# Geologic Time Relative vs. Absolute 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

Figure 7.1

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) 9:00 AM, Oct. 23, 4004 BC Earth is ~6000 years old Geologic eventscatastrophism

How Old is the Earth Relative Dating
Geochronology is the study of time in relation to earth’s existence Relative Dating Determines how old a rock is in relation to its surrounding Absolute (Numerical) Dating Determines actual age in years

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
Distinct layering Beds Sedimentary Rock Bed 1 Bed 2 Different thicknesses Color, & other characteristics

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 didn’t form that way

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
Faults are younger than what they cut. Crosscutting igneous rocks are younger than what they intrude.

Relative Age Inferences Inclusion
Units that include bits of another came later (are younger) younger older older younger

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)

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

Nonconformity Horizontal 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

Fossil Record Recent Older Very Old

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

Correlation of Rock Units: Index Fossils
Common occurrence Wide geographic distribution Very 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 Bryce Zion Grand Canyon

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

Precambrian Geologic Time Scale

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 Algae Single-celled organisms range? ?
(J. William Schopf; © 1993 AAAS)

Fossils Mollusks

Fossils Dinosaur range? Dinosaurs: 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?

Relative Dating: Correlations

Inclusion Units that include bits of another came later (are younger)

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

Geologic Time Scale RECAP: Relative time relationships
Fossil assemblages

Relative Time Scale Worldwide changes in fossils give break points
When did dinosaurs go mostly extinct?

Fossils Algae Single-celled organisms range? ?
(J. William Schopf; © 1993 AAAS)

Fossils Mollusks

Fossils Dinosaur range? Dinosaurs: 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?

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

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.

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. Time=0 Time=Today Cooling off by conduction Earth Solid Earth molten No more heating Theory of heat conduction Experimental data (melting temp. of rocks, size of Earth) Answer: million years Too Young for Geologists!

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 ( million years)

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

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) T1/2= 1 hour Time # of Parent atoms # of daughter atoms 1000 1 hr 500 2 hr 250 750 3 hr 125 875

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

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 A B A B Count the daughter atoms Count the Parent atoms A A B A Calculate the time since the atomic clock started ticking

Mass spectrometer A Minnesota Connection Alfred Nier

Potassium-Argon Dating
40K-40Ar half-life = 1.3 billion years Crystallization 40K40Ar 40K 40K40Ar 40K40Ar K-mineral Closed system Magma Clock is ticking Open system Rock clock is reset Age of crystallization

Closed system no leakage or addition of K or Ar
K-mineral Closed system Closed system no leakage or addition of K or Ar Geological processes can allow material to be added or lost date will be incorrect Rock clock is reset Age of crystallization Cross-check with other radiometric systems using different minerals

Carbon-14 Method Atmosphere Neutron + 14-Nitrogen  14-Carbon  CO2
After Death Time=0 14-C  decays  14-N T1/2= 5730 yrs Surface CO2 Water, Plants, & Animals

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 material Short t1/2 useful for dating young material

Dating the Geologic Time Scale Sedimentary and Igneous Rocks
Granite is older than OLD RED SANDSTONE Volcanic is younger than OLD RED SANDSTONE

Absolute Geologic Time Scale
Eon Era Period Starting Age (Ma) Phanerozoic Cenozoic Quaternary 65 Tertiary Mesozoic Cretaceous 248 Jurassic Triassic Paleozoic Permian 540 Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian Precambrian - Proterozoic 2500 Precambrian - Archean 3800 Precambrian - Hadean 4500

Age of Earth Oldest dated rocks 3.94 by Oldest dated material 4.2 by
Moon Rocks & Meteorites by

A Year of Geologic Time 1second ≈ 200 years 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

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