Docsity
Docsity
Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity

Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan

Guidelines and tips
Guidelines and tips
Sell on Docsity
Sell on Docsity
Docsity AI
Docsity AI
Log inSign up

Prepare for your exams

Study with the several resources on Docsity

Find documents

Prepare for your exams with the study notes shared by other students like you on Docsity

Search for your university

Find the specific documents for your university's exams

Docsity AINEW

Summarize your documents, ask them questions, convert them into quizzes and concept maps

Explore questions

Clear up your doubts by reading the answers to questions asked by your fellow students

Earn points to download

Earn points by helping other students or get them with a premium plan

Share documents
download point icon20 Points

For each uploaded document

Answer questions
download point icon5 Points

For each given answer (max 1 per day)

All the ways to get free points
Get points immediately

Choose a premium plan with all the points you need

Study Opportunities

Choose your next study program

Get in touch with the best universities in the world. Search through thousands of universities and official partners

Community

Ask the community

Ask the community for help and clear up your study doubts

Free resources

Our save-the-student-ebooks!

Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors

Geologic Time: Early Estimates and Radiogenic Decay for Absolute Age Dating, Study notes of Geology

University of Illinois - Urbana-ChampaignGeology

Early estimates of earth's absolute age, including hutton's principle of uniformitarianism, theological clocks, and cooling rates. It also covers radiogenic decay as the basis for absolute age dating, with a focus on common dating systems and the limitations of dating all rocks.

Typology: Study notes

Pre 2010

Uploaded on 03/10/2009

koofers-user-gdu
koofers-user-gdu 🇺🇸

9 documents

1 / 7

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
Geol 104 Lecture 6
Geologic Time
1/15/05 Dr. Stewart 1
I. Early estimates of Earth’s absolute age
A. Old Earth
1. Hutton recognized that the Earth must be very old
a. Principle of Uniformitarianism – the processes that acted to shape the Earth in the
past are the same as those that operate today (“the present is the key to the past’)
b. If so, then the Earth must be very old – great amount of time needed for slow
processes to produce what we observe today.
2. This opened the door for Darwin’s Origin of Species and the Theory of Evolution
B. Theological clocks
1. Based on ‘divine’ texts – e.g. Genesis
2. Christian clocks
a. ~ 10,000 year old Earth
b. Ussher calculated earth began October 23, 4004 B.C.
- used average length of generation & multiplied by # of generations in Genesis
C. ‘Uniformitarianism’ Clocks
1. Sediment Accumulation
a. age of Earth = thickness of all Earth’s sediment / rate of sediment accumulation
b. Flaw: assumed constant sedimentation rate!
c. Flaw: gaps in sedimentary record caused by erosion (unconformities)
2. Salt Accumulation: John Joly, 100 m.y. old earth
a. assumed oceans initially filled with fresh water
b. assumed river input is source of salt in oceans
c. Calculated time required to reach current salinity
d. Flaw – river input is not constant supply
e. Flaw: removal of salt by pcpt. of evaporates
f. Flaw: Assume closed system by neglecting mantle input, hydrothermal exchange
& subduction
pf3
pf4
pf5

Partial preview of the text

Download Geologic Time: Early Estimates and Radiogenic Decay for Absolute Age Dating and more Study notes Geology in PDF only on Docsity!

Geologic Time I. Early estimates of Earth’s absolute age A. Old Earth

  1. Hutton recognized that the Earth must be very old a. Principle of Uniformitarianism – the processes that acted to shape the Earth in the past are the same as those that operate today (“the present is the key to the past’) b. If so, then the Earth must be very old – great amount of time needed for slow processes to produce what we observe today.
  2. This opened the door for Darwin’s Origin of Species and the Theory of Evolution B. Theological clocks
  3. Based on ‘divine’ texts – e.g. Genesis
  4. Christian clocks a. ~ 10,000 year old Earth b. Ussher calculated earth began October 23, 4004 B.C.
  • used average length of generation & multiplied by # of generations in Genesis C. ‘Uniformitarianism’ Clocks
  1. Sediment Accumulation a. age of Earth = thickness of all Earth’s sediment / rate of sediment accumulation b. Flaw: assumed constant sedimentation rate! c. Flaw: gaps in sedimentary record caused by erosion (unconformities)
  2. Salt Accumulation: John Joly, 100 m.y. old earth a. assumed oceans initially filled with fresh water b. assumed river input is source of salt in oceans c. Calculated time required to reach current salinity d. Flaw – river input is not constant supply e. Flaw: removal of salt by pcpt. of evaporates f. Flaw: Assume closed system by neglecting mantle input, hydrothermal exchange & subduction

