“The Present is the Key to the Past”

James Hutton, 1700’s

History of Geology

• The concept and measurement of geologic time has changed through human history.

• Planet Earth,The Living Machine p1.mp4 7:15 – 8:00

• James Ussher (1581-1665) in Ireland – calculated the age of Earth based on recorded

history and genealogies in Genesis• he said that Earth was created on October 22, 4004 B.C.• it was considered heresy to say Earth was more than

about 6000 years old.

Catastrophism

• Catastrophism– proposed by Georges Cuvier (1769-1832)

– Huge catastrophies caused major changes to Earth’s geology and biology in a short period of time.

– six major catastrophes occurred, corresponding to the six days of biblical creation, the last one was the biblical flood.

Uniformitarianism• Developed by James Hutton, advocated by Charles Lyell

(1797-1875)

• “The Present is the Key to the Past”, James Hutton, 1700’s

• What happens today is what happened in the past. – Earth changes very slowly.

• Supports a very old age of Earth.

• Earth is believed to be about 4.6 billion years old

NeoCatastrophism

• Blends Uniformitarianism with some Catastrophism.

• Earth developed and changed at a very slow rate like today, but had some catastrophic event s that caused some sudden changes. – Eg: extinction of dinosaurs

• Asteroid collision, Gamma-ray Bursts . . .

Relative Dating• This is not about dating your sister!! • Tells us the sequence in which events occurred

in a rock.– Which formed 1st, 2nd, etc. . . – NOT how long ago they occurred.

Laws and Principles

1. Law of SuperpositionIn an undisturbed sedimentary rock, each bed is older than the one above it and younger than the one below it.

Youngest

Oldest

Laws and Principles2. Principle of Original Horizontality

•Layers of sediment are generally deposited in a horizontal position.

•Rocks that are folded or tilted must have been moved into that position AFTER their formation.

1. What happened first?

2. What happened second?

Horizontal deposition

Folding

Laws and Principles

3. Principle of Cross-Cutting Relationships• When a fault or igneous intrusion cuts through another rock, the fault or intrusion is YOUNGER than rock it cut through.

Igneous Intrusion

Laws and Principles

3. Principle of Inclusions• Inclusions are pieces of one rock found in another.• The rock containing the inclusions is YOUNGER than the

rock the inclusions came from.

Older

YoungerInclusions

Laws and Principles

4. Principle of Unconformities– Represents a long period during which deposition is stopped,

erosion removed rock and then deposition started again.– http://www.wwnorton.com/college/geo/animations/unconformity.htm

• Angular unconformity– The first layers were tilted before the deposition started again.

• Disconformity– When two layers of sedimentary rock are separated by an

erosional surface but are still parallel to eachother.

• Nonconformity– Erosional surface separates older igneous rocks from younger sedimentary rocks.– http://www.wwnorton.com/college/geo/animations/types_of_

unconformity.htm

Principle of Unconformity

Disconformity Angular Unconformity Nonconformity

• Which type of Unconformity is which???

Relative Dating• Place the following layers in order of occurrence from

OLDEST to YOUNGEST. Include the Fault and unconformity.

8.

7.

6.

5.

4.

3.

2.

1.

H

R

M

Unc

Fault

F

B

I Oldest

Youngest

Fossils: Evidence of Past Life

• What is a Fossil?– Preserved remains or traces of prehistoric life.– The type of fossil formed is determined by the

conditions the organism died and was buried.

Fossils: Evidence of Past Life

The two conditions necessary for the formation of fossils is:

1. Having HARD parts2. Rapid BURIAL

Fossils: Evidence of Past Life

• Forms of Fossilzation1. Mold and Cast– Mold: the hollow shape of the organism left

after water dissolved the hard part (shell)– Cast: the solid COPY of the shape of the

organism

CastMold

Fossils: Evidence of Past Life

• Other forms:– Petrified: Minerals replace all or part of the

organism. Shows interior detail• (Quartz) Ex: Petrified Wood

– Carbon Films: A thin coating of carbon on the rock. The carbon is left from the organism itself. (plants and insects)

– Impressions: a copy of the surface of organism– Trace Fossils: clues that an organism was there.

(Foot prints)

Fossils: Evidence of Past Life

• Fossil Succession– Fossils succeed one another in a definite order.

(Simpler organisms = older rocks)– Any time-period can be recognized by its fossil

content.

Jurassic Period

Laws and Principles

• Fossil Correlation– Matching similar rock types

found in different areas.

– Index fossils: Fossils that are• Widely distributed• Lived a short time

Rock Correlation with Fossils

• Activity Answer Key

Unconformity

Radiometric Dating

• Radioactive Elements– Unstable elements break

down or DECAY.

