GEOlogic TIME GEO 2
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Birth of Modern Geology
- A. JAMES HUTTON
- B. UNIFORMITARIANISM- the present is the key to the past
Relative Dating (Placing events in order of occurence)-Key principles
- A. LAW OF SUPERPOSITION
- 1. In an undeformed sequence of sedimentary rocks, each bed older than the one above and younger than the one below it.
- B.PRINCIPLE OF ORIGINAL HORIZONTALLY
- 1. Layers of sediment are deposited in a horizontal position
- 2. If rocks are folded or steeply dippin, they must have been moved to that position after deposition
- C. PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS
- -fault or igneous intrusion cuts across rock layers is younger than the rocks it disrupts
- 1. Pieces one rock contained within another rock
- 2.Inclusions are always older than the rock they are contained in
- a. Xenoliths-inclusions in igneous rocks
- b. Clasts-inclusion in sedimentary rocks
- E. UNIFORMITIES
- 1. Comfortable sequence-sequence of rocks have been deposited w/out interruption
- 2. Unconformities
- a. Buried surfaces of erosion or nondeposition
- b.Time during which depostion stopped, erosion occured, then deposition resumed
- 3. Types of unconformities
- a. Angular unconformity- Erosional surface developed on titled or folded rocks overlain by younger, flat lying strata.
- b. Disconformity- Strata on either side of the unconformity are essentially parallel
- c. Nonconformity-Unconformity that separates underlying igneous or metamorphic rocks from overlying younger sedimentary rocks
- F. PRINCIPE OF FOSSIL SUCCESSION
- 1. Fossils - remains of ancient animals and plants
- 2.Types of fossils and fossilization
- a. Body fossils
- i. Petrification-small internal cavities are filled with precipitated minerals
- ii. Mold- shell is buried, then dissolved by groundwater
- iii. Cast -mold is filled in with mineral matter
- b. Trace fossils- tracks, trails, burrows, etc.
- 3. Conditions favoring preservation
- a Rapid burial
- b. Perservable hard parts
- 4. Principe of fossil succession
- a. Fossil organisms succeed one another in a definite order and anytime period may be rec by its fossils
- b. Fossils document the evolution of life on earth
- c. Index fossils
- i. Widespread geographically
- ii. Lived for a short interval of geologic time
- iii. Easily recognizable
Absolute Geologic Time
- A. PARENT AND DAUGHTER PRODUCTS OF RADIOACTIVE DECAY
- 1.Parent-unstable isotpe
- 2. Daughter product-result from radioactive decay
- B.HALF LIFE-TIME IT TAKE IN YRS FOR ONE HALF OF PARENT TO DECAY TO DAUGHTER
- C.EXAMPLES OF PARENT, DAUGHTER, AND HALF LIFE
- 1. C14=>N14 5730+/-30 YEARS
- 2.U235=>PB207 713 MY
- 3.U238=>PB206 4.56 BY
CORRELATION OF ROCK LAYERS
- A. MATCHING ROCKS OF SIMILAR AGE IN DIFFERENT REGIONS
- B. OVER LIMITED AREAS, ROCKS MAY BE MATCHED BASED ON ROCK TYPE, POSITION IN A SEQUENCE OF ROCKS.
- C. OVER LONG DISTANCES , FOSSILS ARE USED FOR CORRELATION
THE GEOLOGIC TIME SCALE
- A. STRUCTURE OF THE TIME SCALE
- 1. Eons- largest time units
- a. Precambrian- reps about 88% of geo time
- b. Phanerozic Eon- "visible life"
- 2. Eras-eons are divided into eras
- a.Phanerozoic Eon
- i. Paleozoic Era- "ancient life"
- ii. Mesozoic Era-"middle life"
- iii. Cenozoic Era- " recent life"
- 3. Periods-eras divided into periods
- 4. Epochs- periods are divided into epochs
- b. Eons
- i. Eras
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