GEOL 101 Lecture 8: Introduction to Geological History Chapter 8 Add

GEOL 101

Lecture 8: Introduction to Geological History

Chapter 8

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• 14 C lab •19th Cent smoke stacks •A-Bomb

Geologic Time Scale

Relative Dating (Logic) & Absolute (Radiometric) Dating

1 General Rules for Interpreting Sedimentary Rocks • Law of Original Horizontality • Law of Superposition • Law of Cross-Cutting Relationships • The Meaning of Unconformities • Concept of Facies = (Environments of Formation) • Correlation Tools

Original Horizontality • Horizontal Beds vs. Vertical Beds

Up What’s It All Mean?

Horizontal Bedding: U.S. Rt. 19, Powell Mtn., WV J.S. Kite Photo

2 GEOL 101 Lecture #

General Rules for Interpreting Sedimentary Rocks • Law of Original Horizontality • Law of Superposition • Law of Cross-Cutting Relationships • The Meaning of Unconformities • Concept of Facies = (Environments of Formation)

3 Superposition

• Stratified Sediments or Rocks

• Younger “Rocks” - Added to Top of a Sequence • Oldest “Rocks” at the Bottom of Sequence

Horizontal Bedding: U.S. Rt. 19, Powell Mtn., WV J.S. Kite Photo

Superposition • Oldest “Rocks” are at the Bottom. • Younger Rocks are at the top.

Youngest

Oldest

4 Breached (Eroded) Folds & Age of Rocks. Superposition: Youngest on Top

Younger Rocks Older Rocks

Where are older rocks exposed at the surface? Younger Rocks

Cross-Cutting Relationships

• Younger Intrusions or Faults Cut Across Older Rocks .

5 Fault Parts

Cross-Cutting Relationships - Plutons

Explain the geological history represented by an outcrop of basalt that is intruded by a dike of rhyolite porphyry. The 5-10 cm of rock that was basalt along the dike is now hornfels. Which rock was there first? Which was there second? What caused the hornfels?

6 General Rules for Interpreting Sedimentary Rocks • Law of Original Horizontality • Law of Superposition • Law of Cross-Cutting Relationships • The Meaning of Unconformities • Concept of Facies = (Environments of Formation)

Unconformity • Buried Erosion Surface .

Unconformity, Ouray Colorado

7 Unconformity, Ouray Colorado

Late Devonian sandstone Elbert Formation ~360 million years old Unco nform ity: ~ 1,000 ,000, 000 y ears mis sing

Precambrian shale, siltstone & sandstone (Uncompahgre formation ~1.5 billion years old) Vince Matthews, Colorado G.S. source for text information.

8 Maturity Immature Sediments

Mature Sediments

Facies

Tools to Reconstruct Relative Ages in Earth History

• Correlation .

Correlation:

• Marker Beds –Volcanic Ash –Iridium Layer •(Bolide Impact) .

9 Volcanic Ash Marker Beds

Iridium Layer ld o rs ea y 00 ,0 00 ,0 65

Great Relative Dating Tool: Fossils

Trilobite

10 Correlation with Fossils

Strata = Layers

Stratigraphy = Study of Layered Rocks

Stratigraphic Units • Groups • Formations • Members

11 Formation

• Smallest Mappable Stratigraphic Unit • (vs. Outcrop)

Morgantown Stratigraphy

• Monongahela Group • Conemaugh Group • Allegheny Group • Pottsville Group

Bits of Local Stratigraphy • Monongahela Group • Waynesburg Formation • Uniontown Formation • Pittsburgh Formation • Pittsburgh Coal - $$$ • Conemaugh Group • Clarksburg Shale • Morgantown Sandstone

12 Local Stratigraphy

• Pottsville Group • Connequenessing Formation • Upper Connequenessing Sandstone Crops Out at Coopers Rock

Absolute Dating

• E.G. Radiometric Dating

• * Table 8.3, p. 195 * • Plummer et al. 10th ed

• Expect Questions

Meadowcroft Rock Shelter, Avella, PA: Occupied How Do we Know That? 17,000 years ago

13 How Do we Know That?

Carbon-14 Dating = Radiocarbon Dating

Carbon Isotopes

Common, Stable Uncommon, Stable Uncommon, Radioactive

Cosmic Rays “Zap” a Nitrogen-14 Proton into a Neutron to Form C-14

Beta Capture

14 C-14 Dating

• C-14 mixed thru Atmosphere • Plants take in C-14 • Animals Eat Plants • Other Animals Eat Animals ... etc.

• All living things include C-14

C-14 Dating

• Dead Things Do Not Take in New C-14 (or C-12 or C-13) • C-14 Decays w/ Time • C-12, C-13 Does Not.

Carbon - 14 is Radioactive • Decays back into N-14 at Known Rate • Half-Life is 5730 yr

N-14

C-14

15 C-14 Dating

• In 5730 years (1 Half-Life), Half of C-14 will Decay • In next 5730 years, Half of what was left will Decay –3/4ths will decay in 11,460 yr • 7/8ths will decay in 17,190 yr • etc....

