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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.

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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 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 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 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 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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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 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:– 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