Geologic Time

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Geologic Time

• Relative vs. Absolute Time
• Stratigraphy and Relative Time Relationships
• Unconformities and Gaps in Stratigraphic Record
• Stratigraphic Correlation
• Geologic Time Scale

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Figure 7.1
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How Old is the Earth?

• Historical Records, written word
• 5000 years

• Modern View, 4.6 billion years old

Hutton (1795) Uniformitarianism deep time?
very old Earth
Bishop Usher (1664) 900 AM, Oct. 23, 4004 BC
Earth is 6000 years old Geologic
events?catastrophism
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How Old is the Earth
Geochronology is the study of time in relation to
earths existence

• Relative Dating
• Determines how old a rock is in relation to its
  surrounding
• Absolute (Numerical) Dating
• Determines actual age in years

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Geologic Time and the Rock Record

• Rocks record the processes and events and help us
  measure geologic time.
• By studying outcrops with the scientific method,
  we can figure out the relative order of events!
  Even absolute ages!
• How?
• Geometric relationships ? Stratigraphy.
• Fossils ? Biostratigraphy.
• Radiometric dating ? Geochronology.

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• Stratification (Strata)
• Layering of Sedimentary Rocks

Distinct layering Beds
Sedimentary Rock
Bed 1
Bed 2
Different thicknesses Color, other
characteristics
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Relative Age Inferences Original Horizontality
Sedimentary rocks are formed in layers (strata)
which were originally horizontal.

• If layers are inclined at an angle, then
  something tilted them - they didnt form that way

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Original horizontality
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Relative Age Inferences Superposition

• If one layer is on top of another, then it came
  later (its younger).
• Note that layers can be completely upside down,
  and you need something like ripple marks to tell
  which way the layers are facing

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Relative Age Inferences Cross-cutting
relationships
Faults are younger than what they cut.
Crosscutting igneous rocks are younger than what
they intrude.
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Relative Age Inferences Inclusion
Units that include bits of another came later
(are younger)
younger
older
older
younger
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Relative Age Inferences

• Assumptions / Geometric Principles
• Sediments deposited horizontally
• Younger sediments on top of older
• Units that cross-cut (e.g. faults or intrusions)
  came after (i.e., are younger than) those that
  they cut
• Units that include bits of another came later
  (are younger)

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Lets practice

• List events from oldest to youngest (including
  faulting and erosion)
• Deposition of Abo Formation, Yeso Formation,
  Moenkopi Formation, Agua Zarco Formation
• Fault (covered) offsets the four sedimentary
  units
• Erosion (especially of Moenkopi)
• Emplacement of Bandelier Rhyolite (as hot ash
  flow)
• Erosion

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Missing Time-Gaps Happen
Buried and tilted erosional surface
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Conformable Contact

• Layers of rock that have been deposited without
  any interruption.
• No gaps in time.
• No missing record due to erosion, non-deposition,
  etc.

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Unconformity

• 3 types of break in the rock record.
• Such surfaces represent
• A hiatus in deposition and/or
• A period of erosion.
• ?Missing time
• ?Significant events.

Popostosa Fm. Playa deposits post-Santa Fe
Group (Pl) alluvium. N of San Lorenzo Cyn
(Socorro, NM) P. A. Scholle (1999).
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Angular Unconformity

• A sharp discontinuity in the rock record
  separating strata that are not parallel.
• Indicates that during the break, a period of
  deformation occurred.

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Disconformity

• A break in the rock record across which there is
  little change in orientation of strata.
• Often just a pause in deposition (subtle).
• May also be obvious erosion surface.

River Road
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Nonconformity

• Horizontal sedimentary rocks on top of eroded
  crystalline rocks (metamorphic or igneous).

Requires erosion to bring crystalline rocks to
the surface.
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Missing Time Unconformities
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practice relative dating
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Fossils as Timepieces

• Fossils? remains of ancient organisms

trilobites
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Fossil Record
Recent
Older
Very Old
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Relative Dating
faunal succession

• groups of fossil animals and plants occur in the
  geologic record in a definite chronological order
• periods of time recognized by characteristic
  fossils

dinosaurs
trilobites
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Correlation of Rock Units Index Fossils
Common occurrence Wide geographic
distribution Very restricted age range
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Key Beds
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Correlation

• Match rocks between different areas
• Key Beds Index Fossils
• Fossil successions.
• Unique minerals.
• Unique rock sequence.
• Can extend relative age sequence elsewhere.

