Lecture 2 Geologic Time Geologic Time Scale How we know about Earth history Highlights of the Histor

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Lecture 2Geologic Time-Geologic Time Scale-How
we know about Earth history -Highlights of the
History of Life
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Geologic Time
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Geologic Time

• Age of Earth 4.6 billion years old
• Humans 2.5 million years
• Recorded history, a few thousand years
• How do we measure geologic time?

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Rock types tell us about Earth history

• The nature of the rock tells us about the
  conditions in which it formed.
• ExampleBasalt indicates lava flowSandstone
  indicates beachEvaporite indicates a sea that
  dried up, because it is the salts left behind

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Rock structures tell us about Earth history, too

• After a rock is formed it may be tilted, folded,
  faulted, or magma may intrude into it.
• Examining the rocks structures tell us about such
  events

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Geologic Time Measured in Two Ways

• Relative Age
• Absolute Age

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Geologic Time Measured in Two Ways

• Relative Age This is older than that.
• -Tells the order in which events occurred
  relative to each other
• -Law of superposition younger rocks are on
  top of older rocks
• -Principle of Original horizontality The rock
  had to exist before it got tilted, folded or
  faulted

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Example of Relative Age

• Final rock layer that has abundant dinosaurs, of
  the most evolved forms Late Cretaceous
• Next layer up, with few fossils and no dinosaur
  fossils Early TertiaryAge names can be
  assigned to rocks based on their fossils and the
  changes in fossils

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Absolute Ages

• -Measured in years.
• -Dates are derived from measurement of
  radioactive isotopes in the rocks
• -Examples Dinosaurs went extinct 65 million
  years ago
• Permian extinction 248 million years ago

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Fossils

• Oldest known fossils bacteria-like organisms 3.8
  billion years ago
• Tell us what was living at the time the rock
  formed
• Tell us the path of evolution
• Also tell us about the ancient environment,
  because most organisms thrive in certain
  environments and not others

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Fossils as Temperature Indicators

• Corals live in clear, warm, shallow seas

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Fossils as Temperature Indicators

• Today fossil corals are found in rocks of cold
  mountain environments such as the Canadian
  Rockies and Mt Everest
• Rocks exposed in these locations were once
  submerged beneath clear, warm, shallow seas
• Tectonic processes raised the marine rocks to
  form the high peaks

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Fossils and Ancient EnvironmentsExamples

• Clams in limestone rock ? shallow seas
• Mollusks with thick shells capable of pounding by
  waves ? shorelines
• Thin delicate shells ? deep, calm off-shore waters

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Principle of Fossil Succession

• Fossil organisms succeed one another in a
  definite and determinable order
• Therefore any time period can be recognized by
  its fossil content
• The fossils become like a calendar, marking the
  march of time
• Fossils document the evolution of life through
  time

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Index Fossils

• Widespread geographically
• Limited to a short span of geologic time
• Their presence in a rock formation permits
  matching that rock to rocks of the same age
  around the world
• For example Trilobites Paleozoic individual
  species allow you to pinpoint what part of the
  paleozoic the rock is from

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Correlation Matching up rocks from different
areas to make a more complete record

• Distinctive rock types
• Distinctive fossils
• Distinctive events or transitions (such as mass
  extinctions)
• Same absolute age
• Putting together multiple sections of rock, you
  can tell a more complete history

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Geologic Time Scale

• Eons, largest interval, divided into
• Eras, divided into
• Periods, divided into
• Epochs, smallest interval.

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Geologic Time Scale, How it is determined

• Originally constructed on the basis of relative
  age determinations, events and fossils
• Fossils have been important for designating time
  periods Which fossils dominate? eg., Cambrian
  (with Trilobites) came before the Ordovician
  (with fishes)
• Mass extinctions mark many eon or era boundaries
• Radiometric dating has added actual dates to the
  relative time scale
• Absolute dates get revised as research improves

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Pre-Cambrian Time

• From 4.6 to 543 my
• The first 8/9 of Earth History
• Includes the first three Eons-Hadean-Archean-
  Proterozoic

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Pre-Cambrian Rocks

• Relatively few of Earths rocks are Pre-Cambrian
  in ageWhy?

