Geologic Time

1
Geologic Time

• Geochronology the study of the timing of
  geologic events and the ages of geologic
  materials
• Relative dating the space/time relationship
  between two or more geologic events or units
• Numerical dating absolute time or measured
  dates through geochemical analysis

2
Principles of Relative Age Dating

• Uniformitarianism James Hutton The present is
  the key to the past.
• Horizontality/Superposition Layers were
  originally laid down horizontally with oldest at
  bottom/youngest at top
• Cross-cutting relationships any intrusive
  formation must be younger than the rocks it
  intrudes or cuts.
• Inclusions must be older material picked up
  during intrusion
• Faunal succession there is a direct link
  between the procession of fossils and time (e.g.
  once a plant/animal becomes extinct it is never
  seen again)

3
Unconformities

• Gaps in geologic record forming boundaries that
  separate rocks of different ages.
• Non-conformity (pg. 251) boundary between
  unlayered intrusion and overlying sedimentary
  rocks
• Angular unconformity (Fig. 9.6) nonconforming
  boundary between horizontal rocks that are
  rotated and overlying rocks that were deposited
  horizontally
• Disconformity (pg. 251) boundary between
  parallel layers of sedimentary rocks

4
Great Unconformity of the GC(angular
unconformity)
5
Great Unconformity of GC(nonconformity Tapeats
SS over crystalline basement)
6
Great UnconformityBox Canyon Ouray, Colorado
7
Disconformities in GC(gaps between sedimentary
layers)
8
Correlation

• Rocks that are of the equivalent age but are
  separated spatially (Fig.9.9)
• Fossil evidence biota of same age
• Key beds short duration event over a wide area
  (Ft. Apache Lm. In Sedona)
• Facies change within a unit

9
Stratigraphic Sections
10
Correlating Stratigraphic Sections

• Vertical measurement of rock unit thickness
• Section is a hypothetical column of rock units
  for a particular area
• Multiple sections can be correlated by drawing
  lines between similar or identical beds or units
• Facies change can be identified if type of rock
  between correlated units changes between measured
  sections

11
Unit Correlation
12
Stratigraphic Unconformities
13
Numerical Dating Techniques

• Isotope dating rate of decay for radioactive
  isotopes uses known rates to measure the time
  elapsed since mineral formation
• Half life time it takes half the atoms of the
  parent isotope to decay into daughter isotopes
  (Fig.9.15)
• Typically more useful for igneous rocks
• Most reliable in closed system
• Limited by time
• Uranium-thorium-lead 10million to 4.6 billion
  only for oldest rocks
• Potassium-argon 100,000 to 4.6 billion very old
  and very young dates
• Rubidium-strontium 10 million to 4.6 billion
  half-life only rocks gt 10my
• Carbon 14 good for lt 100,000 yrs old

14
Radioactive Parent Stable Daughter Half life

Potassium 40 Argon 40 1.25 billion yrs 1.25 billion yrs

Rubidium 87 Strontium 87 48.8 billion yrs 48.8 billion yrs

Thorium 232 Lead 208 14 billion years 14 billion years

Uranium 235 Lead 207 704 million years 704 million years

Uranium 238 Lead 206 4.47 billion years 4.47 billion years

Carbon 14 Nitrogen 14 5730 years 5730 years
15
Parent/Daughter Ratio
16
How does Carbon-14 dating work?

• Cosmic rays from the sun strike Nitrogen 1 4
  atoms in the atmosphere and cause them to turn
  into radioactive Carbon 14, which combines with
  oxygen to form radioactive carbon dioxide.
• Living things are in equilibrium with the
  atmosphere, and the radioactive carbon dioxide is
  absorbed and used by plants. The radioactive
  carbon dioxide gets into the food chain and the
  carbon cycle.
• All living things contain a constant ratio of
  Carbon 14 to Carbon 12. (1 in a trillion).
• At death, Carbon 14 exchange ceases and any
  Carbon 14 in the tissues of the organism begins
  to decay to Nitrogen 14, and is not replenished
  by new C-14.
• The change in the Carbon 14 to Carbon 12 ratio is
  the basis for dating.
• The half-life is so short (5730 years) that this
  method can only be used on materials less than
  70,000 years old. Archaeological dating uses this
  method.) Also useful for dating the Pleistocene
  Epoch (Ice Ages).
• Assumes that the rate of Carbon 14 production
  (and hence the amount of cosmic rays striking the
  Earth) has been constant (through the past 70,000
  years).

17
Geologic Time Scale(Fig. 9.17)Read Box 22.3

• Eons
• Eras
• Cenozoic Age of mammals, early primates, Ice
  Ages, man (3.4-3.8mya), spread of modern humans
  last 10,000yrs.
• Mesozoic, age of dinosaurs, early flowering
  plants, formation of the Rocky Mountains
• Paleozoic, - Mt. Building in N.A. and Eur., early
  land plants, coal forming swamps
• Precambrian 4.6by Earth forms, 1.5by Pangaea
  forms, .5by first multi-celled organisms

18
Periods Breakdown of Eras

• Paleozoic
• Permian
• Pennsylvanian
• Mississippian
• Devonian
• Silurian
• Ordovician
• Cambrian