TIME, THE HISTORY OF GEOLOGY & THE AGE OF THE EARTH

Geology's great contribution to Science: "Deep Time" [Stephen Jay Gould]

• can mark time from beginning of Universe, or formation of Earth

• Stephen Jay Gould (1941 - 2002) was one of the world's most famous evolutionary biologists.  He was the author of over 20 books, some even best-sellers.  His writing made him the public face of modern ideas of evolution, and he was the center of a storm of academic debate.  In 1997 he attained the ultimate mark of contemporary celebrity - a cameo on The Simpsons. His book, Wonderful Life (The Burgess Shale and the Nature of History) was a finalist for the Pullitzer prize and introduced to a large audience his concept of punctuated evolution. He once said that geology's great contribution to Science was the concept of "Deep Time".  Immense amounts of time!  As we shall see, the history of geology is intimately involved with pushing back the age of the Earth!  And an ancient Earth is key to understanding most of the Earth! Now, of course, we know the universe is several times as old as the Earth, but only a century ago there were still many who believed the Earth was only thousands (not billions) of years old...

Age of the Earth

• 1654 - James (Bishop) Ussher (1581- 1656): 9 am, Oct. 26, 4004 BC
• Geologists: mapped rx, mostly sedimentary rx, particularly in Britain
  • "In 1793 William Smith, a canal digger, made a startling discovery that was to turn the fledgling science of the history of the earth -- and a central plank of established Christian religion -- on its head. He noticed that the rocks he was excavating were arranged in layers; more important, he could see quite clearly that the fossils found in one layer were very different from those found in another. And out of that realization came an epiphany: that by following the fossils, one could trace layers of rocks as they dipped and rose and fell -- clear across England and, indeed, clear across the world." - from Simon Winchester's description of The Map That Changed The World. The map is about 6 feet by 9 feet! It is displayed at the Geological Society in London, although now protected by a curtain.
    
    
• thousands of feet of sediments
  
• deposited slowly
• required old Earth, and led to...

Here's an example of a road cut through the Arbuckle Mountains in south central Oklahoma. The layers of sandstone and limestone are almost vertical; they would have been deposited horizontally, of course, like successive layers of dust on your furniture! During mountain building they were tilted about 90 degrees! It's hard to know the scale, but it looks like you can see roughly 100 feet of rock layers as you walk from one end of the outcrop (left side of photo, for example)  to the other (right side of photo).

Geologists in the 1700s wouldn't have had many road cuts, but they could have seen exposures cut by streams, or along sea cliffs, like this one I recently saw in Ireland. Here, the rock layers are tilted a little towards the upper left part of the photo.

Uniformitarianism

• James Hutton (1726-1797)
• Scot, medical man, gentleman farmer, geologist
• Published massive "Theory of the Earth with Proofs and Illustrations" in 1795; birth date of modern geology
• Uniformitarianism: physical process acting today to change the Earth's surface have also operated in the geologic past; i.e., there is a uniformity of processes, past and present
• Uniformitarianism: in Hutton's own words: "The results, therefore, of our present enquiry is, that we find no vestige of a beginning -- no prospect of an end."
• Uniformitarianism: The present is the key to the past
• Hutton also conceived of the Rock Cycle to explain the processes by which the major rock groups form:

Catastrophism

• Biblical scholars resisted great age:
• God created Earth as we see it today
• created with appearance of age!
• Early on, most geologists, forced by their religious beliefs, tried to fit all that they saw, in those very short 6000 years:
• sedimentary rocks: great flood -- Noachian Deluge
• volcanic rocks: enormous eruption period
• deep river canyons: earthquakes

On closer examination, though, uniformitarianism prevailed:

• sedimentary rocks are composed of ordinary beach sands, lagoonal muds, stream gravels, etc.
• some in floods, but not 1 great flood
• canyons carved by slow but inexorable erosion by streams
  
  
  
• geological age estimates:
  • sedimentation rates, maximum thickness of sedimentary column: 3 m.y. to 1.5 b.y.!
  • ocean salinity: ~100 m.y.

Charles Darwin (1809-1882)

• founder of evolution, geologist
• required a "long lapse of time"
  
  

Lord Kelvin

• physicist
• starting about 1862, did calculations:
  • Earth started out molten
  • between 400 and 20 Ma, would have cooled to present-day temperature gradient
  • by 1899, he reduced estimate to 20 million years!
  • Kelvin neglected two important things

By 1900, geologists leaned toward "limitless" age; 100's of Ma

Radiometric methods: Earth is approximately 4.6 billion years old !

