We can move in two dimensions; Birds & fish get to use three
Imagine being able to use all four
The study of the earth involves immense quantities of time
In our lives, we can comprehend hours, days, weeks, years (maybe)
The rise and fall of Rome are 'ancient' history, and details of most of what came before that are locked up in legend and speculation
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
In any event, it is comforting to be a geologist
That's probably the best thing about being one
Our view of time is sufficiently long to allow for healing of any possible scar
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
Like the roomfull of monkeys & typewriters
Our measurements of time on earth reflect our limited scope of time.
We have seconds, minutes, hours, days, and years
You've been listening to me talk for 10 min. and it feels like an hour
No matter what, we don't commonly deal in millions or billions of years
It can be tough to estimate time intervals throughout geologic history
The age of the earth is a good example
Anyone have any idea how old it is?
This has been open for speculation for quite some time
Archbishop Ussher (1581-1656)
Added up the 'begats' in the old testament
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
Early attempts used measurements of stratigraphic sequences
Lots of problems
How fast are sediments deposited
How fast are they eroded
Is sedimentation continuous
No continuous exposure through all of the earth's history
With all of this in mind (maybe)
William Sollas (1883)
Estimated the earth's age at 26 million years
Far cry from the 6000 years postulated by the church
It's certain that if he had been born 200 years earlier he would have been toast!
Amount of salt in seawater - Joly (1899) - Irishman
Came up with an age of 100 million years
Lots of problems here, too
Is the rate of salt addition constant
This is directly related to rates of mountain building and tectonics
Which certainly seems to be anything but regular
We currently believe that the earth is 4.6 billion years old
Based on Absolute Age Dating methods
This is some real GeoMagic
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
Not available in the past
Geologists often use a Relative Time Scale
Doesn't concern itself with actual ages, only in the RELATIVE order of events
Refer to separate discussion for additional details
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