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Bryce

• Correlation

Zion
Grand Canyon
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The Geologic Column and the Geologic Time Scale

• In 19TH Cent., geologists began to assemble a
  geologic column
• composite column containing, in chronological
  order
• the succession of known strata, fitted together
  on the basis of their fossils or other evidence
  of relative age.
• The corresponding column of time is the geologic
  time scale.

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Geologic Column

• Catalog of all known strata
• Not one physical locality but a chronological
  compilation of all localities

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Precambrian
Geologic Time Scale
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• Eon largest interval into which geologic time is
  divided.
• Hadean Eon
• Some moon samples were formed during the Hadean
  Eon.
• Archean Eon
• Archean rocks, which contain primitive
  microscopic life forms are the oldest rocks we
  know of on the Earth.
• Proterozoic Eon
• Phanerozoic Eon

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Relative Time Scale

• Worldwide changes in fossils give break points
• When did dinosaurs go mostly extinct?

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Fossils

• Single-celled organisms range?

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Fossils
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Fossils

• Dinosaur range?

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Relative Time Scale

• Worldwide changes in fossils give break points
• The relative time scale doesnt give us numerical
  ages.
• Where do these numbers come from?

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Relative Dating Correlations
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Inclusion
Units that include bits of another came later
(are younger)
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Absolute Time

• Early attempts to measure absolute time
• Radioactivity
• Radiometric Dating Methods
• Absolute time and Geologic Time

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Geologic Time Scale
RECAP Relative time relationships Fossil
assemblages
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Relative Time Scale

• Worldwide changes in fossils give break points
• When did dinosaurs go mostly extinct?

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Fossils

• Single-celled organisms range?

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Fossils
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Fossils

• Dinosaur range?

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Relative Time Scale

• Worldwide changes in fossils give break points
• The relative time scale doesnt give us numerical
  ages.
• Where do these numbers come from?

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Early Attempts to Measure Geologic Time
Numerically
Quantity of Something
Time
Rate Quantity changes with time
For example, Rates of sedimentation thickness
of sedimentary rocks
Problem did not account for past
erosion differences in sedimentation rates
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Early Attempts to Measure Geologic Time
Numerically

• Saltiness of Seawater (date the ocean)

Edmund Halley (1715) John Joly (1889)
Salt rivers
Answer 90 million years
Oceans
Incorrect!!!
Salts are added both by erosion and by submarine
volcanism, but salts are also removed by solution.
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Early Attempts to Measure Geologic Time
Numerically

• Lord Kelvin (1870s), a physicist, attempted to
  calculate the time Earth has been a solid body.

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Early Attempts to Measure Geologic Time
Numerically

• Lord Kelvin (1897s), a physicist, attempted to
  calculate the time Earth has been a solid body.

Time0
TimeToday
Cooling off by conduction
Earth Solid
Earth molten
No more heating
Theory of heat conduction Experimental data
(melting temp. of rocks, size of Earth)
Answer50-100 million years
Too Young for Geologists!
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Radioactivity A Little History

• H. Becquerel (1896)
• discovers radioactivity in Uranium
• Marie Curie (1900)
• discovers radium heat is given off as byproduct
  of radioact.
• E. Rutherford (1905)
• Radioactive elements transform from one chemical
  element to another
• B. Boltwood (1907)
• Radiometric dating of minerals (410-2000 million
  years)

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Radioactive Atoms

• Atoms contain Protons, Electrons Neutrons

Carbon Atomic number 6 (6 protons)
Stable Isotope
Unstable Isotope
Isotope atoms of the same element containing
different neutrons
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Radioactive Decay

• It turns out that some elements will
  spontaneously turn into other elements. This is
  called radioactivity

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Half-life (T1/2)

• Time needed for ½ of parent atoms to decay (rate
  of decay)

T1/2 1 hour
Time of Parent atoms of daughter atoms
0 1000 0
1 hr 500 500
2 hr 250 750
3 hr 125 875
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Decay rate is a non-linear process
All radioactive elements follow the same law
But, each element will have different decay rates
(half-life) 1 nanosecond? 49 billion years
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Decay Rates

• Decay rates are unaffected by geological
  processes (mainly chemical)
• Once radioactive atoms are created they start to
  act like ticking clock

