Geologic Time and Plate Tectonics
Read Chapter 8
• Questions
• How do geologists place the events of geologic history in
sequence?
• What is the geologist’s definition of plate tectonics and
what evidence underlies the theory?
• How do we apply plate tectonics?
• Tools
• Principles of stratigraphy
• The geologic timescale
• Radiometric dating
1
Terms to know
GEOLOGIC TIME
SCALE
ERA
EON
TRILOBITE
PRECAMBRIAN
CYANOBACTERIA
PALEOZOIC ERA
MESOZIC ERA
CENOZOIC ERA
PERIOD
2
Terms to know
FOSSIL
INDEX FOSSIL
TRACE FOSSIL
RELATIVE AGE
LAW OF
SUPERPOSITION
MASS EXTINCTION
UNIFORMITARIANISM RADIOACTIVE
DECAY
RADIOMETRIC
DATING
3
Geology deals with a wide
range of times and rates
•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.
4
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.
• 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.
5
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.
6
Original horizontality and superposition
7
Superposition?
Which way is up?
8
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
9
NUMERICAL DATING OF THE EARTH
•Rocks contain radioactive
minerals which are constantly
disintegrating at a steady rate
•Under certain circumstances,
these atomic “clocks” can be
red to give a “time”
Radioactive elements
• Not all elements are radioactive.
Those that are and are the most
useful for geologic dating are:
• U-238
• K-40
• C-14
Half-life = 4.5 By
Half-life = 1.25 By
Half-life = 5.73 ky
• Also, Sm-147, Rb 87, Th-232, U-235
Half-life for decay from U-238 all the way to
Pb-206 is 4.5 b.y. (billion years).
U-238 Decay Series
Decay rates for intermediate daughter
products range from <1 sec (polonium)
to 1,622 years (radium 226).
Fig. 5.4
Note changing ratio of parent/daughter after 2
half-lives. To get an estimate of the geologic age,
you need the ratio of the parent isotope to the
Fig. 5.5
Simple arithmetic plot of a universal isotopic decay curve. After 1 halflife 50% of parent isotope remains; after 2 half-lives, 25% remains.
What happens if the vertical axis is changed from linear to logarithmic?
Model radioactive decay
15
Lithostratigraphy
• The placement of a continuous series of
stratified rocks in chronological order is
based on two axioms
• 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.
16
17
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
18
Geochronology and Stratigraphy
• 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.
• The major divisions of the geological timescale and
their relative sequence are defined mostly by
biostratigraphic horizons, the absolute numbers
attached to the boundaries are determined by
bracketing the boundaries between radiometric dates
The geologic timescale
• Study figure 8.6,
page 212
The geologic timescale
• The base of the Phanerozoic at 543
Mya is 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)
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:
• Broadly speaking, the large fauna of the
Paleozoic is dominated by invertebrates,
the Mesozoic by reptiles, the Cenozoic by
mammals.
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
Devonian
Silurian
Ordovician
Cambrian