Download 21.4 – Key Beds, Absolute Dating, and FOSSILS Key Beds

21.4 – Key Beds, Absolute Dating, and FOSSILS
Key Beds
 Definition: Rock or sediment
in history, and cover a
that can be traced back to a very
area.
Absolute Age Dating
 Four ways:




Radiometric Dating (review)
 Certain elements have radioactive properties.
 That is, they lose atomic parts from their
 We call this losing of parts
 When they lose
, the element changes to a whole
element
 When they lose
, they become a different
of the same element,
with a new atomic mass.
 The radioactive decay happens at a
rate
 So we can use these
to determine how long ago a rock layer was formed.
Radiometric Dating (more review)
We use half-life calculations, given the amount of parent material and daughter material in a sample.
A
is the amount of time it takes for
the element to decay.
Absolute Dating-Tree Rings

tree and seasonal events
= the science of studying
to determine the age of the
Absolute Dating-Ice Cores

– created by drilling hollow tube through ice up to several kilometers thick
 Drilling down through ice gives a record of
events and
events.
 Ice shows seasonal changes, like tree rings do.

ice has
bubbles and
crystals than winter ice does.
Absolute Dating-Varves

: bands of alternating light and dark-colored
of sand, clay and silt
 Rates of sedimentary deposits vary by
(as with ice cores and tree rings)
 Help study
melting patterns
The Fossil Record and Life
 The
record provides evidence about the history of life on Earth. The fossil record also shows
that different groups of
have changed over time.

is the gradual change in species over long periods of time.
 When geologists find fossils in rocks, they know that the rocks are about the
age as the
fossils. Thus, they can
that the same fossils found elsewhere are also of the same age.
Radiolarians
 Microfossils

organisms with hard shells that have populated the oceans since the
Period
 Used by petroleum geologists to determine the age of rocks that might produce
Original Preservation
 The picture on the preceding slide is from the La Brea Tar Pit in California. The soft parts of a mammoth were
preserved in the aforementioned Pit.

: PLANT AND ANIMAL REMAINS THAT HAVE BEEN
ALTERED VERY LITTLE SINCE THE ORGANISM’S DEATH (USUALLY
TISSUE DECAYS BUT IN
ORIGINAL PRESERVATION IT DOES
)
Original Preservation Example
The insect is completely
(hard and soft parts) in amber.
Altered Hard Parts
 The soft portion decays away quickly and the hard portion (bones, shells, cell walls) can become fossils in one of
ways:
1)
:
a) pores in hard parts are filled in with minerals from groundwater
b) groundwater comes into contact with original hard parts mineral and replaces the material with a
different mineral
2)
:
Original mineral retains the same chemical formula although takes on a crystalline structure for greater
long term stability
Molds/Casts


: impression left behind in the sediment where a shell once was
: filled in mold
Trace Fossils
 Provide evidence of how an organism
,
and obtained
 Examples: worm trails, footprints, tunneling burrows, gastroliths (rocks in dinosaur stomachs- left) and
coprolites (fossilized feces- right)