Geological Time
Geology Needs a Time Scale
•  Rocks record geologic events and changing life forms of the past
•  Interpreting Earth history is a prime goal of geology, based on
clues found in rocks
•  The geologic time scale was developed and Earth’s history was
discovered to be exceedingly long
Relative Dating: Key Principles
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Placing rocks (and events) in their proper sequence of formation
Law of superposition
•  Developed by priest/geologist Nicolaus Steno in the 17th
century
•  In an undeformed sequence of sedimentary rocks, each bed is
older than the one above, younger than the one below
Relative Dating: Key Principles
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Principle of original horizontality
•  Layers of sediment are generally deposited in a horizontal position
•  Rock layers that are flat have not been disturbed
Principle of cross-cutting relations
•  Younger features (dykes, faults) cut across older features (pre-existing rocks)
Unconformities
•  An unconformity is a break
in the rock record caused
by erosion and/or nondeposition of rock units
•  When there is no break in
the rock record, the rocks
are considered conformable
Relative Dating: Key Principles
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Unconformities
•  Types of Unconformities
–  Angular unconformity – tilted rocks are overlain by younger flat-lying
rocks
–  Disconformity – strata on either side of the unconformity are parallel
–  Nonconformity – separates older metamorphic or igneous rocks from
younger sedimentary strata
Correlation of Rock Layers
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Correlation by physical criteria
•  Matching rocks of similar ages and features in different regions is known as
correlation
Fossils and correlation
•  William Smith (late 1700s) noted that sedimentary strata in widely separated
areas could be identified and correlated by their distinctive fossil content
•  Principle of fossil succession – fossil organisms succeed one another in a
definite and
determinable
order; any
time interval
can be
recognized by
its fossils
•  Index fossils:
they are
widespread
geographically
and limited to
a short span
of geologic time
Dating with Radioactivity
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Provides numeric ages – specifying the actual number of years that
have passed since an event occurred (also known as absolute age
dating)
Radioactivity
•  Each atom has a nucleus of protons (+) and neutrons, orbited
by electrons (-)
•  Isotope
–  Variant of the same parent atom
–  Differs in the number of neutrons
–  Some isotopes are unstable and their nuclei spontaneously
break apart; this process is called radioactive decay or
radioactivity
–  Unstable isotopes are called parents
–  Isotopes formed by the decay of parents are called
daughter products
Dating with Radioactivity
Half-Life
•  The time required for half of the
radioactive nuclei in a sample to
decay
•  Also the time elapsed when quantities
of parent and daughter are equal
(ratio 1:1)
•  Together with the parent/daughter
ratio, half-life (rate of decay) is used
to calculate the numeric age of a
sample
Dating with Radioactivity
Dating with Radioactivity
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Radiometric dating
•  Sources of error
–  A closed system is required
–  To avoid potential problems, only fresh, unweathered rock samples
should be used
• 
Dating with carbon-14 (radiocarbon dating)
•  Used to date very recent geologic events (half-life is 5730 years)
•  Carbon-14 is produced in the upper atmosphere and absorbed by organisms
•  Useful tool for anthropologists, archaeologists, historians, and geologists who
study very recent Earth history
• 
Importance of radiometric dating
•  Radiometric dating is a complex procedure that requires precise
measurement
•  Confirms the idea that geologic time is immense
The Geologic Time Scale
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Subdivides geologic history into units
•  Originally created using relative dates
Structure of the time scale
•  Eon – the greatest expanse of time
–  Phanerozoic (“visible life”) – the most recent eon, began just over
540 million years ago
•  Era – subdivision of an eon
–  Eras of the Phanerozoic eon
! Cenozoic (“recent life”)
! Mesozoic (“middle life”)
! Paleozoic (“ancient life”)
•  Eras are subdivided into periods
•  Periods are subdivided into epochs
Difficulties in Dating the Time Scale
•  Not all rocks can be dated by radiometric methods
•  Grains comprising detrital sedimentary rocks are not the same
age as the rock in which they occur
• 
The age of a particular mineral in a metamorphic rock may not
necessarily represent the time when the rock formed
• 
Datable materials (such as volcanic ash beds and igneous
intrusions) are often used to bracket various episodes in Earth
history and arrive at ages
• 
Dates change as brackets become narrower and methods
refined
Deep Time
The Earth is Old and continually evolving
True Scale:
Geologic Time is dominated by the Precambrian (87% of all time to now)
Early Earth (4.6 – 4.0 billion years ago)
– Prisocoan Eon
• Earth coalesces from material
captured in proto-solar system
Early Earth (4.6 – 4.0 billion years ago )
– Prisocoan Eon
• Very hot – liquid rock for much of this
time – magma oceans!
