Transcript 2003 Lecture 1: Geologic Time and Plate Tectonics

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 geologist’s 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 and Jordan, 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
science…it 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.
2
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 correlation…within each outcrop the
sequence of colors is fixed by direct observation. Matching
this sequence with what is observed in a different outcrop is
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correlation.
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 Earth’s 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 timetransgressive.
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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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Isotopic and Chemical Stratigraphy
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Isotopic and Chemical Stratigraphy
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Astronomical Stratigraphy(!)
• To a significant extent, climate variations are
modulated by astronomical factors
– Obliquity of Earth’s spin axis,
– Eccentricity of earth’s 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:
–
–
–
–
Phanerozoic (543 Ma–present)
Proterozoic (2.5 Ga–543 Ma)
Archean (~4.5 Ga–2.5 Ga) [also Archaean]
(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 verticallyburrowing 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:
– Cenozoic (65 Ma–present)
– Mesozoic (251 Ma–65 Ma)
– Paleozoic (543 Ma–251 Ma)
• 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 Cenozoic are
– the Quaternary
– the Tertiary
• (now divided into Paleogene and Neogene)
• The periods of the Mesozoic are
– Cretaceous
– Jurassic
– Triassic
• The periods of the Paleozoic era are
– Permian
– Carboniferous
–
–
–
–
• (further divided in N. America into Mississipian
and Pennsylvanian)
Devonian
Silurian
Ordovician
Cambrian
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The geologic timescale
• The epochs of the Quaternary are
– Holocene
– Pleistocene
– 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).
• The epochs of the Tertiary period are
– Neogene:
• Pliocene
• Miocene
– Paleogene:
• Oligocene
• Eocene
• Paleocene
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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.,
Noah’s 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 beginning—no 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 sun’s 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 Earth’s
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
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