Stratigraphy! The study of the rock layers of a planet! (I was going to say: “rock layers of...

Stratigraphy!

The study of the rock layers of a planet!

(I was going to say: “rock layers of Earth”, but…)

GEOLOGIC DATING and STRATIGRAPHY

There are two basic ways to “date” objects and events:

Relative Dating: What came first? What came second?

Absolute Dating: Actual age in years, months, etc…


Egyptian Galley

Elizabethan Galleon

Pre-WWI dreadnought

Modern Missile Frigate

1970’s

1910

1500s

1000 BC

Absolute Dating: Actual age in years, months, etc…

(The study of Rock Layers:)

SUPERPOSITION: The oldest rock layers are on the bottom…the youngest rock layers are on the top. (If they are undisturbed!)

Oldest

the Principles of Stratigraphy…

Oldest

Youngest

Youngest

HORIZONTALITY:

Rock layers are originally laid down flat. IF they ARE tilted, then they HAVE been disturbed! (See SUPERPOSITION principle)

Oldest?Youngest?

Not sure?





CROSS-CUTTING RELATIONSHIPS:

any rock layer or fault that CUTS ACROSS other rock layers is YOUNGER than those layers. (What came first? The paper or the tear? You can’t tear a piece of paper before you have the paper!)

The Faults happened LAST (except for…?)

UNCONFORMITIES: (“missing rock”)

An unconformity represents a time that is missing because rocks were removed by erosion… (or were never deposited)

Geologists read the layers of rocks like pages in a book. Sometimes…there are pages torn out!

Unconformity!

“Missing rock”

In most of Illinois, a major unconformity represents the almost complete ABSENCE of all Mesozoic history (Dinosaur times).

CORRELATING ROCK LAYERS:

How do we know if the rocks layers we see here…

…are the same layers we see there?


We can often use a “Key Bed” (a distinctive rock unit) to “link together” (correlate) one rock layer to another.

“Here” “There”

Perhaps a distinctive ash layer from a convenient volcano?

FAUNAL SUCCESSION:

Life Changes: Old species die; new species evolve…

( Dead species don’t come back! )

The Fossil Record therefore acts as a kind of time marker…

Each life-form exists only during a certain time, and its fossils “label” the rocks that they are found in as been formed at that time!

INDEX FOSSILS:

These are fossils useful for “finding” yourself in time.

A good Index Fossil is:

Common (or you won’t find it often!)

Wide Spread (or you will only find it in one place!)

Short-Lived (as a species…if a species lived for hundreds of millions of years, how would its presence mean anything?)

Short-lived Species versus Long-lived Species:

This species is found in rock layers of many ages.

This species is found only in rock layers that formed during a very limited time.

“Here” “There”

We also find a few examples of this fossil in one layer.

NOW we know how to correlate the rock layers.

We don’t find many fossils. We do find some of these

But that doesn’t help discover the age of the rocks!


Remember our rocks that we found “Here” and “There”?

We find the same fossils in THIS rock layer…

…that we find in THIS rock layer. We can similarly correlate

other rock layers.

The rock layers correlate like this.

The strata in this area are strongly folded.

…and others.

Using these principles, geologists explored the rock layers in many places, and built an understanding of the history of the earth.

Some rock layers were found in some places…

Some rock layers were found in other places…

Geologists worked out “what came first” and “what came next”…

…and worked out the main sequence of events that happened during the history of the Earth.

Cenozoic Era

Mesozoic Era

Paleozoic Era

Lower rock layers were also named…we will just refer to them as the “Pre-Cambrian”

The more recent layers are subdivided into “PERIODS”

Cretaceous Jurassic Triassic

Subdivided, but usually just called the “Cenozoic”.

Right “at the top”… the “Quaternary” (the “Ice Age”!)

The only “Period” BEFORE the Paleozoic Era.

Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian

“Ediacaran”“Pre-Cambrian”

Fossils were abundant in the upper rock layers…

Geologists divided these rocks into 3 major groups

But! NONE of that tells us HOW OLD anything actually is! It’s all “Relative Dating”.

How is “Absolute Dating” accomplished? How do we add “dates” to our history?

Tuesday, Oct 23

68,000,540 Years BC

Things to do today:

Do NOT become extinct!

Attack & eat something!

ArchBishop Usher used the Bible to determine that the Earth was only 6,000 years old!

Scientists soon realized that the Earth was MUCH older.

Early estimates of the Age of the Earth:

Absolute Dating: How old is the Earth?

Evidence indicated that the Earth had been molten at one time…

Better calculations by Kelvin made that 40 million years old

Early calculations revealed that the Earth had to be at LEAST 75,000 years old to have cooled down.

…and then cooled down.

Other calculations involving the rate of deposition of sand, gravel, clay, etc., and the amount of salt in the ocean, gave an age of about 100 million years.

By the late 1800’s, scientists realized that the Earth was very old…

But HOW old?

The discovery of radiation by Madame Curie changed everything…

…some elements are radioactive and break down!