C-14 Dating

• <0.1 % of C-14 is Left After 57,000 yr • Most Labs “poop out” at >40,000 to >50,000 yr

C-14 Dates

• Years Before Present (yr B.P.) • Present = A.D. 1950 • +/- Measurement Error • example: 22,500 +/- 450 yr B.P.

16 What Can C-14 Dating Be Used For? • Dating Dead Things • <100,000 years

Bristlecone Pine Tree-Ring Calibration of C14 Dating

http://www.sonic.net/bristlecone/Images2.html

Dendrochronology Another Absolute Dating Method

17 Carbon - 14 Basic Assumptions #1: Predictable Decay Rate #2: Constant C-14 Production Rate #3: All Living Things have Same Portions of C-14, C-12, & C-13

Obvious Problems w/ C-14 Dating

• No Good for Most Minerals • Useless on Really Old Stuff –Find Another Method

Problems w/ C-14 Dating

• Basic Assumption # 3 : – All Living Things DO NOT have Same % of C-14, C-12, & C-13 • Bottom Waters • Antarctic Ocean Diluted by Glacial Meltwaters

18 Problems w/ C-14 Dating Basic Assumption # 2 : –C-14 not Constant in Atmosphere –Sun’s Cosmic Ray Output not Constant –More C-14 in Past –10,000 yr BP = 11,500 Calendar yr

Man’s Impact

• Industrial Revolution - Fossil Fuels diluted C-14 • Atmospheric Nuclear Testing increased C-14

Problems w/ C-14 Dating • Biggest Problem = –Stupid Geologist - • 2nd Biggest Problem = –Contamination of Samples • Young Carbon • Old Carbon

19 Geologic Cross Section - Hawaii

Highlights of Earth History and Absolute Geologic Time

• ~13.7 BY (BY = billion years): Universe forms

• 4.55 - 4.60 BY: Earth forms • (Rest of Solar System, too) – Molten Magma Surface, – Methane, Ammonia, Hydrogen Atmosphere – High UV Radiation – Amino Acids etc.

Geologic History Zircon Photo: J. Valley • 4.4 BY – Oldest Mineral (Zircon xl) • 3.8 - 4.1 BY – Oldest Known Rocks, Chemical Fossils • 3.5 BY Stromatolites, – Algae - Stromatolites South Africa • 1.8 - 2.0 BY – Oxygen "Crisis" • 1.5 - 1.7 BY – Sex

20 Geologic Time • 600-650 MY (MY = million years) – First Complex Animals

Hallucigenia sparsa Anomalocaris canadensis

Big Changes

• ~551 MY – First Hard-Shelled "Critters" - Trilobites fossils common • ~500 MY – all phylum of higher animals exist, including chordates (vertebrates)

551-245 MY

• Paleozoic (=“Old Animals”) Era

21 245 - 65 MY Mesozoic Era Age of Reptiles

Allosaurus fragilis, a large Jurassic theropod, one step away from capturing a Dryosaurus altus. www.dinosaursinart.com/allosaurus/ IMAG0009.JPG

65 - 0 MY

• Age of Mammals (Cenozoic Era)

Horses through Time

22 The Geologic Time Scale

• See p. 195 –in Plummer et al, 9th edition • Dates to Know : • 65 MY, 245 MY, 551 MY, 4.5-4.6 BY

Question of Time

If you plotted all of geologic time on a time line as long as the circumference of the Earth (25,000 mi or 40,000 km), what distance would represent 1 year?

Earth Image Source: www.ameritech.net/users/ paulcarlisle/earth.jpg

Question of Time • If you plotted all of geologic time on a time line as long as the circumference of the Earth (25,000 mi or 40,000 km), what distance would represent 1 year?

• What distance would represent 75 years?

• What distance would represent 1 semester (4 months = 0.33 year)?

• What distance would represent this lecture?

23 The Whole Earth Time Line • Circumference of the Earth is 25,000 mi (40,000 km) • What distance would represent 1 year? –Figure this out in small groups. –Easiest if we use metric units • Divide Distance by # of Years (4,600,000,000 yr)

The Whole Earth Time Line 40,000 km / 4,600,000,000 yr = distance represented by 1 year?

40,000,000 m / 4,600,000,000 yr =

4,000,000,000 cm / 4,600,000,000 yr =

0.87 cm / yr (0.34 inch / year)

The Whole Earth Time Line 0.87 cm / yr (0.34 inch / year)

X 75 yr = 65 cm (Lifetime = 25.7 inches)

X 0.33 yr = 0.29 cm (Semester = ~1/8 inch)

24 Whole Earth Time Line

40,000,000,000,000 microns / (4,600,000,000 yr X 365 day/yr X 24 hr/day) = 0.99 micron / hour

This Lecture would be just over 1 micron long on the Whole Earth Time Line. The paper you are writing on is about 100 microns thick