Earth History, Dating Methods,
and Geological Time
Geological Background to Primate
Evolution
Geological Background: Rocks
Igneous rocks are
formed by volcanic
processes
Often by cooling of
molten rocks (at
QuickTime™ and a
TIFF (Uncompressed) decompre ssor
are neede d to see this picture.
surface or within
Earth’s interior)
They are important
for radiometric
dating (see K-Ar
dating)
Geological Background: Rocks
Sedimentary rocks are
formed from deposition of
sediments into layers
Deposited by wind, water,
gravity into layers
QuickTime™ and a
TIFF (Uncompressed) decompre ssor
are neede d to see this picture.
Sediments formed by
erosion of other rocks
Layers are gradually
hardened over time
Sedimentary rocks are
important because fossils
are found in these rocks
Geological Background: Rocks
Metamorphic rocks have
“morphed” into another
kind of rock
Formerly sedimentary
QuickTime™ an d a
or igneous
Changed by heat and
TIFF (Uncompressed) decompressor
are need ed to see this picture .
pressure
Not particularly important
in paleontology
But pretty!
How Rocks are formed
Dating Methods
Absolute and Relative Dating Methods
Absolute methods provide ages in years BP
Relative methods provide only relative sequence
of events
Often used in combination
Direct and Indirect Dating Methods
Direct methods date the object itself
Indirect methods date something associated with
the object of interest
Stratigraphy & Superposition
The key to geology is
understanding the layers of
rock in the field
Law of Superposition
QuickTime™ and a
More recent strata are laid
TIFF (Un compressed) decompressor
are neede d to se e this picture.
down on top of older strata,
unless disturbed
Faulting and folding can
complicate matters
Biostratigraphy is a relative
dating technique that relies on
correlation and the principle of
superposition with respect to
fossils embedded within strata
Radiometric Dating Techniques
Rely on “decay” of radioactive isotopes
Decay constant
Probability per unit time of a molecule of a radioactive
element decaying
Half-Life
Time required for half the mass of radioactive mineral to
decay to byproduct
Different isotopes of same element have different
atomic mass but same atomic number
Atomic mass = number of protons + neutrons
Atomic number = number of protons
Different elements have different atomic number
Radioactive Decay
Radioactive decay
occurs at exponential
y
or geometric rate
Equal proportions are
.5y QuickTime™ an d a
TIFF (Uncompressed) decompressor lost per unit time
are need ed to see this picture.
.25y What is radioactive
1 2 3
decay actually?
Click here to find out
X axis = time in half lives
Y axis = amount radioactive material remaining
Potassium-Argon Dating
40K decays to 40Ar (and 40Ca)
Half life is 1.25 billion years
Ratio of 40K to 40Ar is measured to yield date
Potassium is one of most common elements in
Earth’s crust
39K is stable and most common isotope
Potasssium is common in igneous rocks
K-Ar clock is set with volcanic eruption
High temperature releases any 40Ar and begins
decay of 40K to 40Ar
Argon gas is captured in crystal lattice of hardened
lava (basalt, obsidian, etc)
Radiocarbon (C-14) Dating
14C decays to 14N
Half life is 5740 years
Works back to 50 or 75,000 years BP
created in atmosphere when cosmic rays
14C is
bombard atoms of 14N
14C is a small percentage of atmospheric carbon
It is incorporated into living things thru photosynthesis &
then heads up the food chain
When an organism dies, it no longer incorporates 14C
The clock starts ticking at death, ratio of 14C to 12C changes,
until all the 14C is gone
Radiocarbon dating only works for organic remains
Bone, teeth, wood or charcoal, etc.
Web Sites to Study
Two excellent web sites on dating
methods in anthropology & paleontology
many other informative websites exist on
these topics: use your favorite Search
Engine to find others
Dating Methods, from UCSB
Record of Time, from Palomar College
Geology and Dating Methods
QUIZ
Describe, compare and contrast
the K-Ar and C-14 dating methods
in a concise, informative and well-
organized paragraph of complete
sentences and good grammar.
Alfred Wegener and
Continental Drift
1880-1930
German astronomer,
meteorologist, and QuickTime™ and a
polar explorer TIFF (Un compressed) decompressor
are neede d to see this picture.
1915 “The Origin of
Continents & Oceans”
Wegener’s Theory
Continents and oceans have moved
across the face of the Earth in the past
Complimentary contours of continents and
continental shelfs
Similar geological formations separated by
oceanic basins
Geographic distribution of past fossil forms
across oceanic gaps
South America & Africa
QuickTime™ and a QuickTime™ an d a
TIFF (Un compressed) decompressor TIFF (Uncompressed) decompressor
are neede d to see this picture. are need ed to see this p icture .
These continents share all three kinds of data
suggesting they were once joined.
Why Was Wegener’s Theory
Soundly Rejected?
Built on mostly circumstantial evidence,
most of which could be explained by
other hypotheses
Land bridges and sunken continents
Major weakness was that he had no
mechanism that could explain how
continents could move across or
through the earth
Plate Tectonics: A Revolution
in the Earth Sciences
Paleomagnetism
Earth’s magnetic field has switched many times in
past from Normal to Reversed
Apparent polar wandering
Igneous rocks retain remnant magnetism from
Earth’s magnetic field and bear the record of
magnetic reversals and “polar wandering”
Deep Sea Oceanography
Parallel stripes of magnetism along both
sides of mid-oceanic ridges, or spreading
centers
K-Ar dating shows older ages further from
ridge
Plate Tectonics: A Revolution
in the Earth Sciences
Sea Floor Spreading
New crust created at spreading centers
New crust cools and spreads laterally
Plates and plate boundaries
Oceanic and continental plates in lithosphere
ride upon “plastic” asthenosphere
Convection currents in asthenosphere,
powered by heat from radioactivity deep
within Earth’s mantle
Plate Tectonics: A Revolution
in the Earth Sciences
Plate boundaries and geologic features explained by
plate tectonics
Mid-oceanic ridges or spreading centers
Mid-Atlantic ridge
Subduction zones, where oceanic plates are pulled down below
continental plates
Island arcs and marine trenches
Volcanoes
Earthquakes along many plate boundaries
Transform faults, where plates slide by each other
San Andreas fault
Mountain building, where continental plates collide
India and Asia form Himalayas
Plates and Plate Boundaries
QuickTime™ and a
TIFF (Uncompressed) decompressor Plate tectonics & earthquakes
are neede d to see this picture.
Paleomap Project
USGS on plate tectonics
QuickTime™ and a
TIFF (Uncompressed) decompressor
UCMP on plate tectonics are neede d to see this picture.
Earthquakes are Associated
with Plate Boundaries
QuickTime™ and a
TIFF (Uncompressed) decompre ssor
are neede d to see this picture.