Transcript Chapter 29

Chapter 29: Geologic
Time
THE RULES OF THE GAME
Time
What
is
time?
Time is…
Either
relative or
absolute
Time is: a measured period
during which an event,
process, or condition exists
or continues.
Dating Techniques
One way in which geologists
look at time: Relative
Dating
Relative Dating is..
…the measure
What happened first?
What happened next?
of a sequence
of events
without
knowing the
exact date at
which the
events
occurred.
Absolute Dating
• Absolute age—
age, in years,
of a rock or
other object;
determined by
properties of
atoms.
Principles of Relative Dating
• Original Horizontality
• Superposition
• Cross-Cutting
Relationships
• Inclusions
• Unconformities
• Fossil Succession
Principle of Original Horizontality
Layers of sediment
are generally
deposited in a
horizontal position.
Thus, if the rocks
are still flat, then
they have not been
disturbed and still
have their original
horizontality.
But, if the layers are folded or inclined at a
steep angle they must have been moved into
that position by crustal disturbances
sometime after their deposition.
Principle of Superposition
In an undeformed
sequence of
sedimentary
rocks, each bed
is older than
the one above
and younger
than the one
below.
In other
words, the
older rocks
on the
bottom were
formed
before the
ones above.
1 = Oldest
5 = Youngest
5
4
3
2
1
Principle of Cross-Cutting
This is
• Any feature
because you
that cuts
can't cut
across another
something until
rock or
that
structure is
“something”
younger than
exists!
what it cuts
across.
Q#2
Principle of Cross-Cutting
• What is
younger?
• The rocks
or the
fault line?
Law of Included Fragments
• Rock layers which contain
other rocks are younger than
the inclusion.
The
inclusion
(the other
rocks)
is older!
Law of Included Fragments:
1st - Igneous Intrudes
Granite Worn Down
Sediments Deposited
Over Fragments
Principle of Unconformities
Unconformities are surfaces that
represent gaps in the geologic
record. Usually represents long
periods of erosion.
Principle of Unconformities
missing rock layer = missing
time
There are 3 types of
unconformities:
Angular Unconformity
1. Sedimentary
rocks
deposited
horizontally
2. Sedimentary
rocks tilted
and eroded
3. New
sedimentary
rocks
deposited.
Disconformity
1. Sedimentary
rocks
deposited
horizontally
2. Long period of
erosion occurs
3. New
sedimentary
rocks
deposited.
Nonconformity
1. Igneous or metamorphic rock forms
2. Erosion occurs
3. Sedimentary rocks deposited.
Principle of Fossil Succession: Fossil
organisms follow one another in a specific
order, and therefore any time period can be
recognized by its fossil content.
What happened first?
How
do
you
know?
Where is the youngest rock?
How
do
you
know?
Where is the unconformity?
What type is it?
How do
you
know?
What rock layer is the
oldest?
How
do
you
know?
Does this diagram show
an unconformity?
If so,
which
type?
Stop Here!
WHAT ARE FOSSILS?
Fossils are any remains,
impressions, or traces of an
organism preserved in the Earth’s
crust. Fossils have been found on
every continent on Earth.
Here's a flow chart of fossil
formation:
Four Types of Fossils
• mold fossils: a fossilized impression made
in the sediment - a negative image of the
organism
• cast fossils: formed when a mold is filled in
• trace fossils = ichnofossils: fossilized
evidence of organism activity nests,
gastroliths, burrows, footprints, etc.
• true form fossils: fossils of the actual
animal or animal part
Ways that organisms can turn into
fossils:
1. unaltered preservation:
• Wooly Mammoth in Siberia
• Sloth in Desert Cave
• Mosquito in Amber
• Saber Toothed Tiger in Tar Pit
2. permineralization=petrification: Rock-like
minerals seep in slowly and replace the original
organic tissues with silica, calcite or pyrite, forming a
rock-like fossil - can preserve hard and soft parts most bone and wood fossils are permineralized
3. carbonization=coalification: Only the carbon
remains in the specimen - other elements, like
hydrogen, oxygen, and nitrogen are removed
TRACE FOSSILS or
(ichnofossils)
• Record the movements and behaviors
of an organism.
• Even the lack of trace fossils can
yield information; the lack of tailfurrow fossils indicates an erect tail
stance for dinosaurs that were
previously believed to have dragged
their tails.
Examples of Trace Fossils
• Trackways (sets of footprints)
• Toothmarks - Toothmarks generally appear
in bones
• Gizzard Rocks - Some dinosaurs swallowed
stones to help grind their food (modern
birds do this also).
• Coprolites (fossilized feces) - Coprolites
yield information an organism‘s diet and
habitats.
• Burrows and Nests - Fossils burrows and
nests can reveal a lot about behavior.
Index Fossil: Any fossil used to date strata.
Index fossils
have formed
from
organisms
which had a
wide
geographic
distribution
and existed
for a short
period of
time.
Absolute Ages of Rocks
• Absolute age—
age, in years,
of a rock or
other object;
determined by
properties of
atoms.
Radioactive Decay
• Isotope: An atom
having the same
number of protons in
its nucleus as other
varieties of the
element but has a
different number of
neutrons.
Radioactive Decay
• Unstable
isotopes
break down
into other
isotopes and
particles.
Beta Decay
• An isotope’s
•Now
the
neutron
atom
hasinto
an a
breaks
extra
proton and an
proton--it
electron. is
a new
• The electron
element!
leaves the
atom.
Alpha-Decay
• An isotope gives off two
protons and two neutrons as
an alpha particle
• The atom is now missing
protons and neutrons—it is a
new element!
Half-life
• The time it
takes for
half of the
atoms in an
isotope to
decay.
Time
Amount
0
100
1
50
2
25
Radiometric Dating
• Is the calculation of the
absolute age of a rock using
the ratio of parent isotope
(starting element) to
daughter product (ending
element) and the half-life of
the parent.
How to Do a ½ Life Problem
• An isotope of manganese
(manganese-137) has a half-life of
30 years. If 2.0 mg of manganese137 disintegrates over a period of
90 years, how many mg of
manganese-137 would remain?
Isotope Name
Step 1: Make a Table
Time Elapsed Amount of
Substance
Step 2: Fill in the Time Elapsed
Add 1
Half
Life
Time Elapsed Amount of
Substance
0 years
30 years
60 years
90 years
Step 3: Plug in Initial Amount
Time Elapsed
0 years
30 years
60 years
90 years
Amount of
Substance
2 mg
Step 4: Calculate Substance
Loss
Time
Elapsed
0 years
Amount of Divide by 2
Substance
2 mg
30 years
1 mg
60 years
0.5 mg
90 years
0.25 mg
Final Answer!
Time
Elapsed
0 years
30 years
60 years
90 years
Amount of There will be
Substance 0.25 mg
of
2 mg
manganese1 mg
137
left over
0.5 mg
after 90
0.25 mg years!
Radiometric Dating
• Potassium-Argon dating is
used to date ancient rocks
millions of years old.
• Carbon-14 dating is used to
date bones, wood, and
charcoal up to 75,000 years
old.