Geologic
History
Ms. Susinno’s favorite topic
Please view as a slideshow
GEOLOGIC TIME scale
http://www.nhm.ac.uk/nature-online/life/dinosaurs-other-extinct-creatures/mass-extinctions/end-permian-mass-extinction/
ESRt Challenge!
Hint: you need many pages of the ESRT!
Meet My friend “eury”!
The Anthropocene is a proposed geologic
chronological term for an epoch that begins
when human activities have had a significant
global impact on the Earth's ecosystems.
http://www.smithsonianmag.com/science-nature/what-is-the-anthropoceneand-are-we-in-it-164801414/?no-ist
Reading the geo time scale
1. What is the oldest living organism?
2. When did oxygen enter the atmosphere?
3. Explain how this might have happened.
4. How long have humans been on Earth
compared to Earth’s history?
5. If Earth’s history were compared to one day
(24 hours) how long do you think man would
have been here?
GEOLOGIC TIME scale
Geologic History
stromatolites
How and why do geologists date rocks?
Geologists date rocks using:
Relative age- older and younger
Actual age- rock layer can be assigned a
time period or definite numerical age.
Geologists make Some assumptions about
strata In order to interpret rock layers:
1. Uniformitarianism- “the key to the present is
the past ”~James Hutton
This idea says that Earth’s shaping processes are basically
the same as they have been in the past (earthquakes,
volcanoes, deposition and erosion) and can help interpret
the present rock formations.
2. The principle of Original Horizontality- Sediment is usually deposited in
flat layers. If it is no longer flat, something happened to it!
3. Principle of Superposition- the bottom layer of a series of sedimentary
layers is usually the oldest
**unless it has been overturned (from folding) or older rock has been thrust
over it (from faulting)
Relative
Dating
What is an unconformity?
Unconformity a buried erosional surface representing a
gap in the sedimentary rock record. They are usually
represented by
in Earth Science diagrams.
What’s
missing?
Rock
layer(s)
and
Time
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm and
http://www.bioygeo.info/Animaciones/Unconformity.swf to see an unconformity form.
Oldest
1. Deposition of layer 1
2. Deposition of layer 2
3. Deposition of layer 3
4. Intrusion of 4 with contact
metamorphism
5. Weathering and erosion of 3 and 4,
=unconformity
(an unconformity only happens if there is submergence and
deposition on top, otherwise it is just an exposed erosional
surface)
6. Deposition of layer 5
Youngest
List the sequence of events from oldest to youngest
1st deposition of shale
2nd deposition of sandstone
3rd deposition of limestone
4th weathering and erosion
of limestone = unconformity
5th deposition of sandstone
6th igneous intrusion with
contact metamorphism
Follow this example:
How do you know the sandstone (step4) and
the shale came before the intrusion?
(on the next slide)
on the next slide
Do not draw on your monitor 
82. Siccar Point
Aug 2009
List the sequence of events from oldest to youngest.
Include time period (and range in years) for each layer.
Actual
Dating
What are ISOTOPES?
This is how many protons rubidium has
Atoms with the same
number of protons, but
different numbers of
neutrons.
This is protons + neutrons.
The number of protons
defines an atom.
Isotopes are unstable and
“decay” into other eleMents.
The number of protons
can change in a few
different ways:
When the # of protons
(atomic number)
changes the atom
becomes a new element!
Watch me to see a cartoon of
radioactive decay:
https://youtu.be/o-9yt7OAYmE
Radioactive Elements used in
Actual dating:
Half-life- the time it takes for one half of the molecules
in a sample to decay to the stable isotope.
What is
the half
life of
Carbon
14?
*After about 5 half-lives, the amount of the original material is
too negligible to be detected and therefore, cannot be tested.
Carbon 14 is used to date things that were once alive because when they
die, they stop producing the isotope and it begins to decay to Nitrogen 14.
How many
Half- lives?
How much of the
original left?
Original:Decay
Product
1 half life
½ (.5) 50%
1 part Original:1 part
Decay Product
2 half lives
¼ (.25) 25%
1:3 (1 quarter : 3
quarters)
3 half lives
1/8 (.125) 12.5%
1:7
4 half lives
1/16 (.0625) 6.25%
1:15
5 half lives
1/32 (.03125) 3.125%
1:31
Original
Decay Product
PRACTICE:
a. Fred the poor dead fish kicked the
bucket 11, 460 years ago, how many
half –lives have passed?
b. How much carbon14 remains?
c. How much of the 14N is left after that
many half- lives?
1. I found a rock that was 4.5 Billion years old.
a. How much 238U remains in it?
b. How many half –lives have passed?
c. How much of the Lead208 is left after that many
half- lives?
***If I cut the rock in half, what will its half life
be?
2. I found a rock which was 3.9 x 109 years
old. How much 40K was left?
a. How many half –lives have passed?
b. How much of the Potassium 40 is left
after that many half- lives?
3. An igneous rock contains 10 grams of 40K and 10
grams of its decay products. During which
geological time interval was it formed?
1. Middle Archaen 2. Late Archean
3. Middle Proterozoic 4. Late Proterozoic
5. According to the Earth Science Reference Tables, which
radioactive element formed at the time of the Earth's origin
has just reached about one half-life?
1. carbon-14
2. potassium-40
3. uranium-238
4. rubidium-87
6. A sample of rock contained 100 grams of potassium-40
(40K) when it was formed. Today the rock contains 50 grams
of potassium-40 (40K). According to the Earth Science
Reference Tables, what is an approximate age of the rock?
1.4 x 109 years
2.8 x 109 years
4.5 x 109 years
5.6 x 109 years
Radiocarbon Dating
Radioactive carbon-14 (C14), because of its short half-life, is used for the
absolute dating of organic remains that are less than 70,000 years old.
Carbon-14 is an isotope of carbon that is produced in Earth’s upper
atmosphere.
High-energy cosmic rays from the Sun hit nitrogen-14 (N14), producing
radioactive
C14. This C14 is unstable and will eventually change back into N14 through
the process of radioactive decay. The proportions of C14 and ordinary C12
in Earth’s atmosphere remain approximately constant.
Radioactive C14, just like ordinary C12, can combine with oxygen to make
carbon dioxide. Plants use CO2 during photosynthesis. The proportion of
C14 to C12 in the cells and tissues of living plants is the same as the
proportion of C14 to C12 in the atmosphere. After plants die, no new C14 is
taken in because there is no more photosynthesis. Meanwhile, the C14 in
the dead plant keeps changing back to N14, so there is less and less C14. The
longer the plant has been dead, the less C14 is found in the plant. The age
of organic remains can be found by comparing how much C14 is still in the
organic remains to how much C14 is in a living organism.
Radioactive C14 was used to determine the geologic age of old wood
preserved in a glacier. The amount of C14 in the old wood is half the normal
amount of C14 currently found in the wood of living trees. What is the
geologic age of the old wood?
State one difference between dating with the radioactive isotope C14 and
dating with the radioactive isotope uranium-238 (U238).
The End!
Helpful Links:
Help w/ pgs8-9 ESRT:
https://www.youtube.com/watch?v=y9LhrG5b8CM
and
https://youtu.be/arz-r0weWXg and https://youtu.be/5fbx4TBIZPE
Half-life animation:
http://www.absorblearning.com/media/attachment.action?quick=185&att=3167
How does Carbon dating work: https://youtu.be/phZeE7Att_s
Unconformity animations:
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm and
http://www.bioygeo.info/Animaciones/Unconformity.swf and
http://www.wwnorton.com/college/geo/egeo2/content/animations/10_4.htm
Up next- our last topic:
Fossils!