Stratigraphy and Geologic Time Flashcards
Why document Earth history?
The past may be the key to the future: changes occur in cycles, patterns repeat
How has the Earth changed?- e.g.CO2 levels, climate, sea level, landmasses, biological evolution & extinctions, Wilson cycles
HOWEVER: Almost all of Earth’s history predates humans
How is Earth history documented?
The Rock Record
Sediments laid down in layers (strata) form a….
stratigraphy of events: a record of earth processes over geological time
what are the main concepts for reading the stratigraphic record?
1) Beds (strata, sedimentary units) – horizontal – bound by bedding planes
2) Multiple beds make up a stratigraphic ‘sequence’ - bound by erosional episodes due to fluctuations in sea-level, uplift
3) Strata contain fossilised flora and fauna
4) The rock record is incomplete at any one place (gaps in time)
what is biostratigraphy?
Fossils
Used in correlation of strata
Occur over a stratigraphic range
Organisms: also provide info on environment–some organisms tolerate very wide environmental conditions; others do not. e.g., different trilobite species: bottom dwellers vs. floaters vs. swimmers
what are the two ways to view geologic time?
(1) Relative time
- dating by sequence of events
- “older” vs. “younger”
- placed in order
(2) Absolute time
- numerical dating using radioactive decay in minerals
what are the principles of relative age dating?
1) Superposition
2) Original horizontality
3) Faunal (& floral) succession
4) Inclusions
5) Cross-cutting relations
what is the principle of superposition?
In an undisturbed stratigraphic sequence, the rocks at the top are youngest
what is the principle of original horizontality?
Sedimentary layers are deposited in horizontal units/beds
Inclined layering -> layers were tilted from initial horizontal orientation some time after deposition
what are unconformities?
large gaps of time between deposition of layers (strata)
what is the principle of faunal (& floral) succession?
Sedimentary layers contain fossilized flora & fauna
Organisms succeed each other vertically in a specific, reliable order -> fossil record
Rocks with similar fossils are (generally) of similar age e.g., Neanderthal bone (young) never found in same strata as a T-Rex (much older)
what are some useful index fossils?
dinosaurs
trilobites
what is the principle of inclusions?
Older rocks “included” in younger rocks – e.g., blocks eroded from country rock by intruding magma
what is the principle of cross-cutting relations?
Older rocks are “cross-cut” by younger rocks or features (e.g., dykes, faults, erosion surface)
what is radioactive decay?
Many atoms decay spontaneously Decay produces energy (heat) & stable daughter products Solved 2 problems: (1) An ongoing source of heat for Earth (2) Absolute dating of igneous rock (discovered by Becquerel in 1896)
