Class 8 : Geological ages, life on
Earth, and mass extinctions
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Geological ages and some important
geological terminology.
Development of life on Earth.
Introduce idea of mass extinctions.
I: Geologic Time
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Divided into Eons, Eras, Periods, and
Epochs.
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E.g. We are currently in the Holocene Epoch
of the Quaternary Period of the Cenozoic
Era of the Phanerozoic Eon!…
Smaller (regional) subdivisions possible, i.e.
Ages and Chrons.
Based on appearance (or disappearance) of
fossils in rock, and/or radiometric dating.
II : Life on Earth
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Before 3.8 billion years ago, heavy
bombardment continuously vaporized
surface of oceans…
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First life appeared 3.5-3.8 billion years
ago.
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Any emergent life would be wiped out.
Simple, single-celled organisms (like
modern bacteria).
First multi-cellular life: 1 billion years
ago.
After that, things went quite quickly...
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Life moved onto the land 570 million
years ago – start of Cambrian period.
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Microbes first, then plants, then animals.
Spread rapidly across the lands, adapting to
different environments.
Dinosaurs were walking the Earth by
about 225 million years ago…
III : Dinosaurs
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Most successful species to inhabit Earth.
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Amongst biggest animals to ever live.
Adapted to most environments.
Top of food chain.
Then, 65 Myrs ago, they all died!
IV : The geological time-machine
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The geological record.
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Digging deeper into the Earth’s surface
corresponds to going back in time…
Older rocks/fossils are deeper.
65 million years ago, an event occurred
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Ended the Cretaceous Period (and Mesozoic
Era).
Started the Tertiary Period (and the
Cenozoic Era – the current era).
Called the K-T transition.
A lot of evidence suggests that a 10 km
asteroid hit the Earth at this point in time…
V : The K-T extinctions
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At the K-T boundary, 70% of all species
became extinct.
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Was extinction sudden or gradual?
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Many geologists believe that dinosaurs were
already starting to become extinct even
before any possible asteroid impact.
If so, impact may have helped extinction
but was not the primary cause.
But it’s very difficult to say whether
extinction was gradual or sudden.
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Ideally, if extinction were sharp, there would
be a depth above which fossils suddenly
disappeared.
But several things can disrupt/blur the ideal
time-ordering of strata…
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Bioturbation (sedimentary rock disturbed by
tunneling animals).
Reworking (sedimentary rock disturbed by nonbiological factors – waves etc.)
Erosion (can result in missing strata).
Debate rages on… but the hypothesis of
sudden extinction at KT boundary has yet to
be refuted.