Transcript Unconformity

Mountain Building
Folding
vs.
 Faulting

Geologic
time
&
dating
Relative Dating

Process of placing
events in the
sequence in which
they occurred

does NOT identify
actual dates
Geologic History Sequence
Uniformitarianism
George Hutton

concept that the same
processes that occur today
also occurred in the past

changes were slow over
time, not quick
catastrophic events

“the present is the key to
the past”
Original
Horizontality
concept that most sedimentary rocks
are deposited as a horizontal layer
Superposition
youngest
oldest
concept that in an undisturbed sequence of
sedimentary strata, the oldest rock layer will
be at the bottom, and the youngest on top
Cross-cutting
concept that an igneous intrusion or fault is
always younger than the rock it cuts across
4
3
2
1
5
Embedded Fragments
concept that rocks embedded in another rock
must be older than rock in which it is found
5
4
3
2
1
Pieces of igneous rock, left
when the igneous was
partially eroded, are in the
green sedimentary rock.
Logically, then, the igneous
pebbles must be older than
the sedimentary rock they
are included in.
Uplift
Over time, rocks can be tilted or deformed by
regional or local metamorphism
5 - tilting
4
3
1
2
6
If rocks are tilted, then the metamorphism that
caused it is younger than the rocks
Unconformity- gap in
geologic record
caused by erosion
9
8
7
5 - tilting
4
3
1
2
6
New, younger rocks form on top of the tilted strata
Unconformity
Practice
D
C
B
A
Layer A is the oldest. Layer D is the
youngest.
Practice
E
unconformity
D
C
B
A
Layer A is the oldest. Layer E is the
youngest. Tilting & erosion occurred after D,
but before E.
Practice
F
unconformity
E
C
D
B
A
Layer A is the oldest, then Layers B, C,
D and E. Folding & erosion occurred
after E but before F.
Practice
F
E
unconformity
D
C
B
Anticline /
Dome Mtn.
A
Layer A is the oldest. Layer F is the
youngest. Tilting & erosion occurred after
D, but before E.
Practice
5. sandstone
7. Igneous
Intrusion
4. limestone
3. shale
2. sandstone
1. limestone
6. reverse fault
Practice
Geologic
Column
7.
6. Unconformity
5. Fault
4.
3.
2.
1.
Youngest
F
B
K
N
A
J
D
M
H
C
Oldest
L
G
E
Oldest
M E
G
A
K
Z
C
F
D
N
B
Youngest
L T R J
Oldest
F P B
R
M
H
A
E
X
D
Youngest
K J S
Oldest
V C O M X K E B S Z
Youngest
J G L F T A D P H N R
Fossils
help geologists study Earth’s past & determine…
Approximately when life
began
What plants & animals were 1st
to live on land
When organisms
appeared/disappeared
How organisms live
Trilobite
Fossils
Can be in the form of:
REMAINS
IMPRINTS
BONES
TEETH
TRACES
How Fossils form
The body must be protected from
scavengers & bacteria
Hard parts have a better chance of
becoming fossils
Fossils are found in sedimentary rocks
INDEX FOSSILS
Certain fossils can help find the relative age of rocks.
Since each organism comes from a specific time
period, we can use a fossil to determine the age of the
rock it is found in.
These special fossils are called index fossils.
They are abundant, widely spread and must have
lived only during a short part of earth's history.
INDEX FOSSILS
Ex: Graptolites an
ancient plant alive 350-450
mya.
Ex: Trilobite an
ancient animal alive 500-600
mya.
Some Index Fossils
ABSOLUTE
Dating
Absolute
Dating
Determining the
actual dates for
events
 Gives ages of
rock in number
of years

Counting the Years

Tree rings (up to 10,000 yrs)
Varves
(yearly glacial layers)

How does this help us understand
the past?
Radioactive dating
The most accurate method for
determining age.
 Radioactive elements give of particles
and energy as they decay and new
elements form.
 Scientists know how long it takes for
them to wear down, so they are like
natural clocks.

Radioactive decay
100 kg
50 kg
0
1
25 kg
12.5 kg
2
3
Half-life: the time it takes for half the
radioactive atoms in a sample to decay
to a stable product
Absolute dating

Most absolute dating is done by
measuring the amount of…
– Carbon 14 left over in fossils of plants &
animals less than 50,000 years old.
– Uranium 238 left over in rocks older than
50,000 years old.
Isotopes Used in Radiometric Dating
Parent Isotope
Daughter Isotope
Half-life
(years)
Effective Range
(yrs)
Carbon-14
Nitrogen-14
5730
100-70,000
Once-living matter
Uranium-238
Lead-206
4.5 billion
>10 million
Uranium-bearing minerals (zircon)
Rubidium-87
Strontium-87
47 billion
>10 million
Micas, feldspars, metamorphic rocks
Potassium-40
Argon-40
1.3 billion
>50,000
The instrument that measures
radioactivity is a Geiger
counter.
Possible Materials for Dating
Micas, amphiboles, feldspars, volcanic
rocks
Bibliography
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http://www.geologyrocks.co.uk/geopics/image8.jpg
http://wrgis.wr.usgs.gov/docs/parks/gtime/
http://www.physicalgeography.net/fundamentals/images/Lyel
l.jpg
http://wwwrohan.sdsu.edu/~rhmiller/geologictime/GeologicTime.htm
http://web.utk.edu/~grissino/images/pemberton%20oak%20
01.jpg
http://www.kernchemie.unimainz.de/~pfeiffer/home_for_old_atoms.jpg
http://www.geo.arizona.edu/Antevs/ecol438/geochron.html
Modified by J Fehr & L Bell