2003 Lecture 1: Geologic Time and Plate Tectonics

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2003 Lecture 1 Geologic Time and Plate Tectonics

• Questions
• How do geologists place the events of geologic
  history in sequence? What is correlation?
• What is the geologists definition of plate
  tectonics and what evidence underlies the theory?
• How do we apply plate tectonics to understand the
  geodynamic settings of the major rock suites?
• Tools
• Principles of stratigraphy
• The geologic timescale
• Reading Grotzinger et al., chapters 1, 2 8

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Geology deals with a wide range of times and rates

• Much of science deals only with the possible and
  the present, asking only what can happen.
• Geology is a historical scienceit asks what did
  happen and when.
• When considering events in the unobservable past,
  two basic needs are to establish the relative
  order of events and to fix the absolute age of
  events.

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Time and stratigraphy

• Stratigraphy is the branch of geology that places
  events in history and the preserved products of
  those events (rocks, fossils, structures) in
  chronological order.
• Placing absolute dates on those events is
  geochronology.
• All stratigraphy begins by constructing a local
  sequence, putting in order those rocks among
  which the temporal relations can be directly
  observed by contact in the field.
• Relating sequences or ages measured in one place
  to events in other places requires correlation,
  the basic tool for building up a global sequence
  of events and a globally useful timescale.

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Time and stratigraphy

• Local sequences and correlationwithin each
  outcrop the sequence of colors is fixed by direct
  observation. Matching this sequence with what is
  observed in a different outcrop is correlation.

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Time and stratigraphy

• There are many kinds of evidence that can be
  measured in the field and used to place rocks and
  events in local order and to correlate sequences
• Lithostratigraphy
• Biostratigraphy
• Magnetic stratigraphy
• Isotopic stratigraphy
• Astronomical chronometry
• Radiometric (absolute) chronometry

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Time and stratigraphy

• Radiometric dating is the only sure way to
  establish absolute ages, but
• it is a comparatively recent development in the
  history of geology
• many rocks (e.g., essentially all sedimentary
  rocks) cannot be dated because their formation
  did not reset isotopic indicators
• the errors on radiometric dates are of a
  different kind from the errors that can be made
  in relative stratigraphy
• All the other methods are therefore needed to
  leverage ages known from radiometric measurements
  to learn the ages of every other rock on earth.

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Lithostratigraphy

• The placement of a continuous series of
  stratified rocks in chronological order is based
  on two axioms (due to Nicolaus Steno, 1666)
• The principle of superposition
• In a sequence of undisturbed layered rocks, the
  oldest rocks are on the bottom.
• The principle of original horizontality
• Layered strata are deposited horizontally or
  nearly horizontally or nearly parallel to the
  Earths surface.
• A corollary is the recognition of cross-cutting
  relationships, where the planes associated with
  one rock type or stratum are seen to truncate the
  planes associated with a therefore necessarily
  older rock or stratum
• Together, these establish the sequence of age
  within one continuous outcrop or within the
  distance across which recognizable rock horizons
  can be traced.

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Original horizontality and superposition
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Original horizontality and superposition
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Original horizontality? and cross-cutting
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Superposition?
Which way is up?
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Original horizontality? and cross-cutting
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Lithostratigraphy

• Lithostratigraphy is local because at constant
  time (for example, the present), as we move
  geographically we encounter different sedimentary
  environments, where different kinds of rocks are
  forming
• Even where one rock horizon can be traced over a
  long distance, it does not in general represent
  the same time everywhere.
• During an episode of sea-level rise, e.g., a
  sandstone characteristic of the beach environment
  will move across the landscape. Such a rock layer
  is said to be diachronous or time-transgressive.
• Certain rock horizons, however, are verifiably
  isochronous (same time everywhere) and very
  widespread. The best are volcanic ashes, which
  fall across a wide region in a (geologic)
  instant. They are also (see below)
  radiometrically datable.
• We will discuss the principles of
  lithostratigraphy and sedimentary environment
  reconstruction in more detail in lecture 8.

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Biostratigraphy

• Life evolves over time and leaves recognizable
  traces in rocks called fossils
• actual preserved body parts, casts or impressions
  of body parts, or traces left by the passage of
  an organism (e.g., a worm burrow or footprint)
• A distinctive species or assemblage with a
  limited age range and a wide geographic range is
  an index fossil and can be used for correlation
• In general, biostratigraphy is a vastly better
  tool for correlation than lithostratigraphy,
  since evolution imprints a timestamp on fossils,
  whereas rock deposition environments move around
  but do not really evolve with time (except where
  biologically controlled!).
• Some care is required organisms migrate, and
  biostratigraphic zones can be time-transgressive.

