geologic time (rev.) chapters 12 & 13. scientists john powell –explored the grand canyon...
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Geologic Time (Rev.)
Chapters 12 & 13
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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
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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
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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
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Which layer came first?
A
B
C
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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
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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.
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Steno’s Observations
• Inclusions– Pieces of one type of rock that are contained
within another
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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
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1
2
3
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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)
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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
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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
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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
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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!
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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!
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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
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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
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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
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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
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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
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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
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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
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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.
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• Chixculub Crater, Yucatan Peninsula
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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
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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
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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