• Heavy bombardment phases – many
water-rich comets and iron-rich
meteorites, including very large impacts
probably
Early Earth (4.6 – 4.0 billion years ago ) –
Prisocoan Eon
• Slow sinking of many heavy metals to form
iron rich core
Early Earth (4.6 – 4.0 billion years ago ) –
Prisocoan Eon
• Too hot for plate tectonics or life
probably (atmosphere also oxygen poor
and toxic)
Archean Earth (4.0-2.5 billion years ago)
• Slowly cooling Earth – 1st definite continent
rock masses and water oceans, but still pretty
hot and ocean chemistry very different (e.g.
very iron rich)
• Atmosphere still very toxic and oxygen poor
Archean Earth (4.0-2.5 billion years ago)
• First life by about 3.8-3.9 billion years –
cyanobacteria only (like blue-green algae)
Life strongly influences
the composition of the Atmosphere
Our atmosphere formed
by outgassing (gases
released from solidifying
rocks) which continues
to this day
Water vapour formed
condensation and led to
rain creating water
bodies
Bacteria and later other
life forms generated free
oxygen
Archean Earth (4.0-2.5 billion years ago)
• Algal blooms flourish, produce oxygen
by photosynthesis in oceans and use up
much of ocean Fe
Archean Earth (4.0-2.5 billion years ago)
•  First good evidence of plate tectonics by about 3.2 billion years –
suggests stable mantle with convection happening by then
Proterozoic Earth (2.5 billion to 540 million years ago)
• Free oxygen in atmosphere by about 2.2 billion years
Proterozoic Earth (2.5 billion to 540
million years ago)
•  First cold climate evidence (glaciations) –
probably climate becoming similar to
present Earth with cold (icehouse) and
warm (greenhouse) cycles; though perhaps
more extreme and overall a very warm
Earth
•  Plate tectonics much like today’s Earth
Proterozoic Earth (2.5 billion to 540
million years ago)
•  Still just bacterial life until about 700
million years, then more complex
lifeforms evolve (though still softbodied, leave impressions only
•  Ediacara fauna)
Paleozoic Earth (540-250 million years)
• First complex lifeforms with skeletons
– rapid radiation of types
• “stable” Earth with climate very
similar to present
Paleozoic Earth (540-250 million years)
• Mostly stable climate
Paleozoic Earth (540-250 million years)
Paleozoic Earth (540-250 million years)
•  All life in oceans until about 420+
million years, then rapid evolution of
land plants and fauna
•  Why?
What do you know about the “Ozone Hole”?
Formation of Ozone
Sustaining Ozone
The Ozone Layer and
Chlorofluorocarbons (CFCs)
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Ozone (O3) is a ‘chemically out of place pollutant’
–  CFCs attack ozone and can destroy, or thin, the ozone layer
–  CFCs build up their concentration in the upper atmosphere, have a
residence time of about 100 years, and breakdown into various by-products
including chlorine monoxide (ClO)
–  One ClO can destroy many ozone molecules
The Ozone Layer and
Chlorofluorocarbons (CFCs)
• 
Ozone (O3) is a ‘chemically out of place pollutant’
–  In upper atmosphere (stratosphere) the ozone layer absorbs harmful
ultraviolet radiation
–  Ozone forms as O2 + ½ O2 = O3
–  Ozone layer is beneficial to life
The Ozone Layer and
Chlorofluorocarbons (CFCs)
The Ozone Layer and
Chlorofluorocarbons (CFCs)
Global Ozone Recovery Predictions
Paleozoic Earth (540-250 million
years)
• Major extinction event at end
of Paleozoic (Permian extinction)
•  85-90% of all organisms
wiped out (cause much
debated, probably extreme
climate change due to major
volcanic events, possibly
meteorite impact?)
