Geologic Time

Absolute Dating and Geologic Time

Absolute Age•It is the age of a rock unit, fossil or geologic event expressed in units, such as years

• For example, your birthday . . .

– You were born on a specific day, month and year

• Absolute age can be determined by Radiometric Dating

Radiometric Dating• Uses the known decay rate (half-life) of

elements in rocks to determine the absolute age of the rock

• U238Pb206 – useful in dating old igneous rocks• K40Ar40 – useful for intermediate igneous rocks

Radiometric DatingThe Atom

• Fundamental unit of matter• Made up of components called subatomic particles

– Protons (positive charge)– Neutron (no electrical charge)– Electron (negative charge

Radioactive DecayStable Isotopes

• An atom is generally stable if the number of protons equals the number of neutrons in the nucleus

• Atomic Number

• Mass Number

– An element’s identifying #– Equals # of protons in atom’s

nucleus

– Equals # of protons and neutrons in an atom’s nucleus

Radioactive DecayIsotopes

• Variant of the same parent atom

• Differ in the # of neutrons• Result in different mass # than parent

• For example:

– Carbon-14 (C-14)– Types of carbon

• C-12 (stable)• C-13 (unstable)• C-14 (unstable)

Radioactive Decay The nucleus of an atom (decays) changes

into a new element.

The proton number (atomic number) changes

14C 14N6 7

Parent – unstable radioactive isotope

Daughter – isotopes resulting from decay of parent

How long does this take?

Radiocarbon DatingHalf-Life

• Half Life - time required for one-half of radioactive nuclei in a sample to decay

The half life of C-14 is 5,730 years

The Carbon Cycle

When Does the Clock Start?

When a plant or animal dies, the clock starts.

Organism diesNo more C-14 intake

C-14 begins to decay

How the Carbon Clock Works

There are two types of carbon used in dating

A living starfish has the same ratio as the atmosphere

A fossilized starfish has a

different ratio

– C-12 (stable does not decay)– C-14 (radioactive decays)

When organism is living, it has same ratio (C12:C-14) found in the atmosphere

How the C-14 Ratio Works

Amount of stable N-14

Amount of unstable C-14

Ratio Years dead # of half-lives

0 100 0 0

50 50 1:1 5,730 1

75 25 4:1 11,460 2

87.5 12.5 8:1 17,190 3

94 6 16:1 22,920 4

97 3 32:1 28,650 5

Carbon-14 can date elements up to approximately 100,000 years

– Used to date very recent events

– Important tool for anthropologists, archeologists and geologists

The ratio of C-14:N-14 can tell us how many half-lives

Radiometric Dating Uses the known decay rate (half-life) of elements

in rocks to determine the absolute age of the rock

U238Pb206 – useful in dating old igneous rocks K40Ar40 – useful for intermediate igneous rocks

C-14 ExampleTurin Shroud

Believed to be burial cloth of crucified Christ

ExampleTurin Shroud

artistic impression

Untouched x-rayed

ExampleShroud of Turin

• Evidence:– Blood Stains– Wounds – Weave of cloth– Pollen grains– Serum albumin

• Skeptics:– Painting– Blood - red

Carbon-14 Results

Carbon-14 dates the Turin Shroud to:

1356 A.D.

When was Jesus born?

Is the Shroud authentic?

More ExamplesDead Sea Scrolls

Qumran Caves

Aerial view Caves

More ExamplesDead Sea Scrolls

Fragment of Dead Sea Scroll

Dead Sea Scrolls were found in jars

Evidence:• >500 manuscripts.• 15,000 fragments describe

every book of Old Testament• Some Papyrus samples give

exact dates w/in text coincide with C-14 results

Skeptics:• Some of the scrolls appear

to be work of early Christians.

Carbon-14 Results

Carbon-14

150 B.C – 5 B.C.

Is it Authentic?

Pre-dates Christianity

One More ExampleSolomon’s Temple Stone Tablet

Evidence:• Confirms existence of

Solomon’s Temple – 9th Century B.C.

• 10 lines of Phoenician script describes life of King Jehoash.―Repair to temple.―War, etc.

Carbon-14 Results

Carbon-14 dates stone tablet to:

9th Century B.C.

It’s authentic!!

Radiometric Dating Inconsistencies

• In 1993, scientists found wood (trees) buried in basalt flows (69 feet deep).

• Wood samples sent to two labss to be carbon-14 dated 44,000 years old.

• Basalt sent to two laboratories to be potassium-argon dated 45,000 million years.

69 feet deep Wood Wood encased in encased in

basaltbasalt

Known Carbon-14 Inconsistencies

• Mollusks that are living dated at 2,300 years

• Seal skins (fresh) dated 1,300 years

Okay . . .Back to Geology!!

Earth’s HistoryBased on evidence preserved in rocks

Immensity of time span difficult to appreciate

– Changes occur over millions to billions of years

– Human life span = 70-80 years

Reconstruct history through relative dating

Geologic Time Scale• Organized into

meaningful, manageable units

• Oldest units at bottom, most recent at top

• Larger units (eons) are at left, with smaller units to right (epochs)

• Each unit is identified by rock type & fossils

Geologic Time ScaleScale is divided by Eons, Eras, Periods and Epochs

– Eon is the largest division Pre-Cambrian Eon

Phanerozoic Eon

• 7/8 of all time

• Represents 1/8 of time

• Before abundant life

• Visible life

Phanerozoic Eon

• Boundaries defined by massive extinction

– Paleozoic Era (ancient) Age of Fishes

– Mesozoic Era (middle) Age of Dinosaurs

– Cenozoic Era (recent) Age of Mammals

• Life explodes • Divided into 3 eras

PaleozoicAge of Fishes

Major Events• Supercontinent – Pangaea• Appearance of 1st life forms• Invertebrate (contain shells)• Life restricted to seasExtinction Climate change

MesozoicAge of Dinosaurs

Major Events• Break up of Pangaea• Seas invaded west N. America• Gymnosperms (cone-bearing)• Dinosaurs dominant• Some mammalsExtinction Large meteorite

CenozoicAge of Mammals

Major Events• Angiosperms (flowering

plants)• Marsupials (kangaroos,

opossums) • Placentals (humans)Extinctions humans

Large mammalsSaber tooth tiger, etc.