40
1.3 billion
How do we know the Earth can’t
be older than about 6-7 b.y.?
• Moderate half-life isotopes (1 b.y.)
• If Earth was > 6-7 b.y. old, there wouldn’t
be many parents left
Radiocarbon Dating Method
• Carbon is found in all life
• It has 3 isotopes
– carbon 12 and 13 are stable but carbon 14 is not
– carbon 14 has a half-life of 5730 years
– carbon 14 dating uses the carbon 14/carbon 12 ratio of
material that was once living
• The short half-life of carbon 14 makes it suitable for dating
material < 70,000 years old
• It is not useful for most rocks, but is useful for archaeology and
young geologic materials
Carbon 14
• Carbon 14 is constantly forming
in the upper atmosphere
– when a high-energy neutron, a type
of cosmic ray , strikes a nitrogen 14
atom it may be absorbed by the
nucleus and eject a proton changing
it to carbon 14
• The 14C formation rate
– is fairly constant
– and has been calibrated against tree
rings
Carbon 14
• The carbon 14 becomes part of
the natural carbon cycle and
becomes incorporated into
organisms
• While the organism lives it
continues to take in carbon 14
– when it dies the carbon 14 begins
to decay without being replenished
• Thus, carbon 14 dating
measures the time of death
Tree-Ring Dating Method
• The age of a tree can be determined by counting the
annual growth rings in lower part of the stem (trunk)
• The width of the rings are related to climate and can be
correlated from tree to tree
– a procedure called cross-dating
• The tree-ring time scale now extends back 14,000 years!
Tree-Ring Dating Method
• In cross-dating, tree-ring patterns are used from
different trees, with overlapping life spans
Summary
• Uniformitarianism holds that
– the laws of nature have been constant through
time
– and that the same processes operating today
have operated in the past
– although not necessarily at the same rates
Summary
• The principles of superposition
–
–
–
–
–
original horizontality,
lateral continuity
and cross-cutting relationships
are basic for determining
relative geologic ages and for interpreting Earth
history
• Radioactivity was discovered during the late
19th century
– and lead to radiometric dating
– which allowed geologists to determine absolute
ages for geologic events
Summary
• Half-life is the length of time it takes for onehalf of the radioactive parent isotope to decay
to a stable daughter isotope of a different
element
• The most accurate radiometric dates are
obtained from long-lived radioactive
isotope/daughter pairs
– in igneous rocks
Summary
• The most reliable radiometric ages are obtained using two
different pairs in the same rock
• Carbon 14 dating can be used only for organic matter
such as
– wood, bones, and shells
– and is effective back to about 70,000 years