Download 1_ Earth_s History - St. Raymond High School for Boys

Geologic Time
I. Principles for interpreting the geologic past:
 Relative age: age compared w/that of other rocks or events using such terms as older or younger.
 Absolute age: actual age of a rock or event given in years
A. Uniformitarianism: events that shape the earth’s surface today are the processes that occurred in the
geologic past.
B. Law of Superposition: in an undisturbed sequence of rocks, it’s assumed the bottom layer is the oldest,
the top layer the youngest.
C. Original Horizontality: sediments are generally deposited in horizontal layers, when they are lithified
(transformed into stone) form rocks in horizontal layers.
1. If sedimentary rocks are found at an angle, it’s assumed that they were deformed by tectonic events
(folding or faulting).
- The event would be younger than the top layer being deformed
II. Igneous formation
A.
1.
a)
B.
Extrusive: molten rock reaches the surface.
Extrusive igneous rocks will be younger than the rock below it & older than the rock layer above it.
Magma will cause contact metamorphism on rocks that are OLDER.
Intrusive: molten rock that cools below the earth’s surface.
1. It may squeeze its way between already existing layers
2. Intrusive rocks will be younger than the rocks that they intrude.
- You can tell the difference between intrusive & extrusive b/c intrusive rocks will be surrounded by
contact metamorphism; the extrusive will NOT have contact metamorphism.
III. Deformational Event
A. Fold-bend in the rock layers: may result in an older rock layer being on top of itself
B. Faults: crack in the earth’s surface where movement occurs; if there’s a great amount of offset (or
displacement) this may result in an older layer being above a younger layer.
C. Ersosional surface: layer of earth’s surface has been weathered & sediments were removed.
1. In a cross section it’s indicated by an uneven surface.
2. Before erosion can occur there must be a period of uplift & emergence (out of water)
D. Deposition: layer of sediments that’s been dropped by agents of erosion.
1. For this to occur there must be a period of submergence & subsidence (going down below water) b/c
deposition mostly occurs under water.
E.
1.
2.
3.
Unconformity: a gap in the geologic record.
may be caused by a period of erosion before the sediments are protected by overlying layers
may also be caused by a period of non deposition
an unconformity is identified by an uneven surface or wavy line
***
I. Geologic Time Line- ESRT p. 8-9
A. Divisions of geologic time
EONS-ERAS-PERIODS-EPOCHS
1. no two units are the same duration
ex. every minute is 60 seconds
 not every era is the same duration
2. Precambrian makes up 85% of Earth's past
-little or no fossil evidence because the few organisms that were alive were soft bodied=no bones, teeth or
shells
B. Life on Earth
1. Organisms have evolved throughout time
2. Charles Darwin- Theory of evolution: organisms that have traits that better adapt them to their environment
would survive longer and have more offspring to whom they would pass on these traits: Natural Selection
C. Rock Record in NYS
1. This column indicates by a dark solid line the presence of rock from that time period.
-The locations of the rocks found in NYS are give on page 3 in the ESRT
D. Distribution of Fossils
1. Each black line is labeled with the name of an organism.
- the length of the line indicates the period of time the organism was alive.
-the small letters on the line indicate the presence of a specific example of that species, and the picture is given
at the top of the ESRT.
E. Tectonic Events
1. This column lists the different types of plate boundaries that existed along the east coast of North America
II. Fossils-traces or evidence of past life.
A. Found in sedimentary rocks, not igneous and rarely metamorphic.
B. Reveals evidence of the environment that existed at that time.
Example- coral fossil in upstate NY=the area was once under water and a tropical climate
Mastodon in NY= the climate was much colder.
III. Outcrop correlation
A. process of showing that a geologic event or a rock layer from different locations were formed at the same time.
1. Similarity of rocks-specific characteristics
2. Walking the outcrop-follow the layer from one location to the next.
3. Volcanic time markers: when a volcano erupts it will deposit ash that covers a large area of land. When a
geologist finds the layer of ash they know exactly how old the rock is.
4. Index Fossils-used to assign an age to a rock layer containing a specific fossil
a. Requirements
-organism was alive for a short period of time which means that it will be found only in a few layers: small
vertical distribution
-organism was found at many locations: can be used to date and correlate rocks from different continents
large horizontal distribution
IV. Radioactive Dating: the use of the rate of decay of a radioactive element to assign an absolute age.
A. Decay- the spontaneous and natural breakdown of an unstable radioactive isotope.
1. radioactive-when a nucleus of an element has more or less than the normal number of neutrons.
Example- carbon 12 = 6 protons 6 neutrons
carbon 14 = 6 protons 8 neutrons
2. isotope -the same element but a different number of neutrons.
B. Results of Decay
1. Release of energy in the form of radiation
-this process is responsible for maintaining the high temperatures at the center of the Earth.
2. Produces a stable decay product or daughter product.
-a lighter more stable element
Example- C14 =>Energy + N14
K40=>Energy +Ar40+Ca40
C. Half life-the time required for half of the remaining radioactive isotope to change into a decay product.
1. Decay rate is constant and cannot be affected by environmental conditions.
2. The half life of an isotope never changes
Example- carbon 14-half life = 5.7 X103which is 5700 years
There are 3 others on the front of the ESRT p1
3.Decay product ratio-the ratio between the mass of a radioactive element and the mass of
the decay product.
Sample Problems:
1. If a 200g fossil was found and it had equal volumes of U238 and Pb206, How old is it?
How many half lives has it gone through?
2. A fossil of a trilobite was found and scientists want to determine the age,
Why can't they use C14?
3. An igneous rock was found that is thought to be older than the oldest rock.
Give two reasons why C14 could not be used?
4. A rock was found and it contained 45g of K40 and 315 g of Ca40,
How big was the original sample?
How many half lives has it gone through?
How old is this sample?
5. A rock sample has gone through 5 half lives.
What fraction has remained unchanged?
What fraction has turned onto decay product?
6. What is the half life of C14?
7. How old is a sample of C14 if it has gone through 5 half lives?
8. What is the decay product for Rb87?
9. How long will it take for 3/4 of the sample of Rb87 to turn into its decay product?
10. If a sample contains 28.125g of U238 and 871.875g of Pb206,
How many half lives has it gone through?
How old is this sample?
Regent Sample Questions:
1. If a rock is found and it contains 75g of K40 and 525g of Ca40,
How many half lives has it gone through?
How old is this sample?
2. If a sample has gone through 4 half lives,
What fraction remains unchanged?
What fraction has turned into decay product?
3. If a sample contains equal volumes of decay product and radioactive isotope
How many half lives has it gone through?
4. If a sample of K40 is 3.9 X10 9 years old, How many half lives has it gone through?