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Magnetic Stratigraphy

• The magnetic field of the Earth occasionally
  reverses polarity, undergoes significant
  departures from the normal dominantly axial
  dipole, or changes dramatically in intensity.
• Many rocks, both igneous and sedimentary, acquire
  a remanent magnetic field at the time of their
  deposition that can be measured today.
• To the extent that the terrestrial magnetic field
  is a simple dipole, all rocks formed anywhere on
  earth at a given time record the same field
  polarity and intensity, and geographically
  consistent orientation information
• The establishment of a history of field reversals
  and events therefore provides a tool for global
  correlation, wherever particular reversals can be
  identified in a stratigraphic sequence (or, in
  the case of the oceans, as a function of
  horizontal distance from a spreading center).

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Isotopic and Chemical Stratigraphy

• The isotopic composition of terrestrial
  reservoirs, particularly the ocean, varies over
  time, and rocks or fossils deposited from the
  ocean record shifts in isotope ratios.
• For time-scales on which the ocean is well-mixed
  with respect to a given tracer, these isotope
  ratios can be used for global correlation of
  widely separated marine sequences.
• This technique applies both to stable isotopes
  and initial ratios of radiogenic isotopes.
• More rarely, global shifts in the concentration
  of some component, rather than an isotope ratio,
  in the ocean can be used for correlation.
• Also, even when the ocean remains constant in
  composition or isotope ratio, the fossils and
  chemical sediments may be fractionated in a
  temperature-dependent way, and global temperature
  shifts may thus lead to global isotope shifts
  that can be correlated.

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Astronomical Stratigraphy(!)

• To a significant extent, climate variations are
  modulated by astronomical factors
• Obliquity of Earths spin axis,
• Eccentricity of earths orbit,
• Precession of the perihelion.
• These effects are global, so when a climate proxy
  can be extracted from a rock sequence, it may be
  possible not only to correlate various sequences
  by aligning the astronomical cycles but to
  measure the absolute passage of time within a
  sequence using astronomical cycles as a local
  clock.

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Geochronology and Stratigraphy

• Although sedimentary rocks can rarely be dated
  directly by radiometric techniques, with the
  principle of superposition or cross-cutting
  relations ages can be bracketed between
  underlying and overlying datable horizons.
• Although the major divisions of the geological
  timescale and their relative sequence are defined
  by biostratigraphic (or occasionally
  lithostratigraphic, magnetic, or even isotopic)
  horizons, the absolute numbers attached to the
  boundaries are determined by bracketing the
  boundaries between radiometric dates

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The geologic timescale

• The need to establish a global timescale into
  which any rock sequence could be correlated
  predated the development of accurate absolute
  chronometers and remains today a separate issue
  because of errors and uncertainties in fixing
  absolute dates.
• Hence Earth history is organized in hierarchical
  fashion into named subdivisions. The names of
  these divisions, at the highest levels, form the
  standard vocabulary of geology. You must learn it
  to talk to geologists.
• As we get closer to the present, the time spans
  at each level of division tend to get shorter.
  The major levels of the hierarchy are eon, era,
  period, epoch, and stage.

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The geologic timescale

• The eons are
• Archean (4.5 Ga2.5 Ga) also Archaean
• Proterozoic (2.5 Ga543 Ma)
• Phanerozoic (543 Mapresent).
• (Sometimes the term Hadean (4.5 to 4 Ga), is
  used to refer to the time of heavy bombardment,
  before the stabilization of crust or the hope of
  preservation in the rock record)
• The (Hadean,) Archean and Proterozoic are
  together called pre-Cambrian.
• The base of the Phanerozoic (evident life)
  marks the appearance of shelly fossils in the
  rock record. The exact definition of the base of
  the Phanerozoic at 543 Ma is the sudden global
  appearance of vertically-burrowing trace fossils
  in the stratigraphic record.
• The boundary between Archean and Proterozoic is a
  matter of convenience it is not keyed to any
  particular event at exactly 2.5 Ga but is
  generally associated with a dramatic increase in
  atmospheric oxygen levels.

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The geologic timescale

• The sudden global appearance of
  vertically-burrowing trace fossils in the
  stratigraphic record

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The geologic timescale

• The base of the Phanerozoic at 543 Ma is also the
  first widespread appearance of easily-fossilized
  hard parts (shells).
• It is NOT the first appearance of multicellular
  organisms that was at least 100 Ma earlier

Edicaran fauna (pre-Cambrian)
Trilobite (Cambrian)
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The geologic timescale

• The eras of the Phanerozoic eon are
• Paleozoic (543 Ma251 Ma)
• Mesozoic (251 Ma65 Ma)
• Cenozoic (65 Mapresent)
• These are marked by major, first-order changes in
  marine and terrestrial fossil assemblages, and
  the boundaries between them are the largest mass
  extinctions of species in the fossil record
• Paleozoic-Mesozoic or Permian-Triassic extinction
  at 251 Ma, 95 of species go extinct
• Mesozoic-Cenozoic or Cretaceous-Tertiary (K-T)
  extinction at 65 Ma, 50 of species go extinct
• Broadly speaking, the large fauna of the
  Paleozoic is dominated by invertebrates, the
  Mesozoic by reptiles, the Cenozoic by mammals.
• It is generally possible to recognize at a glance
  which era a fossil assemblage is from.

