contamination
How Reliable are the Rock dating Methods?
Dr. Andrew snelling
How Are Rocks dated? Q. How are the ages of rocks measured? a) b) c) d) e) f)
By color or appearance? Rocks come already labeled? By observing rocks form? By the minerals in the rocks? By the fossils in the rocks? By the chemistry of the rocks?
How Are Rocks dated? Q. How are the ages of rocks measured? f) By the chemistry of the rocks? Answer. By the chemistry of the rocks assuming/utilizing radioactive decay
the Radioactive dating methods • The decaying radioactive atoms (radioisotopes) are called the parent atoms (isotopes), and the resultant stable atoms (isotopes) are called the daughter atoms (isotopes) • Minerals, rocks, and fossils contain some of these radioactive parent atoms (radioisotopes) and daughter atoms (isotopes)
the Radioactive dating methods Parent Atom
Daughter Atom
Carbon-14 Uranium-238 Uranium-235 Potassium-40 Rubidium-87 Samarium-147
Nitrogen-14 Lead-206 Lead-207 Argon-40 Strontium-87 Neodymium-143
the Radioactive dating methods • A rock is chemically tested for these parent and daughter atoms (isotopes) • If the rate of radioactive decay has remained constant at today’s measured rate • Then it can be calculated how long it has taken the measured amount of daughter atoms (isotopes) to accumulate — the rock’s age
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
The simple Hourglass analogy • Starting with all the sand grains in the top bowl, it takes one hour for all the red sand grains to fall to the bottom bowl as green sand grains • A rock is chemically tested for these isotopes, that is, the red and green atoms are measured
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
the Radioactive dating methods • If the rate of radioactive decay (falling) has remained constant at today’s measured rate • Then it can be calculated how long it has taken the measured amount of daughter green atoms to accumulate from the parent red atoms—how long the hourglass has been operating (the rock’s age)
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
the Radioactive dating methods • BUT the use of an hourglass to tell the time requires three crucial assumptions • So even more are these three assumptions crucial to the radioactive dating methods
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
THREE CRUCIAL ASSUMPTIONS
Assumption 1 The amounts of parent and daughter atoms at the beginning, when the rock formed, that is, the initial conditions, must be known
Assumption 1 There were no green atoms in the bottom glass bowl at the beginning (or there was a known amount), that is, no inheritance – this requires an observer
Assumption 2 All daughter atoms measured today must only have been derived by in situ radioactive decay of parent atoms (a closed system)
Assumption 2
All the green atoms must have only been derived from the falling red atoms, that is, there has been no contamination – this requires an observer
Assumption 3 The radioactive decay rate must have been constant at today’s measured rate (a constant decay rate)
Assumption 3
The rate at which the red atoms fall to produce green atoms must have been constant at today’s measured rate (a constant decay rate) – this requires an observer
Three crucial assumptions • Assumption 1: all the daughter atoms derived from parent atoms • Assumption 2: no other process has affected the parent-daughter relationship • Assumption 3: constant decay rates
Three crucial assumptions • BUT none of these assumptions are provable • The past cannot be observed and measured (tested) • So these assumptions are not even reasonable
Three crucial assumptions • Daughter atoms may be inherited when the rock forms • Subsequent contamination is common
Three crucial assumptions • The quality of the chemical analyses is not disputed, just their interpretation based on these three assumptions • The need for Assumption 1 (the initial conditions) can be seemingly overcome by using the isochron method
Three crucial assumptions • When Assumption 2 (a closed system) is violated by inherited isotopes or contamination it can often be detected • But even if it is not detected does not mean contamination is not present
Three crucial assumptions • Assumption 1: all the daughter atoms derived from parent atoms – NO INHERITANCE • Assumption 2: no other process has affected the parent-daughter relationship – NO CONTAMINATION • Assumption 3: constant decay rates
Mt St Helens, Washington (USA) New lava dome
New lava dome
1986 Dacite Lava Flow
Mt St Helens 1986 Dacite Lava Flow K-Ar model “ages”:•whole rock 0.35 (±0.5) million years •feldspar 0.34 (±0.06) million years •amphibole 0.9 (±0.2) million years •pyroxene 1.7 (±0.3) million years •pyroxene concentrate 2.8 (±0.6) million years Conclusion: Inherited excess
40Ar*
inheritance Hualalai basalt, Hawaii (AD 1800-1801) 1.4 – 1.6 million years “old” Mt. Etna basalt, Sicily (122 BC) 0.25 million years “old” Mt. Etna, Sicily (AD 1792) 0.35 million years “old” Mt. Lassen plagioclase, California (AD 1915) 0.11 million years “old” Sunset Crater basalt, Arizona (AD 1064-1065) 0.25 – 0.27 million years “old”
inheritance Potassium–argon ages at various sample points inside a basalt “pillow” get younger because 40 extra Ar* was inherited as the outer rim cooled rapidly
43 my 33 my
4.3 my 1.5 my 1.0 my
Centimeters from Rim
inheritance
inheritance • 10 Zaire diamonds yielded a K-Ar isochron “age” of 6.0±0.3 billion years • At 6 billion years old, how could these diamonds which came from inside the earth be older than the earth?!?!? • Obviously these diamonds inherited extra Ar
inheritance • Two volcanic centers 100 miles apart in the African Rift Valley whose lavas are
Dr. Andrew snelling
How Are Rocks dated? Q. How are the ages of rocks measured? a) b) c) d) e) f)
By color or appearance? Rocks come already labeled? By observing rocks form? By the minerals in the rocks? By the fossils in the rocks? By the chemistry of the rocks?
