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 U 238  Pb 206 – useful in dating old igneous rocks K 40  Ar 40 – useful for intermediate igneous rocks

Radiometric Dating The Atom Fundamental unit of matter Made up of components called subatomic particles – Protons (positive charge) – Neutron (no electrical charge) – Electron (negative charge

Radioactive Decay Stable 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 Decay Isotopes 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 14 C  14 N 67 Parent – unstable radioactive isotope Daughter – isotopes resulting from decay of parent How long does this take?

Radiocarbon Dating Half-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 dies No 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 RatioYears dead# of half- lives :15, :111, :117, :122, :128,6505 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 U 238  Pb 206 – useful in dating old igneous rocks K 40  Ar 40 – useful for intermediate igneous rocks

C-14 Example Turin Shroud Believed to be burial cloth of crucified Christ

Example Turin Shroud artistic impression Untouched x-rayed

Example Shroud 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 Examples Dead Sea Scrolls Qumran Caves Aerial viewCaves

More Examples Dead 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 B.C – 5 B.C. Is it Authentic? Pre-dates Christianity

One More Example Solomon’s Temple Stone Tablet Evidence: Confirms existence of Solomon’s Temple – 9 th 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: 9 th 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 encased in basalt

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 History Based on evidence preserved in rocks Immensity of time span difficult to appreciate – Changes occur over millions to billions of years – Human life span = 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 Scale Scale 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

Paleozoic Age of Fishes Major Events Supercontinent – Pangaea Appearance of 1 st life forms Invertebrate (contain shells) Life restricted to seas Extinction  Climate change

Mesozoic Age of Dinosaurs Major Events Break up of Pangaea Seas invaded west N. America Gymnosperms (cone-bearing) Dinosaurs dominant Some mammals Extinction  Large meteorite

Cenozoic Age of Mammals Major Events Angiosperms (flowering plants) Marsupials (kangaroos, opossums) Placentals (humans) Extinctions  humans Large mammals Saber tooth tiger, etc.