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The geologic timescale

• major, first-order changes in marine and
  terrestrial fossil assemblages

Paleozoic trilobites, brachiopods, crinoids,
rugosan reefs
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The geologic timescale

• major, first-order changes in marine and
  terrestrial fossil assemblages

Mesozoic ammonites, belemnites, sponge-reefs
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The geologic timescale

• major, first-order changes in marine and
  terrestrial fossil assemblages

Cenozoic -bivalves -gastropods -scleractinian
reefs
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The geologic timescale

• the largest mass extinctions of species in the
  fossil record (?)

Sepkoski
Alroy
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The geologic timescale

• The periods of the Paleozoic era are
• Cambrian
• Ordovician
• Silurian
• Devonian
• Carboniferous
• (further divided in N. America into Mississipian
  and Pennsylvanian)
• Permian.
• The periods of the Mesozoic are
• Triassic
• Jurassic
• Cretaceous.
• The periods of the Cenozoic are
• the Tertiary
• (sometimes divided into Paleogene and Neogene)
• the Quaternary.

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The geologic timescale

• The epochs of the Tertiary period are
• Paleogene
• Paleocene
• Eocene
• Oligocene
• Neogene
• Miocene
• Pliocene
• The epochs of the Quaternary are
• Pleistocene
• Holocene
• The Pleistocene marks the beginning of the ice
  ages, and the Holocene (the last 11000 years)
  marks the time since the end of the last ice age
  (so far).

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The geologic timescale

• Paleocene small mammals

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The geologic timescale

• Oligocene big mammals

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The geologic timescale

• Miocene grasslands, grazing behavior

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History of Thought about the Age of the Earth

• We now know the Earth to be 4.6 Ga old. This is
  a 20th century item of knowledge. In the past
  there was an important two-way interchange
  between ideas about the age of the Earth and
  knowledge in physics, organic evolution,
  geochemistry and of course geology.
• Catastrophism and Neptunism
• If one accepts Judeo-Christian biblical writings
  literally, the Earth was 5767 years old last
  weekend. This is clearly insufficient time for
  the processes we see operating normally on the
  earth to shape the landscape or deposit the
  rocks, so it follows that the earth was shaped by
  extinct and presumably sudden processes.
• In particular, the prevailing view in the 18th
  century was that all rocks on earth were
  deposited in order from a global ocean that then
  receded (i.e., Noahs Flood, more or less
  literally).
• This is actually the origin of the terms Tertiary
  and Quaternary. In the Neptunist view,
  Metamorphic and Plutonic rocks are Primary,
  Volcanic rocks are Secondary, followed by
  sedimentary rocks, and then soils.

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History of Thought about the Age of the Earth

• Uniformitarianism
• Formulated by James Hutton in the 18th century
  and propagated by Charles Lyell in the 19th, the
  uniformitarian philosophy holds that the earth
  was shaped by the same processes that can be
  observed operating today, slowly, over an
  essentially infinite time span.
• no vestige of a beginningno prospect of an
  end.
• Uniformitarianism is clearly closer to the modern
  view than Neptunism, but for many decades was
  held as an excessively rigid dogma that allowed
  no extraordinary past events at all.
• It was into a uniformitarian world view that
  Darwin released Origin of Species in 1859 and it
  was clear to all that evolution required a vast
  amount of time.

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History of Thought about the Age of the Earth

• Kelvin and 19th century physics
• Based on measured heat flow from the Earth and
  the assumptions of no internal heat sources and
  conductive heat transfer only (the interior of
  the Earth is solid, after all), Kelvin
  demonstrated with absolute rigor that, cooling at
  its present rate, no more than 100 Ma can have
  passed since the Earth was completely molten
• Likewise, the Sun emits a huge amount of energy,
  and all the sources known to 19th century physics
  (gravitational contraction and chemical burning)
  are inadequate to maintain the suns current
  energy output for more than 40 Ma.
• In their time, these arguments were unanswerable,
  and proved a major hindrance to the general
  acceptance of evolution.
• Kelvin was in fact wrong for two reasons
• (1) the existence of radioactive decay as a heat
  source for the Earth and of nuclear fusion as an
  energy source for the Sun
• (2) solid-state convection as a heat-transport
  mechanism in the Earths interior (which leads us
  to our next topic, plate tectonics).

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History of Thought about the Age of the Earth
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History of Thought about the Age of the Earth