How Are Rocks dated? Q. How are the ages of rocks measured? f) By the chemistry of the rocks? Answer. By the chemistry of the rocks assuming/utilizing radioactive decay
the Radioactive dating methods • The decaying radioactive atoms (radioisotopes) are called the parent atoms (isotopes), and the resultant stable atoms (isotopes) are called the daughter atoms (isotopes) • Minerals, rocks, and fossils contain some of these radioactive parent atoms (radioisotopes) and daughter atoms (isotopes)
the Radioactive dating methods Parent Atom
Daughter Atom
Carbon-14 Uranium-238 Uranium-235 Potassium-40 Rubidium-87 Samarium-147
Nitrogen-14 Lead-206 Lead-207 Argon-40 Strontium-87 Neodymium-143
the Radioactive dating methods • A rock is chemically tested for these parent and daughter atoms (isotopes) • If the rate of radioactive decay has remained constant at today’s measured rate • Then it can be calculated how long it has taken the measured amount of daughter atoms (isotopes) to accumulate — the rock’s age
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
The simple Hourglass analogy • Starting with all the sand grains in the top bowl, it takes one hour for all the red sand grains to fall to the bottom bowl as green sand grains • A rock is chemically tested for these isotopes, that is, the red and green atoms are measured
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
the Radioactive dating methods • If the rate of radioactive decay (falling) has remained constant at today’s measured rate • Then it can be calculated how long it has taken the measured amount of daughter green atoms to accumulate from the parent red atoms—how long the hourglass has been operating (the rock’s age)
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
the Radioactive dating methods • BUT the use of an hourglass to tell the time requires three crucial assumptions • So even more are these three assumptions crucial to the radioactive dating methods
The simple Hourglass analogy red sand grains (atoms) falling
green sand grains (atoms)
THREE CRUCIAL ASSUMPTIONS
Assumption 1 The amounts of parent and daughter atoms at the beginning, when the rock formed, that is, the initial conditions, must be known
Assumption 1 There were no green atoms in the bottom glass bowl at the beginning (or there was a known amount), that is, no inheritance – this requires an observer
Assumption 2 All daughter atoms measured today must only have been derived by in situ radioactive decay of parent atoms (a closed system)
Assumption 2
All the green atoms must have only been derived from the falling red atoms, that is, there has been no contamination – this requires an observer
Assumption 3 The radioactive decay rate must have been constant at today’s measured rate (a constant decay rate)
Assumption 3
The rate at which the red atoms fall to produce green atoms must have been constant at today’s measured rate (a constant decay rate) – this requires an observer
Three crucial assumptions • Assumption 1: all the daughter atoms derived from parent atoms • Assumption 2: no other process has affected the parent-daughter relationship • Assumption 3: constant decay rates
Three crucial assumptions • BUT none of these assumptions are provable • The past cannot be observed and measured (tested) • So these assumptions are not even reasonable
Three crucial assumptions • Daughter atoms may be inherited when the rock forms • Subsequent contamination is common
Three crucial assumptions • The quality of the chemical analyses is not disputed, just their interpretation based on these three assumptions • The need for Assumption 1 (the initial conditions) can be seemingly overcome by using the isochron method
Three crucial assumptions • When Assumption 2 (a closed system) is violated by inherited isotopes or contamination it can often be detected • But even if it is not detected does not mean contamination is not present
Three crucial assumptions • Assumption 1: all the daughter atoms derived from parent atoms – NO INHERITANCE • Assumption 2: no other process has affected the parent-daughter relationship – NO CONTAMINATION • Assumption 3: constant decay rates
Mt St Helens, Washington (USA) New lava dome
New lava dome
1986 Dacite Lava Flow
Mt St Helens 1986 Dacite Lava Flow K-Ar model “ages”:•whole rock 0.35 (±0.5) million years •feldspar 0.34 (±0.06) million years •amphibole 0.9 (±0.2) million years •pyroxene 1.7 (±0.3) million years •pyroxene concentrate 2.8 (±0.6) million years Conclusion: Inherited excess
40Ar*
inheritance Hualalai basalt, Hawaii (AD 1800-1801) 1.4 – 1.6 million years “old” Mt. Etna basalt, Sicily (122 BC) 0.25 million years “old” Mt. Etna, Sicily (AD 1792) 0.35 million years “old” Mt. Lassen plagioclase, California (AD 1915) 0.11 million years “old” Sunset Crater basalt, Arizona (AD 1064-1065) 0.25 – 0.27 million years “old”
inheritance Potassium–argon ages at various sample points inside a basalt “pillow” get younger because 40 extra Ar* was inherited as the outer rim cooled rapidly
43 my 33 my
4.3 my 1.5 my 1.0 my
Centimeters from Rim
inheritance
inheritance • 10 Zaire diamonds yielded a K-Ar isochron “age” of 6.0±0.3 billion years • At 6 billion years old, how could these diamonds which came from inside the earth be older than the earth?!?!? • Obviously these diamonds inherited extra Ar
inheritance • Two volcanic centers 100 miles apart in the African Rift Valley whose lavas are
