GEOLOGIC TIME

Question 1

2 dating techniques geologists use to

Answer 1

• relative ages = geologic events placed in chronological order based on their position in the geologic record
• absolute ages = ages calculated for geologic events

Question 2

examples of geologic events in earths history

Answer 2

• deposition of sediment and formation of rock strata
• erosion of crust such that a gap is formed in the record of past events
• intrusion of igneous rocks
• faulting of crust rocks
• deformation of crust rocks

Question 3

principles used to determine the relative age of geologic events

Answer 3

1. principle of uniformitarianism - physical processes observed today operated in the same way as geological past
2. principle of superposition - in a sequence of layered rocks, the oldest lies on bottom and youngest on top
3. principle of original horizontality - most sedimentary rocks were originally horizontal so if tilted its due to subsequent geological event
4. principle of original lateral continuity - sedimentary beds originally laterally continuous so if in separate units its due to erosion
5. principle of cross cutting relations - younger features cut across older ones
6. principle of inclusions - an older rock fragment within another
7. principle of baked contacts - thermal metamorphism occurs when country rock is invaded by a plutonic igneous intrusion - baked rock must be older

Question 4

what 2 assumptions does fossil succession depend on

Answer 4

1. no two species are exactly alike
2. species never reappear once extinct

Question 5

whats an index fossil

Answer 5

• distinctive
• wide geo distribution
• restricted to a narrow time interval
• provides reliable correlations between stratigraphic units

Question 6

whats an unconformity

Answer 6

a time gap in the rock record
- non deposition or soil formation
- erosion

Question 7

3 types of unconformities

Answer 7

• disconformity - parallel strata bracketing nondeposition - interruption in sediment
• nonconformity - sedimentary strata overlay metamorphic / igneous rocks
• angular unconformity - rocks below the unconformity were folded / tilted

Question 8

2 types of stratigraphic correlation used by geologists

Answer 8

• lithologic correlation - based on rock type - limited to nearby regions
• fossil correlation - based on fossils within rocks - can apply to much broader areas

Question 9

4 time blocks that the geologic column is divided into

Answer 9

• eons (largest subdivision of time)
• eras
• periods
• epochs
  *each one is a subdivision of the one above it

Question 10

what era makes up the majority of earths geological history

Answer 10

Precambrian (88%)

Question 11

significant geological events on earth

Answer 11

• 3.8 Ga : life first appear on earth - anaerobic single celled orgs
• 2 Ga : oxygen from cyanobacteria
• 700 Ma : evolution of multicellular life
• 542 Ma : hard shells
  ___precambrian - Cambrian boundary___
• cambrian explosion

Question 12

names of the eons

Answer 12

• Hadean - “hell” (oldest): formation of ocean + second atmos
• Archean - “ancient”: continents + early life formed
• Proterozoic - “ealrier life” : tectonic plates + atmospheric O2 for multicellular life
• Phanerozoic - “visible life”: Cambrian boundary + hard shells

Question 13

names of the ears

Answer 13

• paleozoic - “ancient life”: life diversified rapidly
• mesozoic - “middle life”: age of dinosaurs
• cenozoic - “recent life”: age of mammals

Question 14

relative vs absolute age

Answer 14

• relative = chronostratigraphic (but doesn’t tell us how long ago they occured)
• absolute = numerical age before present - most common used is radiometric age dating (use of radioactive decay to date rock)

Question 15

isotopes

Answer 15

same protons, different neutrons
- unstable ones decay overtime

Question 16

what is radiometric dating

Answer 16

measure amount of parent + daughter isotopes to determine age

Question 17

what is half time when referring to isotopes

Answer 17

time it takes for half of unstable parent atom to decay to stable daughter atoms
- happens exponentially

Question 18

how does radiometric dating vary with different rocks

Answer 18

• best rock type for radiometric dating = igneous rocks - contains both parent and daughter isotopes
• metamorphic rocks = can be tricky - drives daughter isotopes out as it recrystalizes
• ## sedimentary rocks = not possible - its grains derive from weathering of pre existing rocks, not the sedimentary rock itself

Question 19

other ways to determine numerical ages without radiometric dating

Answer 19

• growth rings - trees, shells
• rhythmic layering - annual layers in ice / sediment
• magnetostratigraphy - compare megnetic signitures in strata to global reference scale

Question 20

3 oldest rocks on earth

Answer 20

• Acasta Gneisses
• Issua supracrustal belt
• Single zircon crystals

Question 21

how was age of earth determined

Answer 21

4.57 Ga
- based on correlation with moon rocks + meteorites