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• When layers of rocks have been deposited without
interruption for very long periods of time, we call
them conformable.
• However, throughout Earth’s history the deposition of
sediment has been interrupted again and again.
• These breaks in the rock records are called
unconformities.
• An unconformity represents a long period of time in
which deposition of sediments ceased, crustal uplift
occurred and erosion removed the previously formed
rocks, and then deposition resumed.
• Unconformities are important features since they
represent significant geologic events in Earth’s past.
• There are three main types of unconformities.
• The most recognizable is angular unconformity.
• It consists of folded sedimentary rocks that are
covered by younger and flat sedimentary strata.
More common, but less recognizable are disconformities.
This is because the strata of rock on each side are essentially parallel.
• The third type of unconformity is nonconformity.
• In this case, the break separates older metamorphic
or igneous rocks from younger sedimentary strata.
• Igneous and metamorphic rock originate far below
the surface, hence for nonconformity to develop,
there must have been major uplift and erosion of
overlying rocks.
• Once exposed, the renewal of sedimentation
occurred.
Absolute Dating
• Throughout the centuries mankind has
striven to try to determine the exact age
of Earth.
• What these people were seeking was a
numerical date of Earth’s age.
• Numerical dates pinpoint the time in
history when an event took place, such
as the extinction of the dinosaurs 65
million years ago.
• Our current understanding of radioactivity
allows scientists to use the natural
radioactivity of certain elements in rocks to
accurately determine their numerical dates.
• Therefore, we can obtain numeric dates of
many rocks that represent important events in
Earth’s distant past.
Radioactive Half-life and Decay
• To determine the absolute ages of fossils and rocks,
scientists analyze isotopes of radioactive elements.
• Isotopes are atoms of the same element that have the
same number of protons, but have different numbers of
neutrons.
• Isotopes of the same element display the same chemical
attributes, but sometimes display very different physical
attributes.
• Most isotopes are stable, meaning that they stay in their
original form.
• Isotopes that are unstable are radioactive. For instance
carbon-12 is a stable isotope while carbon-14 is radioactive
because it has two extra neutrons.
• Radioactive isotopes have unstable atomic nuclei
that have a tendency to change, or decay, over
time.
• This process is called radioactive decay.
• The figure below shows an example of how
radioactive decay can occur.
• The starting form of the element is called
the parent isotope and the form that it
changes into is called the daughter isotope.
• Because radioactive decay occurs at a
steady rate, scientists can use the relative
amounts of stable and unstable isotopes
present in a rock to determine the rock’s age.
• If you know the rate of decay for a
radioactive element in a rock, you can figure
out the absolute age of the rock.
• Determining the absolute age of a sample,
based on the ratio of parent material to
daughter material, is called radiometric
dating.
• The half-life is the time that it takes one-half of a radioactive
sample to decay.
• If you have a rock sample that contains an isotope with a halflife of 10,000 years, that means that in 10,000 years, half the
parent material will have decayed and become daughter
material.
• If you analyze the sample and find equal amounts of parent
material and daughter material, this means that half the
original radioactive isotope has decayed and that the sample
must be about 10,000 years old.
• If you find that ¼ of your sample is parent material and ¾ is
daughter material, it took 10,000 years for half the original
sample to decay and another 10, 000 years for half of what
remained to decay.
• The age of your sample would be 20,000 years.
Potassium - Argon Method
• One isotope that is used for radiometric
dating is potassium-40.
• Potassium-40 has a half-life of 1.3 billion
years, and it decays to argon and calcium.
• Geologists measure argon as the daughter
material.
• This method is used primarily to date rocks
older than 100,000 years.
Uranium - Lead Method
• Uranium-238 is a radioactive isotope that
decays in a series of steps to lead-206.
• The half-life of uranium-238 is 4.5 billion
years.
• The older the rock is, the more daughter
material (lead-206) there will be in the rock.
• Uranium-lead dating can be used for rocks
more than 10 million years old.
• Younger rocks do not contain enough
daughter material to be accurately measured
by this method.
Rubidium - Strontium Method
• Through radioactive decay, the unstable
parent isotope rubidium-87 forms the
stable daughter isotope strontium-87.
• The half-life of rubidium-87 is 49 billion
years.
• This method is used to date rocks older
than 10 million years.
Carbon-14 Method
• The element carbon is normally found in three
forms, the stable isotopes carbon-12 and carbon13 and the radioactive isotope carbon-14.
• These carbon isotopes combine with oxygen to
form the gas carbon dioxide, which is taken in by
plants during photosynthesis.
• As long as a plant is alive, new carbon dioxide
with a constant carbon-14 to carbon-12 ratio is
continually taken in.
• Animals that eat plants contain the same ratio of
carbon isotopes.
• Once a plant or an animal dies, however, no
new carbon is taken in.
• The amount of carbon-14 begins to decrease
as the plant or animal decays, and the ratio
of carbon-14 to carbon-12 decreases.
• This decrease can be measured in a
laboratory.
• Because the half-life of carbon-14 is only
5,730 years, this dating method is used
mainly for dating things that lived within the
last 50,000 years.