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Pre-Cambrian Rocks

• Relatively few of Earths rocks are Pre-Cambrian
  in age
• Most of Earths very old rocks have been
  metamorphosed, deformed, and eroded
• Traces of earlier history largely lost

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Hadean Eon

• Hadean Greek for Beneath the Earth
• Earliest time in Earth History
• 4.6-3.8 by (billion years)
• Few Hadean rocks on Earth
• No fossils of this age known no life?
• No subdivisions based on fossils

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Archean Eon

• Archean Greek for Ancient
• Some contain microscopic fossils of single-celled
  organisms
• 3.8 by Oldest known fossilsMarks beginning of
  the Archean
• Few fossils, poorly preserved
• Few subdivisions

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Earliest Fossils 3.8 By, Archean

• Stromatolites, bacteria-produced pillars like
  these modern ones in Shark Bay, Australia
• Pre-Cambrian mostly microfossils

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Proterozoic Eon

• Youngest of the Pre-Cambrian Eons
• Protero Greek for Early
• Zoic Greek for Life
• Diverse groups of fossils
• Some multi-cellular, with differentiated cells
  arranged into tissues and organs

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Proterozoic Eon

• Look similar to jellyfish, corals, worms
• A few shell-bearing organisms
• Life diverse but not HUGELY abundant
• Marine life dominates

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Highlights of History of LifePre-Cambrian

• 4.6 by Age of Earth
• 3.8 by first fossils, simple cells, stromatolites
  
• 3 by photosynthesis
• 2 by complex cells
• 1 by multicellular life
• 600 my Snowball Earth
• 600 my simple animals

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Last Part of Pre-Cambrian

• 570 my arthropods (ancestors of insects,
  arachnids and crustaceans)
• 550 my complex animals

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Phanerozoic Eon

• Last 543 my
• The last 1/9 of Earth History
• The 4th final of the Eons
• The oldest period is the Cambrian, hence
  Pre-Cambrian to designate all that came
  before

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Phanerozoic Eon

• Phaneros Greek for evident, visible
• Zoic Greek for life
• Very Visible Life
• Fossils diverse abundant
• Divided into many periods and epochs

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Fossil Preservation

• Any given organism has an infinitesimally small
  chance of being preserved
• The vast majority of all ancient life has left no
  trace in the geologic record.
• Some living things are more likely to to leave a
  fossil hard parts (bones, teeth, shells)
• Rapid burial increases chances

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Changes occurred at the beginning of Phanerozoic

• Increase in the number of species with shells and
  skeletons
• Increase in the average size of individual
  organisms
• Increase in the number of species in fossil
  record
• Increase in the total number of individuals in
  fossil record

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Phanerozoic 3 Eras

• Cenozoic
• Mesozoic
• Paleozoic

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Paleozoic

• Greek for Old Life
• Oldest Phanerozoic Era
• Fossils of early life forms, such
  as-invertebrates-fishes-amphibians-reptiles-f
  erns-cone-bearing trees

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Cambrian Oldest Period in the Paleozoic

• Age of the
• Trilobites
• Index fossil
• The species of trilobite tells you which part of
  the Paleozoic the rock is from

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Cambrian Explosion

• Burgess Shale, Incredibly rich fossil layer

Proliferation of strange and complex creatures
like this 5-eyed opabinia
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Paleozoic Age of Marine Invertebrates, then
Fishes, then Amphibians

• 500 my fish and proto-amphibians
• 475 my land plants
• 400 my insects and seeds
• 360 my amphibians
• 300 my reptiles, dinosaurs 248 Ma Permian
  extinction

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Paleozoic

• Youngest period is the Permian
• 248 my, Permian Ended Abruptly
• Permian Mass Extinction90 marine species2/3
  of reptile and amphibian species30 of insect
  species

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Phanerozoic 3 Eras

• Cenozoic
• Mesozoic
• Paleozoic

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Mesozoic

• Greek for Middle Life
• New types of phytoplankton, microscopic plants
  that float at or near the sea surface
• Swimming cephalopods called ammonoids ?
• Dinosaurs

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Mesozoic Age of Reptiles

• 200 my mammals, got as big as rats
• 150 my birds, from dinosaurs
• 130 my flowers

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Mesozoic most famous for Dinosaurs
?Duckbill dinosaur nurtures babies in their nest
100 million years ago Mesozoic ended abruptly 65
ma with another mass extinction
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Cenozoic Recent Life
Cenozoic is the most recent of the three
Phanerozoic Eras
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Cenozoic Recent Life
Mammals and grasses became abundant. Humans have
evolved and lived wholly in the Cenozoic Era.
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Evolution

• Natural Selection Environmental Pressure, eg.,
  climate change
• Random mutations, most neither help nor hurt the
  organism
• AdaptationDarwin Environment drives evolution

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Cenozoic Age of Mammals

• 65 my Cretaceous Extinction
• 35 my grasses
• 2.5 million years since the appearance of homo
  sapiens (evolved from a common ancestor with the
  apes, not evolved from apes)
• 200,000 years since humans humans started looking
  like they do today

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Our Place on the Geologic Time Scale

• Eon
• Era
• Period
• Epoch

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Our Moment on the Geologic Time Scale

• Phanerozoic Eon
• Cenozoic Era
• Quaternary Period
• Recent or Holocene Epoch
• Some propose that the time in which humans have
  had a major impact on the planet should be
  designated the Anthropocene (Anthro man)

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