Know the decay rate
A
B
A
B
A
Count the daughter atoms Count the Parent atoms
A
B
A
A
Calculate the time since the atomic clock started
ticking
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Mass spectrometer

• A Minnesota Connection

Alfred Nier
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Potassium-Argon Dating

• 40K-40Ar? half-life 1.3 billion years

Crystallization
40K?40Ar
40K 40K
40K?40Ar
40K?40Ar
K-mineral Closed system
Magma Clock is ticking Open system
Rock clock is reset Age of crystallization
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• Closed system? no leakage or addition of K or Ar

Rock clock is reset Age of crystallization
Geological processes can allow material to be
added or lost? date will be incorrect
Cross-check with other radiometric systems using
different minerals
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Carbon-14 Method
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Isotopic Systems Used for Radiometric Dating

• Rubidium-Strontium t1/2 47 billion yr
• Uranium-lead t1/24.5 billion yr
• Potassium-Argon t1/21.3 billion yr
• Carbon-14 t1/25730 yr

Long t1/2 useful for dating old
material Short t1/2 useful for dating young
material
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Dating the Geologic Time ScaleSedimentary and
Igneous Rocks
Granite is older than OLD RED SANDSTONE Volcanic
is younger than OLD RED SANDSTONE
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Absolute Geologic Time Scale
Eon Era Period Starting Age (Ma)
Phanerozoic Cenozoic Quaternary 65
Phanerozoic Cenozoic Tertiary 65
Phanerozoic Mesozoic Cretaceous 248
Phanerozoic Mesozoic Jurassic 248
Phanerozoic Mesozoic Triassic 248
Phanerozoic Paleozoic Permian 540
Phanerozoic Paleozoic Pennsylvanian 540
Phanerozoic Paleozoic Mississippian 540
Phanerozoic Paleozoic Devonian 540
Phanerozoic Paleozoic Silurian 540
Phanerozoic Paleozoic Ordovician 540
Phanerozoic Paleozoic Cambrian 540
Precambrian - Proterozoic Precambrian - Proterozoic Precambrian - Proterozoic 2500
Precambrian - Archean Precambrian - Archean Precambrian - Archean 3800
Precambrian - Hadean Precambrian - Hadean Precambrian - Hadean 4500
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Age of Earth

• Oldest dated rocks 3.94 by
• Oldest dated material 4.2 by
• Moon Rocks Meteorites 4.4-4.58 by

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A Year of Geologic Time 1second 200 years
000 AM, Jan 1 Formation of Earth
Late January Formation of Core-Mantle-Crust
Mid February Life Begins, Oldest Know Rocks
Late March First Photosynthetic Organisms
Mid July Evolution of Cells with Nucleus
Mid November First Organisms with Shells
Late November First Land Plants/Fish
Mid December Dinosaurs became Dominant
Dec 26 Extinction of Dinosaurs
Evening of Dec 31 Human-like Animals
115945-115950 Rome Ruled the Western World
115959 Modern Geology Started with Hutton

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Clair Patterson the Age of the Earth

• In early 1950s, Clair Patterson was a graduate
  student at the University of Chicago.
• Wanted to use lead isotope ratios to determine
  the Earths age,
• but the background level of lead contamination
  was too high
• Lead used in gasoline, paints, plumbing, solder
  (cans for food) and pesticides.

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Clair Patterson the Age of the Earth

• To accurately measure very low lead
  concentrations, Patterson created the modern
  laboratory clean room.
• in 1953, published estimate of Earths age as
  4.55 BY(previously estimated at 3.3 BY)

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• By 1960s, Patterson began to worry about the
  extent of lead contamination in our environment.
• Patterson discovered that modern humans had 700
  to 1,200 times as much lead in their bones as
  pre-Columbian Incas.  

over 99 of the northern hemisphere atmospheric
lead originated from human activity. The
average atmospheric lead levels were 10 to 200
times higher than in pre-industrial times and up
to 1,000 to 10,000 times higher in urban areas!
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• First recognition of the global scale and early
  history of lead pollution
• First recognition that essentially EVERYONE in
  1950s-60s society suffered from low-level lead
  poisoning.
• Patterson campaigned extensively for lead
  removal, but was vigorously opposed by industry
  labs and some other scientists.

Eventually, scientific data accumulated by
Patterson and others led to the 1970 Clean Air
Act
By 1991, lead levels in Greenland snow had fallen
by a factor of 7.5