INTERPRETING GEOLOGIC HISTORY
I. Principle of uniformity
A. Clues of the Past
1. Perserved by rock
2. Geologist study rock & infer events of the past
B. Uniformatarianism
1. Events that happened in the past, happen the same way today.
II. The Law of Superposition
A. In undisturbed rock
1. the bottom layer is the oldest
2. the top layer is the youngest
3. This allows geologist to determine relative age
B. Exceptions
1. various crustal activities can cause younger rock to be below older rock
2. rock is always older than the process that changes it
ex.
C. Process
1. Extrusions ­
a. younger than the rock below it
2. Intrusions
a. younger than all rock layers around it
b. creates contact metamorphisms with the rock that it touches
3. Folds ­ sometimes overturn rock layers
4. Faults ­ can push older layers above younger layers
5. joints
a. cracks in the rock
b. may be in areas of movement
c. can be caused by compression or shrinkage which happens when rocks cool or break
Title: May 17 ­ 9:37 AM (1 of 7)
6. veins
a. molten rock is injected into a crack and cools b. rock melts in place
c. groundwater solutions are deposited and evaporate in cracks forming solid veins
7. Natural cements
a. help transform soft sed. rock into hard sed. rock
b. ex. silica and calcite cements
D. Rock fragments within Rock
1. sed. rock is composed of weathered and eroded older rock
2. Xenoliths ­ pieces of rock are picked up by lava flows, so you have igneous rock surrounding older rock
E. Fossils
1. any naturally preserved remains or impressions of living things
2. generally found in sed. rock
3. whole organisms can be preserved in amber,tar or ice
4. generally soft tissue is destroyed hard tissues like bone, teeth and shells are preserved
5. Pretified ­ when hard parts of organisms are replaced by minerals
6. impressions in mud
7. fossilized dung help to determine eating habits
8. fossils reveal information on climate
III. Correlations
A. Correlations 1. Trying to determine if similar rock strata in different locations were laid down at the same time
Title: May 17 ­ 9:58 AM (2 of 7)
B. Different ways to correlate rock strata
1. Walking the outcrop
a. CORRELATION BY CONTINUITY
b. the outcrop must be exposed
c. then follow a layer from one end to the another
d. you can then match rock types at each end
*this is not perfect because the same rock could have been made at different times
*ROCK LAYERS COULD HAVE BEEN ERODED
e. comparative layers are better, but not perfect
* comparing sequences of rock
2. Time correlations
a. index fossils ­ organisms that lived for a short period (20,000,000) yrs.
of time over a large area. Therefore fossils would be in a very narrow vertical space, but in a large horizontal space.
b. index event ­ volcanic eruptions, ex. Tambora, Kracatoa, Mt. St Helen
c. volcanic ash ­ falls for a relatively brief time over a large area
IV. Geologic time scale
Title: May 17 ­ 10:23 AM (3 of 7)
A. Formation
1. A mappable unit of rock of uniform age and composition
2. Can be identified by fossils
3. Formations help establish relative time scale with sequence from oldest to youngest
4. Formation often named for a location where it's characteristic
fossils can be observed.
ex. Devonian fossils 1ST observed in Devonshire, England
5. Worldwide formations have been used to establish geologic
time scale, showing major events in geologic history
6. Divisions of scale
a. eon
b. era
c. period
d. epochs
7. Pages 8­9 can be used in conjunction with page 3
8. orogeny ­ mountain building events
V. Evolution of life
A. gradulism
B. Punctuated equilibrium
C. Earliest rock layers (deepest)
1. Lack fossil evidence
a. no fossils in oldest layers (Precambrian)
b. so probably no life ??????
2. Why are there no fossils for the earliest periods?
D. The theory of organic evolution
1. The older the fossil the less complex
2. some organisms develop to a point & dissappear (extinction)
3. Organisms with traits that are suitable for survival­> continue to
live
Title: May 18 ­ 10:08 AM (4 of 7)
4. Natural selection leads to some extinction of species & the formation of new ones 5. fossil records show variation of trait over time
6. some fossil records are lost, how?
7. Humans evolved over the past 4,000,000 years or about 1% of the
earths age.
VI. Geologic events of the past
A. Unconformity
1. A gap in the geological record caused by erosion of sediments or fossil containing rock before they are protected by layers above
2. no area has a complete geological record
3. geological events can cause change in the rock record
4. Geologist can study outcrops to determine geologic history
5. all of these techniques only give relative age
VII. Radioactive dating ( the ultimate hot date)
A. Absolute dating (measured in years)
1. Geological records only give relative time scale
2. measurement of natural radioactivity in the rock has allowed the geological time scale to become an absolute time scale
Title: May 18 ­ 10:34 AM (5 of 7)
B. Decay Chains
1. isotopes
a. elements with the same number of protons,but different number of neutrons
b. C12 and C14 c. Isotopes are radioactive
d. Isotopes break down naturally to form lighter elements called decay products
e. Decay is random but sample may contain millions of atoms
C. Half lifes
1. The rate of decay of radioactive elements is measured in 1/2
lifes
2. 1/2 life ­ the time required for 1/2 of the elements atoms to change to decay products
3. Always after one 1/2 life ­> 1/2 the sample is radioactive and
1/2 the sample is stable decay
product
4. The older the sample the more decay product it will have
D. Decay product ratio
1. Ratio between the mass of the decay product & radioactive
element
2. After the ratio has been determined the number if 1/2 lifes
can be determined
3. ex., If the sample of materials contains equal amounts of C14
& N14 the the sample has undergone one 1/2 life
4. No enviromental factors effect 1/2 lifes Title: May 18 ­ 11:51 AM (6 of 7)
E. Selecting the best radioactive element for a sample
1. first you must select the right radioactive elements
a. based on sample origin
b. based on quantity of elements in sample
2. Second the sample estimated age
a. less than 50,000 years ­> C14
b. more than 50,000years ­> U238
Title: May 18 ­ 12:07 PM (7 of 7)