– Occurs when the Nucleus changes (Protons and Neutrons change).

– During Radioactive Decay, one element breaks down into another element giving off ENERGY.

Radiometric Dating

– Occurs in IGNEOUS rocks.

– The RATE OF DECAY is CONSTANT!

– Scientists use that rate to determine AGE of a rock.

Radiometric Dating

• Half-Life– The time it takes for half of the radioactive atoms to

decay into another element. • Parent material = Radioactive Element• Daughter product = the stable element the radioactive

element decays into. – Example: Carbon 14 turns into Nitrogen 14

• Carbon 14 = Parent material• Nitrogen 14 = Daughter product

• Carbon-14’s half-life is 5,730 years. (what does that mean?) It takes 5,730 years for half of C14 atoms

to decay into N14.

Radiometric Dating

• Potassium 40 – Argon 40 Dating (K Ar) (Beta capture)

– Parent = Potassium 40 Daughter = Argon 40– Half-life = 1.3 billion years.– Can be used to date rocks because of its long half-

life.– Why do you need a LONG half-life to date rocks?

Because rocks are old so if half-life is short, there will be NO Parent element in the rock, only the Daughter product. You need Both to determine age.

Carbon-14 Nitrogen-14 Cycle

• All living things have C-14 in them.

• This begins to turn to N-14 the moment they die. – Can only be used for dating if

there is some C14 remaining in the organism

Radiometric Dating

• Carbon 14 – Nitrogen 14 (C14 – N14): (Beta emission)

– Called Carbon Dating.– Only used for dating things ONCE LIVING.– Decay begins when the organism dies. – Only works for things YOUNGER than 75,000 yrs.

Iceman found in in the European Alps. He died 53,000 years ago.

Radiometric Dating

• How to determine age of rock using Half-lives

Half-life Parent element Daughter product

0 1 0

1 ½ 1/2

2 ¼ ¾

3 1/8 7/8

4 1/16 15/16

Each Half-life: Parent goes down by ½ Daughter increases by that amount

Radiometric Dating

• This can be done by Percentage too.

Half-Life Parent element Daughter product

0 100% 0

1 50% 50%

2 25% 75%

3 12.5% 87.5%

4 6.25% 93.75%

Mathmatically, does the Daughter product EVER reach 100%?

NO

Radiometric Dating

• Now lets add HALF-LIFE YEARS.Half-Life Parent element

(C14)Daughter product

(N14)0 yrs 100% 0%

5730 yrs 50% 50%

11,460 yrs 25% 75%

17190 yrs 12.5% 87.5%

22920 yrs 6.25% 93.75%

So if we measured a sample of a mummy and found it to be 12.5% C14 and 87.5% N14, how long ago did he die? 17190 years ago!

Radiometric Dating

• Importance of Radiometric Dating

1. Provided dates for events in Earth’s History.2. Supported beliefs of James Hutton, Charles

Darwin and others who said Earth is VERY OLD.3. There has been enough time for major changes

to have occurred geologically.

Geologic Time Scale

• Earth = 4.6 Billion years old (byo)– Earth’s time span is SOOOO GREAT, that scientists

have divided it into smaller units of time making the “Geologic Time Scale”.• This division of time helps people grasp this expanse of

time.

– Scientists used changes in major life-forms and extinctions to divide the time scale.

– The division of the time scale is broken into 4 sections.

Geologic Time Scale

• The 4 major divisions are???

1. Eon2. Era3. Period4. Epoch

Geologic Time Scale

• 88% of Earth’s History– PreCambrian

• Very little evidence of life.

• Other 12%– Phanerozoic

• Sudden development of life in the oceans.

• Hard parts developed which increased the fossil record.

Single-cell

Visible Life

Geologic Time Scale• Eras (Divided by major extinctions)

– Paleozoic (Hard parts developed)

• Marine animals thrived• Pangaea formedExtinction

– 90% of marine animals became extinct.

– Mesozoic• Dinosaurs developed• Pangaea began to break up.

Extinction– Dinosaurs

– Cenozoic• Mammals became dominant• Humans evolved

Marine extinction

Water animals

Single-cell

Reptiles (Dino)

Mammals

Dinosaur extinction

Geologic Time Scale

• Dinosaurs– Lived on Earth for

150 million years.

• Humans– Lived on Earth for

100 -200 thousand years.

Marine extinction

Water animals

Single-cell

Reptiles (Dino)

Mammals

Dinosaur extinction

Humans

Historical Geology

THE END

https://www.youtube.com/watch?v=9Ra9lwKn75s