GEOLOGIC TIME - THE FOURTH DIMENSION

 

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