Transcript Fossils and Rock Record PowerPoint

Fossils and the Rock
Record
The Rock Record
 Rocks
record geological events and
changing life forms of the past
 Planet Earth was formed 4.6 billion
years ago
 Earth’s surface and interior have
been changed by the same
geological processes throughout
Earth’s history
Why study fossils?
 Fossils
are preserved remains,
imprints, or traces of once living
organisms which are preserved in
rocks.
 We can piece together unknown
puzzles of the past by studying
fossils
Almost
all fossils are
found in sedimentary
rock!!!
WHY?
 To
become a fossil, a dead body
must be protected from
scavengers and microorganisms
–Sediments can cover and not
damage a dead organism
–Sediments protect dead
organisms from being destroyed
by other animals
–May also slow down or stop the
decaying process
Usually
fossils are the hard
parts of organisms, such
as: bones, shells, and
teeth
–This is because these parts
preserve better
Ways to Become a Fossil
4
main ways
–Unaltered remains- remains have
not been changed much over time
 Mummification
–Altered remains- remains are
changed over time
 Carbonization
 Mold
& Cast
 Petrification
Petrification
 Petrified
remains
are hard and rock
like
 The original
material is
replaced with
minerals.
 Commonly
Petrified Objects…
– Wood
– Bones
Carbonization
 When
lots of sediments build up, the
body deteriorates and is left as only
a thin film of carbon residue.
 Residue forms an outline of the
organism, a fossil imprint. This can
be of fish or plants.
 This kind of fossil is called a
carbonaceous film.
Carbonization
Molds & Casts
Air and water
reach something
buried in
sediments and
decay it
 It will leave behind
an empty space in
the rock called a
mold

Later other
sediments fill in
the cavity and
harden
 This produces a
cast of the original
object

Molds/Casts
Unaltered Remains

Sometimes actual
remains may be
found preserved in
some way
– Amber- Hardened
tree sap
– Frozen ground
– Ice
– Tar pits
Tar Pits
Mummification
 Drying
 Often
found in desert caves or
buried beneath the desert sand
 Most bacteria cannot survive
without water, no
waterorganism does not decay!
Other Fossil Evidence
 Trace
fossils – evidence of animal
activity
–Tracks
–Burrows
 Tell us about the lifestyles of the
animals.
Trace Fossils
Index Fossils
From species that were on Earth for a
short amount of time but were abundant
 Fossils used to determine age of rock
layers
 Fossils can also be used to determine
ancient environmental conditions
 If a scientist found the same index fossil in
two different areas of the world this
indicates the layers were made at the
same time

Index Fossils
Relative Dating
 Used
to determine order of events
and the relative age of rocks by their
position in a sequence.
 Doesn’t tell you the exact age
 Tells you age in relation to other
rocks
 Gaps in the rock record are called
unconformities.
Telling the relative age of rock beds
Law of Superposition- In an
undisturbed layer of rock, the oldest rocks
are on the bottom and the rocks become
progressively younger toward the top.
 Principle of original horizontalitylayers of sediment are generally deposited
in a horizontal position
 Principle of cross cutting
relationships- states that if a rock layer
cuts through another the layer that cuts
through must be younger than the layer it
cuts

Telling the relative age of rock
beds
Which layer is
the youngest?
Which layer is
the oldest?
Unconformities-Breaks in the
rock record
 Angular
Unconformity –Horizontal
layers are tilted and lifted upwards
Angular Unconformity
 Disconformity
-A gap in the record
caused by rocks being exposed and
eroded. How it works?
 Sediment was deposited then eroded
and new sediment was deposited on
top leaving a gap in the record.
Disconformity
 Nonconformity
–sedimentary rock
forms above another rock type
(igneous or metamorphic). The other
rocks are uplifted and eroded.
Nonconformity
Absolute Dating
 Method
used by geologists to
determine age, in years, of a rock or
other object.
 Uses
the atoms in rocks and other
objects.
Radioactive Decay

RadioactivitySpontaneous decay
of the nuclei of
unstable atoms

When the number
of neutrons in an
atom change, it
becomes a new
element
Half Life
Time it takes for
half the atoms in
the isotope to
decay
 Ex: Carbon-14 has
a half-life of 5730
yrs.

100
90
80
70
60
50
40
30
20
10
0
C-14
N-14
5730
17190
– Every 5730 half of
the C-14 will decay
into N-14
Dating
When C-14 decays
into N-14, C-14 is
considered to be
the parent material
and N-14 is the
daughter product
 C-14 can be used
to date objects up
to 70,000 years

By measuring the
amounts of parent
& daughter
material,
geologists can
calculate the age
of a rock.
 This is called
radiometric
dating

Radiocarbon Dating
 Carbon
14 can be used to date
fossils, bones, and wood as old as
70,000 years
 Organisms take carbon to build
tissues and once they die, it decays
away
 Used mostly for igneous and
metamorphic rocks…can not be used
for sedimentary rocks
Uniformitarianism
Principle that states that
the processes occurring
today are similar to the
processes from the
past.
 Used this theory to
argue against those who
thought the Earth had
only been around for a
few thousand years.
