The study of the earth involves immense quantities of time
In our lives, we can comprehend hours, days, weeks, years (maybe)
In geology, we are dealing in millions and billions of years
This immense scale is possibly one on the most difficult (although important) concepts to be grasped in geology
Geologists have a healthy respect for time
DIGRESS TO: analog vs. digital clocks
Our view of the earth is not a photograph, more like one frame in a movie
Free Question Coupon opportunity
Catch book discussing earth processes in past tense
The passage of large amounts of time is a critical part of geological thought
You will hear me say "Over the course of geologic time..."
A basic assumption of geology is that just about anything can happen if given enough time (monkeys & typewriters)
It can be tough to estimate time intervals throughout geologic history
The age of the earth is a good example
It has been open for speculation for quite some time
Archbishop Ussher (1581-1656)
The earth was formed on the evening of October 22, 4004 B.C.
This was the accepted date for over two hundred years
Additional work led to refinements of this date
More about this later
We currently believe that the earth is 4.6 billion years old
Based on Absolute Age Dating methods
Relies on the regular decay of unstable (radioactive) minerals
Allows us to assign absolute dates to many rocks
Many uncertainties (at least to me)
Absolute Age Dating methods are a fairly recent innovation
Geologists have long used a Relative Time Scale
Doesn't use actual ages, only the RELATIVE order of events
Geologists can use a relative time scale in many situations in geology
Many have to do with sedimentary rocks
Review: Law of Superposition; Law of Original Horizontality
Click here for additional information on the Relative Geologic Time Scale
Click here for a copy of the Relative Geologic Time Scale
Please note that this representation is NOT TO SCALE
This gives an extremely distorted view of geologic time
So, lets make a true scale model of earth's history
Using a scale of 1" = 50 million years, construct a Geologic Time Line
Materials Needed:
Relative Geologic Time Scale
Butcher paper - 3 meters long
Meter stick
Felt pens (several colors)
The best may go on the wall (w/Coupons?)
Important dates to show on your time line include
4.1 billion - oldest rocks found (so far)
3.5 - 2.5 billion - initial formation of large amounts of granitic continental crust
3.2 billion - oldest evidence of life - bacteria & blue-green algae
2.6 billion to 1.8 billion - banded iron formations
1.8 billion - free oxygen begins building in atmosphere
1.4 billion - eucaryotic cells and enough free O2 to allow aerobic metabolism
1.0 billion - first multicelled organisms
600 million - first fossil invertebrates (w/ hard shells)
All periods from the Relative Time Scale
500 million - first fish
350 million - primitive trees
260 million - first dinosaurs
175 million - first birds and mammals
65 million - extinction of dinosaurs
2 million - early humans
Earliest recorded history (2000-4000 years B.C.)
Your Birthday
2nd Time line - 600 million years ago to the present
3rd time line - "Recent" geologic events (since the extinction of the dinosaurs).
(Can you see your birthday yet? If not, what scale would you need to use in order to resolve it?)