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1 Relative Age Dating Absolute Age Dating Age Dating Using Magnetic Records The Age of the Earth
Geologic Time A major difference between geologists and most other scientists is their attitude about time. A "long" time may not be important unless it is > 1 million years.
2 Earth History
Two ways to date geologic events
1) relative dating (fossils, structure) 2) absolute dating (isotopic, tree rings, etc.)
3 Relative vs Absolute Age • Usually geologists first establish relative ages then try to get absolute age dates • Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries
Steno's Laws
Nicolaus Steno (1669) • Principle of Superposition • Principle of Original Horizontality • Principle of Lateral Continuity
Laws apply to both sedimentary and volcanic rocks.
4 Principle of Superposition
In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.
Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion:
Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below
5 Principle of Superposition
Youngest rocks
Oldest rocks
Principle of Original Horizontality
Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface.
6 Principles of original horizontality and superposition
7 Principle of Lateral Continuity
Layered rocks are deposited in continuous contact.
Principle of Lateral Continuity
Map view
8 Principle of Lateral Continuity
Map view
Principle of Lateral Continuity
Map view
9 Correlation • Process used to tie separated strata together
• Based on matching physical features such as – Physical continuity - trace of rock unit – Similar rock types - marker beds, coal seams, rare minerals, odd color
Correlation • Within sedimentary layers there are often the remains of small animals (fossils) • Fossils are quite useful for correlating between two sections that are not laterally continuous
10 Ammonite Fossils Petrified Wood
Correlation • Fossils represent living creatures that have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age
11 CorrelatingCorrelating bedsbeds usingusing indexindex fossilsfossils
Using Fossils to Correlate Rocks
12 Unconformity
A buried surface of erosion
13 Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption
Unconformity represents missing time in the geologic record
Sedimentation of Beds A-D Beneath the Sea
14 Uplift and Exposure of D to Erosion
Continued Erosion Removes D and Exposes C to Erosion
15 Subsidence and Sedimentation of E over C
Unconformity: a buried surface of erosion
Formation of a Disconformity
16 First type of unconformity
South rim of the Grand Canyon
17 South rim of the Grand Canyon 250 million years old
Paleozoic Strata
550 million years old 1.7 billion years old Precambrian
South rim of the Grand Canyon 250 million years old
550 million years old 1.7 billion years old Nonconformity
18 Nonconformity in the Grand Canyon
Nonconformity in the Grand Canyon
Tapeats Sandstone (~550 million years old)
Vishnu Schist (~1700 million years old)
19 Second type of unconformity
Siccar Point, Scotland
20 Siccar Point, Scotland
Buried and tilted erosional surface
Sedimentation of Beds A-D Beneath the Sea
21 Deformation and Erosion During Mountain Building
Erosional Surface Cuts Across Deformed Rocks
22 Subsidence and Subsequent Deposition Buries Erosional Surface
Angular Unconformity
Formation of an Angular Unconformity
23 Third type of unconformity
Principle of Cross-Cutting Relationships • A rock unit must always be older than any feature that cuts or disrupts it – If a rock unit is cut by a fracture • The rock itself is older than the fracture that cuts across it
24 Cross-cutting Relationships
Relative Geologic Dating
25 The Geologic time scale
• Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages
TheThe GeologicGeologic TimeTime ScaleScale
26 Absolute geochronology
• Add numbers to the stratigraphic column based on fossils. • Based on the regular radioactive decay of some chemical elements.
Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons. i.e.: 235U, 238U 87Sr, 86Sr 14C, 12C
27 NaturallyNaturally OccurringOccurring IsotopesIsotopes ofof CarbonCarbon
Half-life
The half-life of a radioactive isotope is defined as the time required for half of it to decay.
28 Proportion of Parent Atoms Remaining as a Function of Time
Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). • The rate at which this decay occurs is constant and known. • Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.
29 Radioactivity and Absolute Time
30 31 Radioactive Decay of Rubidium to Strontium by β emission
Production and Decay of Radiocarbon
32 of Radiocarbon of Radiocarbon Production and Decay Production and Decay
Major Radioactive Elements Used in Isotopic Dating
33 Geologically Useful Decay Schemes
Parent Daughter Half-life (years) 235U 207Pb 4.5 x 109 238U 206Pb 0.71 x 109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x 109 14C 14N 5730
Radiometric Dating
• We can calculate geologic age if – The half-life of a radioactive isotope is known – The parent/daughter ratio can be measured – There is no loss of an isotopes from the system • e.g., 222Rn is an intermediate daughter product in the 238U decay series to 208Pb
34 Oldest rocks and minerals on Earth Narryer Gneiss, Western Australia • Zircons in a metamorphosed sandstone dated at 4.35 to 4.40 Ga Acasta Gneiss, Northwestern Canada and the Nuvvuagittuq greenstone belt, Northern Quebec • Rocks dated at 3.80 to 4.28 Ga Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica.
Age of the Earth Although the oldest rocks found on Earth are 4.4 Ga, we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.4 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.
35 Age of the Earth This is based on the age of rocks brought back from the Moon (4.4 Ga), and meteorites (4.6 Ga), that are thought to be good representatives of the early solar system. These data suggest that the present chemical composition of the crust must have evolved for more than 4.5 Ga.
The geologic timescale and absolute ages Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for fossil transitions
36 The big assumption
The half-lives of radioactive isotopes are the same as they were billions of years ago.
Test of the assumption
Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systems all of which have given the same answer. However, scientists continue to critically evaluate this data.
37 Bracketing ages
Radiometric dates provide absolute ages to the Geologic Column
38 Earth’s Magnetic Field
Magnetostratigraphy • Technique that works best in volcanic rocks • Time scale based on polarity reversal of Earth's magnetic field • Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks
39 Magnetization of Magnetite
Lavas record magnetic reversals
40 MagneticMagnetic reversalsreversals overover thethe pastpast 2020 millionmillion yearsyears
The Geologic time scale • Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages
41 The Geologic Column and Time Scale
42