"Since [Bishop Ussher's] announcement, over 300 years, or nearly 6% of all the time assigned by the archbishop to earth's history, have passed, and our notion of earth time has changed dramatically"

Geologic Time Scale

• Immenseness of time explains how slow, everyday processes can carve canyons, build mountains, open oceans, etc.
• Examples
  • Atlantic opens at 4 cm/yr; how long to reach present width of 6000 km? ____ m.y. A long time, but only __% of Earth's age! (in fact all oceans very young!)
  • evolution: slow, gradual (?) process; Man: 1.6 m.y., or last 0.04%! If age of Earth is 1 mile, man appeared in last 2 feet!

Another way of looking at how vast geologic time is: (from Encarta)

• One of the great accomplishments of early (pre-20th century) geologists, was establishing the Geologic Time Scale, long before we had any means to determine the absolute age of a single rock! A modern version of that time scale is shown below (Figure 7.1, T&L 7th Ed.)

Multicelled organisms

Measuring time; the rock record

• Law of original horizontality (strata, layering, bedding) [Nicolaus Steno, Italy, 1669]
• Law of superposition [Nicolaus Steno, Italy, 1669]
• Law of original continuity (outcrops must be cut off) [Nicolaus Steno, Italy, 1669]
• Law of crosscutting relationships
• Faunal succession
  • sequence of life forms, especially as recorded by the fossil remains in a stratigraphic sequence
  • index fossil: widespread geographically; limited time extent
  • fossil assemblage: use when (or in addition to) index fossils are unavailable (Fig. 8.10, T&L 7th Ed.)
  • microfossils

Together, these principles lead to stratigraphy:

stratigraphy: science of description, correlation, and classification of strata in sedimentary rocks, including interpretation of the depositional environment of those strata

Correlation

example of faunal succession, correlation, fossil assemblage

Location 1

rock	fossil assemblage
A	a
F	f
K	k

Location 2

rock	fossil assemblage
F	f
K	k
M	m

Leading to this stratigraphic column

rock	fossil assemblage
A	a
F	f
K	k
M	m

But what if, at Location 3

rock	fossil assemblage
A	a
K	k
P	p

• F is missing from rock record at this location
• What about P? older or younger than M? Need outcrop w/ both

unconformity

• rock eroded or never deposited
• disconformity: layers above and below are parallel
• angular unconformity: obvious; large time gap required for tilting
• nonconformity: older met or ig rx covered by younger sed rx; obvious; large time gap required for uplift, since met and because ig rx form at depth (Figure 8.6, T&L 7th Ed.)

Illustration of Stratigraphic Concepts (Figure 8.8, T&L 7th Ed.)

Relative vs. Absolute Time: Radiometric Dating

• Other than semi-quantitative methods like sedimentation rates, time scale through about 1913 was relative: knowing which is older, but not how old
• discovery of radioactivity at turn of century provided means to obtain absolute dates, e.g., in millions of years
• based on radioactive decay of radioactive parent isotope to stable daughter isotope
• Example: Radiocarbon (C-14) dating:
  • cosmic rays in upper atmosphere create unstable C-14 from stable N-14
  • ratio assumed fixed over last 75,000 years or so
  • convection mixes upper atmosphere w/ lower
  • plants "fix" (or animals) carbon; countdown begins, C-14/N-14 ratio decreases
  • how much C-14 left after 57,300 years? [Half is left after one half-life, or 5730 years, one-quarter after 2*5730 years, one-eighth after 3*5730 years...]

• Eons:
  • Phanerozoic: 570-0.0 Ma
  • Proterozoic: 2.5 Ga - 570 Ma; } taken
  • Archaean: 2.5 - 3.8 Ga; } together:
  • Hadean: 4.6 - 3.8 Ga } Precambrian
• Eras: Paleozoic, Mesozoic, Cenozoic eras of the Phanerozoic Eon
• Periods: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurrassic, Cretaceous, Triassic, Quaternary
• Epochs: Quaternary: Recent, Pleistocene;  Tertiary: Pliocene, Miocene, Oligocene, Eocene, Paleocene

Relative Time Exercise

Stratigraphy Exercise Print out, fill out, and turn in Thursday, Feb 5, 2009
Stratigraphy Exercise Key
Geologic Time Expectations What you are expected to know about the Geologic Time Scale
Geologic Time Puzzle
Geologic Time Scale Table
Another Time Scale Table
Geologic Time Scale Mnemonics
An Alternative Time Scale!