Mesozoic Earth (250-65 million years ago)
• Rise of reptiles – Dinosaurs and marine reptiles flourish and
dominate large animal forms
Mesozoic Earth (250-65 million years ago)
• First mammals (about 200-210 million years ago) – small
and very minor species only through Mesozoic
Mesozoic Earth (250-65
million years ago)
• A hot climate throughout –
no glaciations, probably no
polar ice – a very long
“Greenhouse” climate
Cretaceous 144 - 65 Ma
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Western Interior Seaway
–  Early to mid-Cretaceous
–  Arctic Ocean transgresses
onto the continent and is
joined by water from Gulf of
Mexico from South separating
the continent in half
–  Shallow sea with abundant
wildlife
75 Ma
115 Ma
65 Ma
Mesozoic Earth (250-65 million years
ago)
•  Rapid extinction of dinosaurs and
many other species at 65 million years
•  Extreme climate change, due to
Meteorite impact and/or major
volcanic events
Chicxulub Impact Structure, Mexico
•  65 Ma, same age as mass extinction
•  Meteorite would have been ~10 km in diameter
•  Iridium layer observed in clays in many places around the world
Consequences of large impact >1 km
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A base surge, like a volcanic PF, generated by impact
Terrestrial impact, rock pulverized and/or vaporized
–  huge amounts of dust into the stratosphere
Huge amounts of water will be vaporized
–  runaway hurricanes, “hypercanes” - winds to 1,000 km/hr?
–  Global tsunamis, Earthquakes
Consequences of large impact >1 km
•  Suspended dust & soot will cause
global winter & global darkness
•  Acid rains, Greenhouse effects
•  Catastrophic crop failure
•  MASS EXTINCTIONS
Cenozoic Earth (65 million years to
present)
• Rapid evolution of mammals to
fill extinct dinosaur and marine
reptile niches
Cenozoic Earth
(65 million years
to present)
•  Variable climate
– mostly warmer
than today except
last 2.6 million
years
Cenozoic Earth (65 million years to
present)
•  Quaternary climate (2.6 million years to
present) – cyclic oscillations between
“icehouse” and “greenhouse” climates –
glacial vs. non-glacial.
•  Have been mostly in an “intraglacial” climate for last 12,000+
years and continuing.
Last Glacial Maximum
Cenozoic Earth (65 million years to present)
•  First Homo genus (direct Homo Sapien ancestors)
about 2.5-3 million years ago (lots of debate still –
controversial evidence for as old as 6 million)
•  Homo Sapiens by about 130,000 years ago in
Africa (could be older)
Human evolution
Homo
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Homo is the genus that includes modern humans and species closely related to
them; 2.3 to 2.4 Ma ago
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Homo sapiens is the only non-extinct species of its genus
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Some of these other species might have been our ancestors, but many were
likely our "cousins", having speciated away from our ancestral line
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No consensus as to which of these groups are species or subspecies; partly due
to lack of fossils or due to the slight
differences used to classify species
in the Homo genus
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Homo arrived at same time as first
evidence of stone tools
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Due to large number of morphological
similarities exhibited, Homo is closely
related to several extinct hominid
genera, and at this stage none of them
is universally accepted as the confirmed direct ancestor of Homo
Homo sapiens
Homo sapiens
Deep Time
The Earth is Old and continually evolving
True Scale:
Geologic Time is dominated by the Precambrian (87% of all time to now)
Archean Earth (4.0-2.5 billion years ago)
• First life by about 3.8-3.9 billion years –
cyanobacteria only (like blue-green algae)
Proterozoic Earth (2.5 billion to 540
million years ago)
•  Still just bacterial life until about 700
million years, then more complex
lifeforms evolve (though still softbodied, leave impressions only
•  Ediacara fauna)
Paleozoic Earth (540-250 million years)
•  All life in oceans until about 420+
million years, then rapid evolution of
land plants and fauna
Mesozoic Earth (250-65 million years ago)
• Rise of reptiles – Dinosaurs and marine reptiles flourish and
dominate large animal forms
Cenozoic Earth (65 million years to
present)
• Rapid evolution of mammals to
fill extinct dinosaur and marine
reptile niches