Geologic Time (Rev.)

Chapters 12 & 13

Scientists

• John Powell– Explored the Grand Canyon– Determined Earth’s history could be found

in rocks– Realized Earth is much older than

previously thought

• James Ussher– Made the first geologic and biologic

timeline– Earth was created in 4004 BC

Scientists

• James Hutton– Published Theory of the Earth, within he

stated that the ‘physical, chemical, and biological laws that operate today have also operated in the geologic past’

– …Uniformitarianism!!– Some processes today may have had less

importance in the geologic past

• Nicolaus Steno– Described geologic observations based

upon relative dating

Concepts – Steno’s Obs.

• Relative Dating– Identifying which rocks formed first– Tells us the sequence in which events

occurred

• Law of Superposition– Each bed of rock in a sequence of layers is

older than the layer above it– Rule also applies to lava flows and layers

of ash

Which layer came first?

A

B

C

Steno’s Observations

• Principle of Original Horizontality– Layers of sediment are generally deposited in

a horizontal position– The Grand Canyon is a prime example– Sideling Hill is a “syncline” (folded, U-shaped

layers) – Other forces acted on the rock layers

Steno’s Observations• Principle of Cross-Cutting Relationships

– When a fault (or magma) cuts through rock layers, we can assume the fault or intrusion is younger than the rocks affected.

Steno’s Observations

• Inclusions– Pieces of one type of rock that are contained

within another

Unconformities

• Disconformity – Two sedimentary rock layers that are separated by an erosional surface

• Nonconformity – Erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks

• Angular unconformity – Tilted or folded rocks are overlain by younger, flat layers

1

2

3

Fossils

• Fossils are the remains or traces of prehistoric life

• Only found in sedimentary rock

• Three types:1. Unaltered: teeth, bones, and shells. Flesh

and other soft parts decompose

2. Altered: Remains can be petrified (turned into stone)

3. Indirect Evidence: Trace fossils (Impressions or footprints)

Fossils

• Two conditions are needed for fossils:– Rapid burial– Hard parts (bones, shells, etc.)

• Fur, flesh, & organs are eaten or decomposed

• Index fossils are used to correlate rocks of similar age in different regions.– An index fossil is a fossil associated with a

particular span of time

Radioactive Dating

• Radioactivity– An atom of the same element can have

different atomic masses– The number of neutrons can vary, the variants

are called isotopes– Radioactivity is a process when the nucleus of

an atom break apart, or decay– Half-life – amount of time necessary for ½ of

the nuclei in a sample to decay to its stable isotope

Radioactive Dating

• Radiometric Dating– Use radioactive isotopes to calculate the age

of a rock– Examples:

Radioactive Daughter Half-life

Parent Product Values (years)

U-238 Pb-206 4.5 billion

U-235 Pb-207 713 million

Th-232 Pb-208 14.1 billion

Rb-87 St-87 47.0 billion

Carbon-14 Dating

• Carbon-14 is the radioactive isotope of carbon and is produced in the upper atmosphere

• When an organism dies the amount of C-14 decreases

• ½ life = 5730 years

• Used to date recent geologic events!

Precambrian Time

• Time = 0 to 4 billion years

• Rocks– Mining of diamond, gold, silver, copper, and

uranium provides rock samples from Precambrian time

• Early Atmosphere– The original atmosphere was composed of

water vapor, carbon dioxide, nitrogen, and other gases

– No oxygen!

Precambrian Time

• Evolution of atmosphere supporting life– The planet cooled torrential rains

increased evaporation increased cooling of the planet oceans formed water and carbon dioxide levels decreased increased nitrogen levels

• First Lifeforms– Primitive plants released oxygen– By the end of Precambrian time ocean-

dwellers required oxygen

Precambrian Time

• Fossils– Stromatolites are

the most common Precambrian fossil

• Mounds of calcium carbonate

• Deposits from algae

– Cyanobacteria• Life beyond 3.5 bya• Found in chert

Paleozoic Era• Early Life (Cambrian, Ordovician, Silurian)

– Cambrian Period• Formation of the Appalachian Mtns.• Age of the trilobites• Hard parts (shells, etc…)

– Ordovician Period• Appearance of brachiopods

– Attached to seafloor

• Cephalopods were the first large organisms

– Silurian Period• North America covered by shallow seas• Corals and coral reefs

Paleozoic Era

• Late Life (Devonian, Carboniferous, Permian)– Devonian Period

• Age of fishes (armor plated organisms)

– Carboniferous (Pennsylvanian, Mississippian)• Formation of tropical swamps• Large forests and trees• Age of Amphibians (frogs, toads, etc…)

– Permian• Formation of Pangaea• Mass extinction ended the Paleozoic Era

Paleozoic Era

• Paleozoic Extinction– Pangaea supercontinent formed– Changes in the world climate

• Drier climate

– Most land and sea animals went extinct– Reptiles prospered

• Each period ends with a mass extinction: – Large and rapid die off of organisms

Mesozoic Era

• 248 million years ago to 65 mya

• Also known as the Age of Reptiles

• The Mesozoic Era includes the Triassic, Jurassic, and cretaceous periods

• This also noted the breakup of Pangaea

• At the start of the Mesozoic Era most of the earth was above sea level– Therefore there are few marine fossils

Mesozoic Era

• Tectonic activity started to increase• As a result the Earth experienced many

mountain building events, aka Orogeny– Orogeny, greek for “mountain generating”

• The orogenies continued through the Cretaceous period

• Gymnosperms – first seed-bearing plants that did not rely on oceans– Dominant plant during the Mesozoic

Mesozoic Era

• Dinosaurs dominated the earth for 100 million years

• Tyrannosaurus – large carnivore

• Brontosaurus – large herbivore

• Archaeopteryx – flying dinosaur

• It is believed a large meteor caused a mass extinction.

• Chixculub Crater, Yucatan Peninsula

Cenozoic

• 65 mya to today

• Known as the age of Mammals or the Era of recent life.

• Two periods: Tertiary and Quarternary– Tertiary lasted 63 million years– Quaternary lasted 2 million years

• This is when mammals started to dominate life on land

Cenozoic Era

• Angiosperms- flowering plants with covered seeds

• Within the tertiary and quaternary periods are smaller periods called epochs– Holocene, Pleistocene, Pliocene, Miocene,

Oligocene, Eocene, and Paleocene

• Large mammals started to dominate the Oligocene epoch– Mammoths, saber-toothed tigers, etc

Cenozoic Era

• Many large mammals went extinct in the late Pleistocene epoch

• Scientists are puzzled over why these animals disappeared

• One theory is early humans hunted them to extinction