Relative Time

Relative Dating

• Relative dating – events are placed in a sequence, but actual occurance dates are unknown.

• Who is older, Mr. Kopp or Justin Bieber?

• What are our actual ages? – Absolute Dating

Relative Dating Geologic Laws

• Principle of Uniformitarianism – idea that the same geologic processes that are occuring today ALSO occurred in the past

Law of Original Horizontality

• SEDIMENT IS DEPOSITED HORIZONTALLY

Law of Superposition

• If sedimentary rocks are undisturbed – THE OLDEST LAYERS ARE ON THE BOTTOM and YOUNGER LAYERS ARE ON TOP

B

A

C

D

oldest

youngest

Law of Crosscutting

• A cutting feature is younger than the layers it cuts through

A

B

C

D

E

oldest

youngest

Law of Included Fragments

• Pieces of rocks are older than the layers of rock in which they are found

e ee

B

D

C

A

oldest

youngest

“Unconformity Rule”• A place in the rock record where layers of rock are missing and gaps in

geologic time exist. The layers were deposited and worn away OR never existed. The result is a large difference between rocks above and below the eroded surfaces.

C

B

A

D

E

Law of Faunal Succession

• Older rocks contain the remains of older and more simple creatures and younger rocks contain younger more complex creatures

Absolute Time

Radiometric Dating

• Calculation of the exact age of rocks using radioactive isotopes

Remember Relative Dating is only a comparison of the ages of rock layers (older or younger)

How Does Radiometric Dating Work?

• Atomic # = ?

• Atomic Mass # = ?

• Isotope – an element with the same number of protons but a DIFFERENT NUMBER OF NEUTRONS = unstable nucleus

• Radioactivity: when nuclei are unstable (radioactive isotopes), they spontaneously break apart or decay

How Does Absolute Dating Work?

• PARENT = radioactive isotope which decays into a stable DAUGHTER product.

• Radioactive decay continues until a stable daughter product is formed….which happens within a predictable time period known as a half-life.

How does Relative Dating Work?

• Half-life – rate of radioactive decay (the amount of time it takes for HALF of the nuclei of a radioactive isotope to decay into its stable daughter product)

• Uranium-238 decays into Lead-206 (4.5 by)• Uranium-235 decays into Lead-207 (713 my)• Thorium-232 decays into Lead-208 (14.1 by)• Rubidium-87 decays into Strontium-87 (47 by)• Potassium-40 decays into Argon-40 (1.3 by)

How does absolute dating work?

• Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.

• Question?• Why have the oldest rocks on Earth been

dated at 3.9 billion years old yet the Earth has been determined to be 4.6 billion years old?

Carbon-14

• Radiocarbon dating – all organisms contain carbon-14 allowing scientists to determine the age of recent fossils

• Carbon -14 has a half life of 5,730 and can help scientists date recent geologic events up to 75,000 years ago making it extremely valuable for human history

Index Fossils

• Used to define and identify geologic periods• Widespread, existed in a specific period of

time

Geologic Time Scale

(Divided based upon major geologic and evolutionary events)

Eons

• Largest divisions of geologic time

1) Pre-Cambrian 4.6 billion – 540 million years ago - Largest period of time 7/8 of the Earth’s History- Formation of the Earth, rise of life (3.7 bya), to the

first invertebrates

2) Phanerozoic (540 million years ago to present day)- Complex life on Earth

Eras

• 2nd Largest divisions of geologic time

1) Pre-Cambrian – rise of early life2) Paleozoic – rise of complex life3) Mesozoic – age of the dinosaurs4) Cenozoic – age of mammals

Periods and Epochs

• Pre-Cambrian 4.6 billion years ago to 540 million years ago (life appears 3.7 bya)

• Earth Formed, Moon Formed, Heavy Bombardment

• Bacteria• Stromatolites (blue-green algae)

Periods and Epochs

• Phanerozoic Eon, Paleozoic Era• Cambrian Period 540 to 490 mya• Most major animal groups appear• Arthropods and mullosks

Periods and Epochs

• Phanerozoic Eon - Paleozoic Era• Ordovician Period – 490 to 443 mya• Massive Marine LifeDiversification

Periods and Epochs

• Phanerozoic Eon, Paleozoic Era• Silurian Period – 443 to 417 million years ago• Life comes on land

Periods and Epochs

• Phanerozoic Eon, Paleozoic Era• Devonian Period – 417 to 354 million years

ago• Age of fishes

Periods and Epochs

• Phanerozoic Eon, Paleozoic Era• Carboniferous Period• Mississippian and Pennsylvanian Epoch – 354 to 290 million

years ago• Age of plants

Periods and Epochs

• Phanerozoic Eon• Paleozoic Era• Permian Period – 290 to 248 million years ago

• MASS EXTINCTION of 90% of species

Periods and Epochs

• Phanerozoic Eon, Mesozoic Era• Triassic Period – 248 to 206 million years ago• Rise of reptiles

Periods and Epochs

• Phanerozoic Eon, Mesozoic Era• Jurassic Period – 206 to 144 million years ago• Dinosaurs rules the land

Periods and Epochs

• Phanerozoic Eon, Mesozoic Era• Cretaceous Period – 144 to 65 million years

ago• Rise of Flowering plants – End of Dinosaurs

Periods and Epochs

• Phanerozoic Eon, Cenozoic Era• Tertiary Period • Paleocene Epoch - 65 to 54.8 million years ago• Age of Mammals

Periods and Epochs

• Phanerozoic Eon• Cenozoic Era• Quarternary Period • Pleistocene Epoch - 1.8 to million years ago to

10,000 years ago

• Modern organisms – RISE OF HUMANS

Periods and Epochs

• Phanerozoic Eon• Cenozoic Era• Quarternary Period • Holocene Epoch - 10,000 years ago to present