Geologic Time

  1. Cooling Rates: de Buffon (1700s, calculated 75,000 yr old Earth) a. Assume early earth molten b. How long to cool to today’s temperature (assumes rate of heat loss & heat capacity)
  2. Cooling Rates: Kelvin, 1846: 20-30 x10^6 yrs a. assumed heat loss by conduction – missed convective heat loss of mantle! b. used geothermal gradient! But missed heat production by decay! c. problem with uniformitarianists who claimed earth must be much older. II. Radiogenic Decay – basis for absolute age dating A. Radioactive Decay: How does it work?
  3. Some isotopes have energetically unstable nuclei.
  • Isotope means same place (on periodic chart), i.e. same element
  1. The nuclear particles ‘decay’ or transform to another particle in order to reach a more stable nucleus (eventually). a. In so doing, a particle is emitted (=radiation) and Heat is produced. i. Major Source of Heat Production in Crust! ii. Parent isotope -> daughter isotope + particle + heat b. The rate of decay is constant (not dependant on environment)
  • Thus: number of daughters is dependant on number of parents and time!
  1. Measuring Time: a. See that the ratio of Parent to Daughter is a function of time!
  • this is the basis of radiometric dating (no details here) b. The equation used is a line (called Isochron) and its slope is a function of time! i. isochron is determined by measuring P/D in different minerals ii. given measured P/D, can calculate age from the slope of the isochron
  1. Common Dating Systems a. 87 Rb- 87 Sr: 47 b.y. half-life b. 238 U – 206 Pb: 4.5 b.y. half-life c. 235 U – 207 Pb: 713 m.y. half-life d. 232 Th – 208 Pb: 14.1 b.y. half-life e. 40 K – 40 Ar: 1.3 b.y. half-life f. 14 C – 14 N: 5730-year half-life

Geologic Time II. Relative Age -Most rocks cannot be dated. But, we can determine their age relative to another rock A. ‘Definition’:

  1. Relative age places an event/rock unit in a sequence of events.
  • Remember: Rocks are produced during geologic events, so when you determine relative ages, you are really determining sequences of events.
  1. Examples: I am younger than my brother – sequence of events (births) and ages B. Principles of relative age dating
  2. Principle of Original Horizontally: Steno (1600’s) a. Sediments are deposited in horizontal sheets b. Lavas too c. Thus, inclined or folded beds are disturbed.
  3. Principle of Superposition: Steno (1600’s) a. Older layers on bottom b. Layer cake model c. Assumes no disruption of layers (faulting etc.)
  4. Principle of Original Continuity a. Sediments or lavas (‘beds’) accumulate in continuous sheets b. Thus, can infer they continue across eroded regions
  5. Principle of Cross-Cutting Relationships a. Faults cut pre-existing (older) rocks b. Igneous rocks intrude (or extrude onto) older rocks.
  6. Principle of inclusions: a. If a rock contains fragments of another rock, the fragments are older. b. Examples: rip-up clasts, xenoliths etc.
  7. Principle of Faunal Succession (William Smith in late 1700’s) ( draw) a. Smith surveyed canals for coal transport – industrial revolution. b. Noticed ii. Fossils occur in groups (assemblages)

Geologic Time iii. Fossil organisms succeed one another in definite order c. Correlation of Fossil assemblages i. Group rocks units from separate regions based on fossil assemblage ii. Place Relative age on units recognizing that loss of fossil = extinction d. Extinction (Baron Georges Cuvier – early 1800’s, Father of Catastrophism) i. Worked in Paris Basin (Cenozoic) ii. Noticed 7 distinct terrestrial fossil organism

  • Each disappeared from strata prior to succeeding fossil (‘pair’)
  • Each ‘pair’ separated by marine fauna iii. Postulated: Cataclysmic event (flood) caused extinction
  • Did not accept evolution (after his time)
  • Rather, thought that ‘latter’ organism migrated to fill niche C. Unconformities = Gaps in rock record = Gaps in time
  1. Hutton’s unconformity (Siccar Point) a. Inclined sediments i. Must have been horizontal at one point ii. Then tilted and eroded b. Red sandstone and conglomerate then deposited c. Contact between Old Red SS and gray shales = angular unconformity
  2. Other two unconformities a. Nonconformity: sediment on igneous/metamorphic rocks (erosion surface between) b. Disconformity: Sediment on sediment (horizontal) separated by erosion surface c. Unconformities imply uplift and erosion prior to resumption of sedimentation D. Correlations of rock units
  3. Correlation a. Determine age relationships in rocks exposed in different places. b. Using principles of relative age dating (and unconformities)

Geologic Time i. Dinosaurs arrive and dominate through Cretaceous ii. 245 m.y. Permian-Triassic >95% of life on earth became extinct.

  • Cause: unknown
  • Supercontinent of Pangaea
  • Possible causes: restriction of environments (single continent), change in sea level, change in climate (favored) with change in CO iii. 221 m.y. first mammals (Triassic) f. 65 m.y. Cretaceous-Tertiary; i. Extinction of last dinosaurs, ammonites, others ii. Likely boloid impact. g. Tertiatry ~60 m.y. Marine mammals (whales, dolphin, seals etc.) h. Humans begin to develop in Holocene (10,000 years ago)