Transcript EARTH AND SPACE SCIENCE

EARTH AND SPACE
SCIENCE
Chapter 8 The Rock Record
8.2 Determining Absolute Age
8.2 Determining Absolute Age
Objectives
• Summarize the limitations of using the
rates of erosion and deposition to
determine the absolute age of rock
formations.
• Describe the formation of varves.
• Explain how the process of radioactive
decay can be used to determine the
absolute age of rocks.
Introduction
• When judging the relative age of a rock unit,
one is only comparing it as older or younger
than other rock units.
• To further understand Earth’s history, it is
necessary to determine the absolute age of a
rock formation.
• Absolute age is the numeric age of an object
or event, often stated in years before present,
as established by an absolute-dating process,
such as radiometric dating.
Absolute Dating Methods
• A variety of methods are used to arrive at
an absolute age.
• Some methods may involve geologic
processes that can be observed and
measured over time.
• Other methods may involve examining
the chemical composition of certain
materials in rocks.
Absolute Dating Methods
• Rates of erosion can be used to estimate the
age of a stream.
• Determination by measuring erosion rates is
only practical for studying geologic features
that have formed within the past 10,000 to
20,000 years.
• Rates of erosion can vary greatly over millions
of years.
• Average rates of deposition for sedimentary
rocks can also be calculated.
Absolute Dating Methods
• One can, in general, expect about 30 cm of common
sedimentary rock to be deposited over 1,000 years.
• A particular sedimentary layer, however, may not have
been deposited at an average rate.
• A flood, for example, can deposit several meters of
sediment in one day.
• Rate of deposition may slow down considerably under
certain circumstances as well.
• Because of variability in rates of erosion and
deposition, measurement of these processes is a poor
way to determine absolute age.
Absolute Dating Methods
• A varve is a banded layer of sand and silt that is
deposited annually in a lake, especially near ice
sheets or glaciers, and that can be used to determine
absolute age (in context).
• Varves appear as light colored bands of course sands
and darker bands of finer silt particles.
• During summer, when snow and ice melt rapidly, a
rush of water can carry large amounts of sediment into
a lake.
• Most of the course particles settle quickly to form a
layer on the bottom of the lake.
• As winter approaches and the water freezes, clay
particles that are still suspended in the water then
settle slowly to form an fine overlying layer.
Radiometric Dating
• Isotopes are atoms of the same element that
have different numbers of neutrons.
• Rocks generally contain small amounts of
radioactive isotopes.
• Isotopes undergo radioactive decay, releasing
particles and/or energy, in order to reach a
more stable state.
• Scientists can use the radioactive decay
process to determine the age of certain rocks
because the rate of radioactive decay for a
particular isotope is constant and cannot be
changed.
Radiometric Dating
• Scientists can measure the amount of radioactive
parent isotope (original radioactive isotope) and
compare that to the amount of newly formed stable
daughter product.
• The relative proportion of radioactive parent isotope to
stable daughter product can then be used to
determine the age of the object.
• Half-life is the time required for half of a sample of
radioactive isotope to break down into the stable
daughter product.
• For example, if you begin with 100 grams of
radioactive parent isotope, after one half-life, you will
have 50 grams of radioactive parent isotope and 50
grams of stable daughter product.
Radiometric Dating
• After the next half-life, you will have 25 grams of
radioactive parent isotope and 75 grams of stable
daughter product.
• After yet another half-life, you will be left with 12.5
grams of radioactive parent isotope and 87.5 grams of
stable daughter product (and so on).
• Comparing parent or daughter isotopes only works
when the sample has not lost or gained radioactive
parent isotopes or daughter product.
• If too little or too much time has passed, an accurate
reading may not be possible with the chosen isotopes
to measure.
Radiometric Dating
• Also, if the isotope you are measuring
wasn’t in the sample to begin with,
accurate results are impossible.
– In order to date an inorganic rock, you
would not use carbon-14 dating – why?
– If you were dating a dinosaur bone, you
would not use carbon-14 dating – why?
– If you were dating a woolly mammoth, you
would not use potassium/argon dating –
why?
Carbon Dating
• Materials containing organic remains less than
70,000 years old can be dated using carbon14.
• When using radiocarbon dating, one looks for
the small percentage of carbon-14 that was
bonded with oxygen to form carbon dioxide
that was taken in by plants and incorporated
into the organic structure of plants (and then
animals as they ate the plants).
• Carbon-14 has a 5,730 year half-life and
undergoes beta decay to become nitrogen-14.
References
• Erosion and Deposition http://wwwbrr.cr.usgs.gov/projects/Burned_Wat
ersheds
• Varve - http://www.runet.edu/~rugs/saga.html
• Radioactive Decay http://spurr.pls.uni.edu/ctst/inspiration.html
• Half-life http://fig.cox.miami.edu/~cmallery/150/chemist
ry/C14dating.htm