Geologic Time
• Relative vs. Absolute TimeRelative vs. Absolute Time• Stratigraphy and Relative Stratigraphy and Relative
Time RelationshipsTime Relationships• Unconformities and Gaps Unconformities and Gaps
in Stratigraphic Recordin Stratigraphic Record• Stratigraphic CorrelationStratigraphic Correlation• Geologic Time ScaleGeologic Time Scale
How Old is the Earth?• Historical Records, written word
– 5000 years
Bishop Usher (1664)9:00 AM, Oct. 23, 4004 BC Earth is ~6000 years oldGeologic eventscatastrophism
Hutton (1795) Uniformitarianism“deep” time very old Earth
•Modern View, ~ 4.6 billion years old
How Old is the Earth
• Relative Dating–Determines how old a rock is in relation to its surrounding
• Absolute (Numerical) Dating–Determines actual age in years
Geochronology is the study of time in relation to earth’s existence
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.
Stratification (Strata)Layering of Sedimentary Rocks
Sedimentary Rock
Distinct layeringBeds
Bed 1
Bed 2
Different thicknessesColor, & other characteristics
Relative Age Inferences Original Horizontality
If layers are inclined at an angle, then something tilted them - they didn’t form that way
Sedimentary rocks are formed in layers (strata) which were originally horizontal.
Relative Age Inferences Superposition
• If one layer is on top of another, then it came later (it’s younger).
• Note that layers can be completely upside down, and you need something like ripple marks to tell which way the layers are “facing”
Youngest
Oldest
Youngest
Oldest
Relative Age Inferences Cross-cutting relationships
Crosscutting igneous rocks are younger than what they intrude.
Faults are younger than what they cut.
Relative Age Inferences Inclusion
Units that include bits of another came later (are younger)
younger
older
younger
older
Relative Age Inferences
Assumptions / Geometric Principles:1. Sediments deposited horizontally2. Younger sediments on top of older3. Units that cross-cut (e.g. faults or
intrusions) came after (i.e., are younger than) those that they cut
4. Units that include bits of another came later (are younger)
Let’s 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
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.
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).
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.
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
NonconformityHorizontal
sedimentary rocks on top of eroded crystalline rocks (metamorphic or igneous).
Requires erosion to bring crystalline rocks to the surface.
Relative Dating
faunal succession
trilobites
dinosaurs
–groups of fossil animals and plants occur in the geologic record in a definite chronological order–periods of time recognized by characteristic fossils
Correlation of Rock Units: Index Fossils
Common occurrenceWide geographic distributionVery restricted age range
Correlation
• Match rocks between different areas:
Key Beds & Index Fossils• Fossil successions.• Unique minerals.• Unique rock sequence.
• Can extend relative age sequence elsewhere.
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.
Geologic Column
• Catalog of all known strata• Not one physical locality but a
chronological compilation of all localities
• 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
Relative Time Scale
• Worldwide changes in fossils give break points
• When did dinosaurs go mostly extinct?
Relative Time Scale
• Worldwide changes in fossils give break points
• The relative time scale doesn’t give us numerical ages.
• Where do these numbers come from?
Absolute Time
• Early attempts to measure absolute time
• Radioactivity• Radiometric Dating
Methods• Absolute time and
Geologic Time
Early Attempts to Measure Geologic Time Numerically
Time = Quantity of Something
Rate Quantity changes with time
For example, Rates of sedimentation & thickness of sedimentary rocks
Problem: did not account for past erosiondifferences in sedimentation rates
Early Attempts to Measure Geologic Time Numerically
• Saltiness of Seawater (date the ocean)
Oceans
Saltrivers
Edmund Halley (1715)John Joly (1889)
Answer: ~ 90 million years
Salts are added both by erosion and by submarine volcanism, but salts are also removed by solution.
Incorrect!!!
Early Attempts to Measure Geologic Time Numerically
Lord Kelvin (1870’s), a physicist, attempted to calculate the time Earth has been a solid body.
Early Attempts to Measure Geologic Time Numerically
• Lord Kelvin (1897’s), a physicist, attempted to calculate the time Earth has been a solid body.
Earthmolten
EarthSolid
Time=0 Time=TodayCooling off by conductionNo more heating
Theory of heat conductionExperimental data (melting temp. of rocks, size of Earth)
Answer:50-100 million years
Too Young for Geologists!
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)
Radioactive AtomsAtoms contain Protons, Electrons &
Neutrons
Carbon: Atomic number =6 (6 protons)
Isotope: atoms of the same element containing different # neutrons
StableIsotope
UnstableIsotope
Radioactive Decay
It turns out that some elements will spontaneously turn into other elements. This is called radioactivity
Half-life (T1/2)• Time needed for ½ of parent atoms
to decay (rate of decay)
Time # of Parent atoms
# of daughter atoms
0 1000 0
1 hr 500 500
2 hr 250 750
3 hr 125 875
T1/2= 1 hour
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
B
BBA
Decay Rates
• Decay rates are unaffected by geological processes (mainly chemical)
• Once radioactive atoms are created they start to act like ticking clock
A A
AA A
Know the decay rateCount the daughter atomsCount the Parent atoms
Calculate the time since the atomic clock started ticking
Potassium-Argon Dating
• 40K-40Ar half-life = 1.3 billion years
40K40Ar
40K40Ar40K40Ar
MagmaClock is tickingOpen system
40K40K
Crystallization
K-mineralClosed system
Rock clock is resetAge of crystallization
Closed system no leakage or addition of K or Ar
40K40K
K-mineralClosed system
Rock clock is resetAge 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
Carbon-14 Method
Surface CO2
Water, Plants, &Animals
After DeathTime=014-C decays 14-N
T1/2= 5730 yrs
Atmosphere
Neutron + 14-Nitrogen 14-Carbon CO2
Isotopic Systems Used for Radiometric Dating
• Rubidium-Strontium t1/2= 47 billion yr
• Uranium-lead t1/2=4.5 billion yr
• Potassium-Argon t1/2=1.3 billion yr
• Carbon-14 t1/2=5730 yr
Long t1/2 useful for dating old materialShort t1/2 useful for dating young material
Dating the Geologic Time ScaleSedimentary and Igneous Rocks
Granite is older than OLD RED SANDSTONEVolcanic is younger than OLD RED SANDSTONE
Absolute Geologic Time ScaleEon Era Period Starting Age
(Ma)
Phanerozoic
Cenozoic Quaternary 65Tertiary
MesozoicCretaceous
248JurassicTriassic
Paleozoic
Permian
540
PennsylvanianMississippian
DevonianSilurian
OrdovicianCambrian
Precambrian - Proterozoic 2500Precambrian - Archean 3800Precambrian - Hadean 4500
Age of Earth
• Oldest dated rocks 3.94 by• Oldest dated material 4.2 by• Moon Rocks & Meteorites 4.4-4.58
by
0:00 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
11:59:45-11:59:50 Rome Ruled the Western World
11:59:59 Modern Geology Started with Hutton
A Year of Geologic Time1second ≈ 200 years
Clair Patterson & the Age of the Earth
In early 1950’s, Clair Patterson was a graduate student at the University of Chicago.
Wanted to use lead isotope ratios to determine the Earth’s age,
but the background level of lead contamination was too high
Lead used in gasoline, paints, plumbing, solder (cans for food) and pesticides.
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 Earth’s age as 4.55 BY(previously estimated at 3.3 BY)
• By 1960’s, 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!
First recognition of the global scale and early history of lead pollution
First recognition that essentially EVERYONE in 1950’s-60’s 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