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

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

Age of the Earth

Arbuckle cross-section
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. 

Glandore, Ireland

 

Uniformitarianism

Catastrophism

On closer examination, though, uniformitarianism prevailed:

Charles Darwin (1809-1882)

Lord Kelvin

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

Geologic Process:  In One Year:
 deposition of deep-sea sediment  0.0002 cm
 deposition of continental-shelf sediments  0.004 cm
 erosional lowering of continents  0.004 cm
 uplift of mountains  0.3 cm
 plate movement  4 cm
 slip on San Andreas fault  4.5 cm
For Comparision:
 growth of fingernails  10 cm
 growth of a sapling  18 cm
From Bucke, David, Physical Geology

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

Life Form Average Life Span Comment
Housefly 1 month Longer than the average pop music career.
Mayfly 1 year But mayflies live less than one day as a full-grown adult.
Mouse 3 years It's shorter for fat mice: Tests prove it.
Elephant 50 years Some elephants remain standing up after death.
Human 76.1 years 122 years recorded maximum.
Giant Tortoise 100 to 150 years 200 years recorded maximum.
Creosote bush 100 years 11,700 years and counting, if you count clones.
Sequoia tree 2,500 years 3,500 possible maximum.

Multicelled organisms

Measuring time; the rock record

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

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

unconformity

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

Relative vs. Absolute Time: Radiometric Dating

 Method Parent Daughter Half-life Limits  Comments
 U/Pb  U-238  Pb-206  4.5 b.y.  > many b.y.  
 K/Ar  K-40  Ar-40  1.3 b.y.  > 100,000 y  K abundant in rx
 Radiocarbon C-14 N-14 5730 m.y.  < 75,000 y  organic materials
 Rb/Sr Rb-87 Sr-87  47 b.y.  > many b.a.  

Relative Time Exercise

relative time example

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!