Absolute

1
Absolute Relative Dating

• Alex Burkett, Eric Francey,
• Juliet Collados

2
Introduction

• Geologists estimate that the earth is about 4.6
  billion years old
• James Hutton
• ? Principle of
  Uniformitarianism current geologic
• processes are the same
  processes that were at work in
• the past
• Before Huttons work, many believed that the
  earth was only about six thousand years old and
  all geological features had been formed at the
  same time
• Huttons observation and conclusions about the
  age of the earth encouraged other to learn more
  about the earths history

3
Relative Age vs. Absolute Age

• Strata layers if rocks
• Stratigraphy the study
• of strata
• Relative age indicates that
• one layer of rock is older or
• younger than another layer
• ? it does not indicate
• the exact age of the rock
• Absolute Age indicates the actual age of a layer
  of rock

4
Relative Dating - Superposition

• Formation of sedimentary rock begins when
• sediments are deposited horizontally
• as sediments accumulate, they are
• compressed and harden into
• sedimentary rock layers
• Law of Superposition an undeformed
• sedimentary rock layer is older than the
• layers above it and younger than the
• layers below it

5
Unconformities

• Some rock layers are deformed, causing
  unconformities
• Caused by movements in the earths crust
• There are three kinds of unconformities
• ?All rocks below an unconformity
  are older than those rocks
• above it
• ? Nonconformity an unconformity in which
  stratified rock rests upon unstratified rock
• ? Rocks deposited in horizontal layers are folded
  or tilted and then eroded the boundary between
  the tilted layers and the horizontal layers is
  called an angular unconformity

6
Unconformities Cont

• ? disconformity on the ocean floor the boundary
  between older, eroded surface, and younger,
  overlying layers is nearly horizontal

7
Absolute Dating Cross Cutting

• Sometimes tectonic activity disturbs rock layers
  and causes either an intrusion of igneous rock
  into the strata, or a fault (break or crack) in
  the earths crust, shifting rocks positions
• ? determining age using the
  law of superposition may be
• too difficult in these
  scenarios
• Law of Crosscutting Relationships a fault or an
  intrusion is
• 
  always younger than the rock layers that
• 
  it cuts through

8
Absolute Dating Rate of Erosion

• If scientists can determine the rate at which a
  stream erodes its bed, the approximate age can be
  estimated
• ? edge of Niagara Falls
  is eroding at an average rate
• of 1.3 m/year
• ? based on the average
  rate of erosion, scientists
• determined the
  falls formed approximately 9,900
• years ago
• This method is only accurate with geological
  features that formed within the past 10,000 to
  20,000 years

9

• Older features that have developed over a
  greater amount of time have varying rates of
  erosion

10
Absolute Dating Rate of Deposition

• Geologists collect data over long periods of time
  and then estimate the average rates of deposition
  for common sedimentary rocks (such as limestone,
  shale, and sandstone)
• On average 30cm of sedimentary rock are deposited
  over a period of 1000 years, however, this method
  is not always accurate
• ? it is possible that any layer may not
  have been deposited at an average
  rate
• ? i.e. floods deposit many metres of
• sediment in just one day

11
Absolute Dating Varve Count

• Varve shows definite annual layers consists of
  a light coloured band of course particles and a
  darker band of fine particles
• ? similar to estimating the age of
  a tree by counting the
• growth rings in its trunk
• Varves usually occur in glacial lakes
• During the summer snow and ice melt
• rapidly this rush of water carries large
• amounts of sediment into the lake
• Coarse particles quickly form a layer on the
  bottom of the lake

12
Varve count cont

• Winter freezes the surface of the lake causing
  fine clay particles to settle slowly and form a
  think layer on top of the coarse sediments
• Each varve represents one year of deposition

13
Absolute Age Radioactive Decay

• Rocks often emit radioactive isotopes particles
  that have a nuclei that emit particles and energy
  at a constant rate
• Radioactive decay occurs when an atom emits
  particles and energy
• The atom changes into a different isotope of the
  same element or an isotope of a different element
• Radioactive decay continues until a stable, or
  nonradioactive, form of an element is produced
• Scientists measure the concentrations of the
  original radioactive isotope and the newly
  created isotopes
• The proportions are compared of the new and the
  original isotopes to determine the absolute age
  of the rock

14
Absolute Age Half Life

• A half life is the time it takes for half the
  mass of a given amount of a radioactive element
  to decay into its daughter elements
• The half life for U-238 (Uranium) to decay into
  P-206 (Lead) is 4.5 billion years
• ?if you were to begin with 10g of U-238, in
  4.5 billion years you would have 5g
• ?after another 4.5 billion years you would
  have 2.5g
• By comparing the amount of the radioactive
  element and its daughter elements in some rock
  samples, scientists can determine the age of the
  sample
• The greater the percentage of the daughter
  element present, the older the sample is

15
Half Life cont

• The amount of time that has passed since the rock
  was formed determines which radioactive element
  will give the more accurate age measurement
• U-238 (Uranium) is most useful in dating geologic
  samples more that 10 million years old
• K-40 (Potassium-40) is used to date rocks between
  50,000 and 4.6 billion years old (has a half life
  of 1.3 billion years)
• Rb-87 (Rubidium-87) it can be used to verify age
  of rocks previously dated with K-40 and has a
  half life of 47 billion years

16
Absolute Dating Carbon Dating

• To determine the age of once living things
  scientists use carbon dating
• Living plants and animals absorb Carbon-12 and
  Carbon-14 in the form of CO2 during
  photosynthesis
• While the organisms are alive, the ratio of
• the two elements remains relatively constant
• When the organism dies, the amount of
• C-14 decreases as the radioactive carbon
• atoms decay to nonradioactive nitrogen-14

17
Carbon Dating cont

• The half life of C-14 is about 5,730 years
• To establish age, scientists determine the
  proportion of C-12 to C-14 in the sample and
  compare it with the proportion of C-14 to C-12
  known to exist in a living organism
• Carbon dating can establish the age of
• samples up to 70,000 years old

18
Application of Absolute Relative Dating

• Olduvai Gorge the Cradle of Mankind

19
Application of Absolute Relative Dating

• The Shroud of Turin

20
Final Summary

• A reoccurring need in scientific areas such as
  archaeology and geology is the need to accurately
  date material
• The different methods of absolute and relative
  dating provide scientists with various tools to
  come to these conclusions
• These methods help us to better understand both
  the world we live in and how we have lived in it

21
Guessing Game!
22
141 to 65 millions years old

• Started to form 17 million years
  ago

11,000 years old
23
All Done!

• Thanks for your time

24
Works Cited

• Fagan, Brian M., People of the Earth An
  Introduction to World Prehistory
• Fifth Edition. Boston Little,
  Brown and Company, 1986.
• Jolly, Clifford J., and Plog, Fred. Physical
  Anthropology and Archaeology
• Third Edition. New York Alfred A.
  Knopf, 1982.
• Pillips, Clifford R., Ramsey, William L., Sager,
  Robert J., and Watenpaugh,
• Frank M. Modern Earth Science.
  Toronto Holt, Rinehart, and Winston,
• 2002.