post 11

What is the correct sequence of events?

Fault A, Fault B, then Dike A, Dike B

Dike B, then Dike A, Fault A, Fault B

Fault B, then Dike B, Fault A, Dike A

Fault A, Fault B, then Dike B, Dike A

Stratigraphy and Geologic Time

Why is it important to document Earth history? How do we know that one rock is older than another

(relative age)?  Principles..  Fossil record..

 How do we know the age of the Earth, and how has our understanding changed over time?

 How can we use radioactivity to determine the (absolute) age of a rock?

 How was the Geologic Timescale put together?

[Text: 8.1-8.6]

How old is the Earth?

Age #1: 6000 yrs + 6 days (early biblical view, catastrophism: Ussher, 1600s)

Age #2: very old (uniformitarianism, Hutton, late 1700s)

[last class]

Understand modern processes and their products

 apply this to rocks/features that likely formed the same way in the past

e.g. how would you interpret:

- Ancient pillow basalts? - Fine versus coarse-grained sedimentary layers? - Offsets in layers?

Using uniformitarianism to read rocks

How old is the Earth?

Lord Kelvin (1890s): (famous for Kelvin temp scale)

Used Earth’s temperature & thermal gradient, assuming cooling from fully molten state

 Earth age #3: ~20 million years BUT: radioactive sources not yet discovered

(Kelvin also predicted radios would not catch on, flying airplanes not possible)

Image & further info: http://apps.usd.edu/esci/creation/age/ content/failed_scientific_clocks/kelvin _cooling.html

How old is the Earth?

Edmund Halley (1700s):

Rivers bring dissolved salts to oceans  Oceans should get more salty

How much time for initial freshwater ocean to achieve current salinity?

 Calculated by John Joly (1899)

Image: https://edukalife.blogspot.ca/201 5/07/biography-edmund-halley- english.html

Image: http://www.research.ie/feature d-title/john-joly-defending-tcd- against-1916-rebels

Halley/Joly Age of Earth Based on Salinity

 Earth age #4: ~80-150 million years BUT did not account for salt recycling  Earth must be older

Salttoday=Saltoriginal + [(Saltadded/year)*(x years)] Solve for x = age of Earth in years

Original:

Add:

Radioactive Decay

Radioactivity discovered by Becquerel (1896):

- Many atoms decay spontaneously - Decay produces energy (heat) & stable

daughter products

Solved 2 problems: (1) An ongoing source of heat for Earth (2) Absolute dating of igneous rocks

 Patterson (1953): first accurate age of the Earth (Canyon Diablo meteorite)

 Earth age #5: 4.55 billion yrs

http://www.nobelprize.org/nobel_prizes/ph ysics/laureates/1903/becquerel-bio.html

Ernest Rutherford developed radioactive dating http://mp.natlib.govt.nz/detail/?id=7208&l=en

Isotopes Isotopes of an element: same number of protons (Z: atomic #)

BUT different number of neutrons (N)

e.g. carbon isotopes: 12C, 13C, 14C

At.#, isotope (mass)

# protons

# neutrons

6, 12C stable

6 6

6, 13C stable

6 7

6, 14C radioactive

6 8 ZX , , A

6C 12

6C 13

6C 14

A (atomic mass) = Z + N = protons + neutrons

Radioactivity

 Decay of unstable parent isotope to a stable daughter – many steps

 Each isotope decays at its own fixed rate – measured in half lives (half life = time for ½ of parent atoms to decay/remain)

What is a radioactive half life?  Half the original atoms decay during one half life  Decay rates are exponential  Heat produced by decay decreases exponentially

Earle, S. (2016): Online text Fig. 8.14

Linear vs exponential?

e.g., decay of Uranium-238 (92 protons) to Thorium-234 (90 protons)

Radioactive Decay

Image: Hamblin & Christiansen (2003): Earth’s Dynamic Systems, 10th edn., Prentice Hall

238U

234Th

Image: http://www.kgs.ku.edu/Extension/geotopics/earth_age.html

measure ratio of parent : daughter isotopes  # half lives # half lives x half-life length  Age

Source: Hamblin & Christiansen (2003): Earth’s Dynamic Systems, 10th edn., Prentice Hall

Which radiogenic isotope system to use for age of:

Meteorites? ~Billion-yr old rocks? Glacial landforms? Groundwater (H2O)?

