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

Figure 7.1

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.

Original horizontality

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

Missing Time-Gaps Happen

Buried and tilted erosional surface

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.

Missing Time: Unconformities

practice relative dating

Fossils as Timepieces

• Fossils remains of ancient organisms

trilobites

Recent

OlderVery Old

Fossil Record

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

Key Beds

Correlation

• Match rocks between different areas:

Key Beds & Index Fossils• Fossil successions.• Unique minerals.• Unique rock sequence.

• Can extend relative age sequence elsewhere.

Correlation

Grand Canyon

Zion

Bryce

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

Geologic Time Scale

Precambrian

• 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?

Fossils

• Single-celled organisms range?

?

Algae

(J. William Schopf; © 1993 AAAS)

FossilsMollusks

Fossils

• Dinosaur range?

Dinosaurs:290 - 65 M.y.a.

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

Mass spectrometerA Minnesota Connection

Alfred Nier

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