…Or the capture or loss of an electron by the nucleus.

Radioactive Decay occurs with:

The breakup of a nucleus…

Whatever happens…the process forms a new element! What you end with is NOT what you start with!

…via the loss of an alpha particle

(2 protons and 2 neutrons)

electron capture “subtracts” a proton.

electron loss “adds” a proton.

The “Parent” decays to form the “Daughter” product.

PARENT DAUGHTER PRODUCT

HALF LIFE: what is it?

Suppose element “X” decays to form element “O”

Over a certain period of time HALF of element “X” breaks down and is gone (forming element “O”).

The amount of time it take for HALF of element “X” to break down is called the “HALF LIFE”.

PARENT--- “X” DAUGHTER--- “O”

Suppose we start out with this much of element “X”…

At the start…

After 1 half-life

After 2 half-lifes

After 3 half-lifes

After 3 half-lifes, only 1/8 of the original element “X” is left

If these elements are trapped in a rock…

Then the OLDER the rock…the LESS parent material and the MORE daughter material!

Radiometric Dating provides an ABSOLUTE DATE…

The parent element decays to form the daughter material…

Each element has a different Half-Life (different rate of decay)

Decay occurs at precise, measurable rates…

So… How does this all work?

For instance: if the half-life is a million years, and only 1\8 of the parent remains (3 “half-lives”), then the age of the rock is 3 million years!

When “rock” is molten, daughter material escapes and is “lost”…

When molten rock cools, the daughter elements are then trapped within the rock and accumulate.

The measurement of the parent/daughter ratio provides the age of the rock in “Half-Lives”. X

X

X

X X

X

X

X

X

X

X

X

X

X

X

X

X

X

XX

O

O

OO

O

OO O

O

O

O

O

Uranium\Lead

Rubidium\Strontium

Potassium \Argon

Uranium\Thorium

Uses TWO different decay chains and compares the ratio of the Daughter Product.

Uranium 234 Thorium (80,000 HL)Uranium 235 Protactinium (34,000 HL)

Measures the “tracks” left by the decay of U-238 atoms.

Fission Track Dating

Dating of crystals of Zircon, Apatite, Garnet and others use this method.

With a half-life of 760 million years, it’s useful for dates of BILLIONS of years!

…it would be useful for dating objects less than an hour old!

Elements with short half-lives disappear in a relatively short time.

Elements with long half-lives persist for VERY long times.

We use elements with LONG half-lifes to measure OLD dates…

We use elements with SHORT half-lifes to measure recent dates.

What time span would Polonium-218 be useful for?

How about Uranium-238?

With a half-life of 3 minutes…

Older

Younger

How do we put it all together?

Strata accumulates…

Older

Younger

How old is the T rex?

Ash Unit 2 radiometric age

70 million years

Ash Unit 1 radiometric age

68 million years

T rex must be between 68 and 70 million years old!

When we “dig” into Illinois… We find different assemblages of fossil critters in different layers!

When we “dig” into Illinois… We find different assemblages of fossil critters in different layers!

And located all through these rocks are units we can date…

505-438 million years ago






Most of the Mesozoic is eroded (gone!) almost everywhere in Illlinois.

CENOZOIC

MESOZOIC

PALEOZOIC

(ICE AGE )

The Carbon-14 dating process is somewhat different…

Carbon-14…

The amount of C-14 remains relatively constant while the organism lives. After the death of the organism, the amount of C-14 becomes less and less.

Is made continuously in the atmosphere.

Is absorbed by living organisms

Is replaced (even though it decays) as long as the organism lives.

Starts to disappear (decay) after the death of the organism.

C-14 C-14

C-14C-14

C-14

C-14

C-14 C-14

C-14

C-14C-14

But…it is VERY useful for the last part of the Ice Age, and more recent history (trees, campfire charcoal, wooden huts).

REMEMBER…it ONLY works on ORGANIC material

With a half-life of about 5,000 years, measurable amounts of C-14 are gone after 50,000 years.

Therefore…we CAN NOT date Dinosaur bones with C-14!

..NOT rock!!!

(They’re TOO old!)

The most recent dates can be confirmed by DENDROCHRONOLOGY…

This involves the careful counting and studying of tree rings.

Tree rings can be counted to determine age…

…and climate patterns can be determined by noting thick and thin annual rings.

Fragment of ancient trunk.

Old, dead tree.

Living tree today!

If the trees lifespans overlap, we can match up the climate patterns.

Perhaps we locate ancient dead wood…

By comparing overlapping segment of rings, scientists can also work out a chronology of climate changes for the last 8,000-9,000 years.

Now we know how long ago that the ancient fragment grew as a living tree… AND…we can compare it to dates obtained by Carbon-14 Dating!

Another method of “telling time” is to use VARVES.

These are all incorporated with the layers of mud and silt…

Seeds and fruit…

Flowers and flying insects…

PollenLakes collect sediment… And…

Which can be read… …like pages in a book!

Stratigraphy! The study of the rock layers of a planet! (I was going to say: “rock layers of...

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