Image: Hamblin & Christiansen (2003): Earth’s Dynamic Systems, 10th edn., Prentice Hall

40K

40Ar

E.g., 40K  40Ar: measure ratio of 40K (parent amount remaining)

to the total 40K + 40Ar (original parent amount)

Earle, S. (2016): Online text Ex. 8.3; Fig. 8.15

0.5 after 1.25 billion years (one half life)

Example 1: U-Pb in zircons Zircon: a resistant, Uranium-rich mineral Ratios of Uranium, Thorium and Lead isotopes  age of mineral

Image: http://imetcal2.une.edu.au/web-content/Media.html

Animation on U- Pb dating of

zircon minerals

Earth’s oldest minerals Jack Hills, Australia 4.4 billion years old

Wilde et al. (2001): http://www.geology.wisc.edu/~valley/zi rcons/Wilde2001Nature.pdf

Photo by Michael John Cheadle: https://www.nsf.gov/news/news_images.jsp?cnt n_id=104546&org=NSF

Image: Wikimedia Commons

[see next topic: Early Earth]

Image: NASA (public domain)

Example 2: U-Pb system  Age of the Earth

Daughter isotopes of lead (Pb) in meteorites

Image by Geoffrey Notkin: Creative Commons

Canyon Diablo iron meteorite

Image: Dalrymple, G.B. (1986): Radiometric dating, Geologic Time, and the Age of the Earth: A reply to “scientific” creationism, U.S. G.S. Open-File Report 86-110, 76 p. https://pubs.usgs.gov/of/1986/0110/report.pdf

Image: http://www.meteorlab.com/METEO RLAB2001dev/murchy.htm

Example 3: Evolution of Homo

Image: http://www.dailytelegraph.com.au/church-apologises-to-charles-darwin-over-theory-of-evolution/story-e6frewsr-1111117484124

Image: http://www.funbodytherapy.com/sample-page/gallery/evolution-of-man-to-computer/

Image: http://donsnotes.com/science/bi ology/evolution.html

Olduvai Gorge, Tanzania

Important fossil site of early Homo species & stone tools

Image: http://cw.routledge.com/text books/9780415448789/colou

rimages.asp

Image: Encyclopaedia Britannica 2009

http://safariporini.com/olduvai-gorge.htm

“Lucy’s” Footprints (in volcanic ash)

K/Ar dating of volcanic ash 1 m below fossil  3.2 million yrs

Image: http://www.ancientdigger.com/2011/07/laetoli-footprints-explained.html

http://www.dailymail.co.uk/sciencetech/article- 3099430/Mysterious-fossils-reveal-new-species- early-HUMAN-jawed-hominid-lived-alongside-Lucy- 3-4-million-years-ago.html

our earliest known direct ancestor

LUCY

Putting dates on humankind’s evolution

Image: http://www.talkorigins.org/faqs/homs/species.html

Brain size of Homo

EOS120 class

Image: https://ncse.com/book/export/html/1764 (Graphic by Nick Matzke)

Example 4: 14C dating – young events (a few hundred to ~40,000 yrs)

 14C produced in upper atmosphere by cosmic ray bombardment of 14N

 14C incorporated in CO2  absorbed by living matter

 14C continually replaced in living organisms, but after death the 14C decays to 14N

 Ratio of 14C to 12C  age of the matter once it stopped replacing 14C

Image: Hamblin & Christiansen (2003): Earth’s Dynamic Systems, 10th edn., Prentice Hall

Caveat - ‘Bomb’ Carbon-14 nuclear weapons testing  simulated atmospheric production of C-14 in unnatural quantities

Image: http://www.thefullwiki.org/Partial_Test_Ban_Treaty

Read more at: https://www.radiocarbo n.com/carbon-dating- bomb-carbon.htm

Image: Kalish, J.M. (1993), Earth and Planetary Science Letters v. 114 (4): 549-554.

Earle, S. (2019): Online text Fig. 8.4.5

A Geologic Time Scale

how to correlate across vast sections of space and time?

Where to start?

Aristotle, da Vinci: fossils as ‘ancient’ life

Steno: 17th century – “Superposition”

Smith, Lyell: 1800s, ID of strata using fossils - correlation

1896 - Radioactivity discovered  absolute ages

Arthur Holmes: 1913 – first time scale

1977: Official Modern Scale

Earth’s timescale: 4.55 billion yrs – a big number

Counted at 3 numbers per second:

 1 million in ~4 days

 over 10.5 years to count to one billion

 4.6 billion would require 50 years of non-stop counting

The Geologic Time Scale - Based on rock sequences in Europe correlated worldwide; includes use of fossils, radiometric dates

- Period divisions mark changes or loss in fauna

www.washingtonpost.com

Faunal successions (coming and going of organisms) divide up periods of time Many divided by extinction (species loss), others by species emergence

Image: Edwards, L.E. & Pojeta, J. Jr. (1994): Fossils, rocks and time, U.S. Gov. Printing Office 1998-675-105, 24p.

Eons  Eras  Periods

Hadean 4000

541

2.6

Download latest geological time scale here (March 2020)

485 444 419

252 201 145

4550

Carboniferous 299

359

Stratigraphy and Geologic Time

 Deposition and the rock record  Biology (fossils) in the rock record  Relative vs. absolute time  Relative ages from principles (e.g., X-cutting relations)  Dating (ages) using radioactivity in minerals and

organic matter  Time - is of immense magnitude  Precise chronology of the ‘tempo’ of earth events and

evolution  Applied in geology and archeology