1. geologic time

2. time is critical for geologic processes
Rockies and Alps are ~3000 m tall
-- mountains grow at ~1 meter per 5000 yrs (0.2 mm/yr)
m x 5000 yr/m = 15,000,000 (yrs necessary)
Atlantic Ocean is ~5000 km across
-- today, seafloor spreading in Atlantic is ~4 cm/yr
km = 6000 km x 1000 m/km x 100 cm/m
= 600,000,000 cm
-- 600,000,000 cm / 4 cm/yr = 150,000,000 years
for comparison: fingernail grows at 1 cm/yr

3. age of the Earth
prior to 19th century, accepted age from religious beliefs
-- 6,000 years for Western culture (Christian)
…Bishop Usher from geneology in the Bible
-- old beyond comprehension (Hindu/Buddhist/Chinese)
-- age not certain (Islam)
during 19th century, length of time required for
geologic processes to occur was recognized
-- fundamental contribution of geology
to scientific knowledge

4. historical developments
James Hutton ( ) “Father of Modern Geology”
native of Edinburgh, Scotland
educated as a medical doctor in Leiden (1749)
passionate about scientific inquiry
“Theory of the Earth” -- processes are slow; take a long time
Charles Lyell ( )
Scotsman who attended Oxford University
father was an avid naturalist
rebelled against prevailing thought of “catastrophism”.
“Principles of Geology” -- popularized Hutton’s views
idea of “uniformitarianism” --
same processes operating today occurred in the past
….the present is the key to the past….

5. the key to the past relative time absolute time
relative time vs. absolute time
relative time
order of events or objects from first (oldest) to last (youngest)
she is older than he is; she was born first and he was born last
absolute time
age of events or objects expressed numerically
she is twenty-one and he is nineteen
study of timing of geologic events and processes is geochronology

6. relative time and relative order
apply simple concepts to determine…
• original horizontality
• superposition
• lateral continuity
• cross-cutting relationships
• inclusions
• unconformities

7. relative age dating concepts
original horizontality
all beds originally deposited in water formed in horizontal layers
sediments will settle
to bottom
and blanket
the sea floor

8. relative age dating concepts
superposition
youngest
within a sequence of undisturbed
sedimentary or volcanic rocks,
oldest rocks are at the bottom
and youngest at the top
….young upward…
lateral continuity
oldest
original sedimentary layers extend
laterally until they thin at edges
continue
continue

9. relative age dating concepts
cross-cutting relationships
a disrupted pattern is older than
the cause of the disruption
e.g. an intrusion is younger
than the rocks it intrudes

10. relative age dating concepts
inclusions
fragments of other rocks contained in a body of rock
must be older than the
host rock
e.g.
xenoliths in granite are older
than granite and
2) pieces of rock in
conglomerate are older
than conglomerate

11. relative age dating concepts
unconformities
a contact between sedimentary formations that represents a gap
in the geologic record -- “gap” represented is variable (i.e. amount of time or the amount of missing section)
different types of unconformities
conformity
• relatively continuous deposition
• deposition of a sequence of parallel layers
• contacts between formations do not represent significant amounts of time

12. conformity
from:

13. relative age dating concepts
different types of unconformities
angular unconformity
• contact separates overlying younger layers from tilted older layers
• sequence of layers
is not parallel
• contacts between formations
may represent significant
amounts of time
angular unconformity

14. angular unconformity
from:

15. angular unconformity

17. relative age dating concepts
different types of unconformities
disconformity
• contact separates beds (formations) that are parallel
• sequence of layers
is parallel
• contacts between
formations
may represent significant
amounts of time
• missing time is difficult to recognize (may need other
information--paleosol?)

18. relative age dating concepts
different types of unconformities
nonconformity
• strata deposited on older crystalline (metamorphic/igneous) rock
• erosion surface on igneous/metamorphic rock covered by
sedimentary rocks
• large gap in
geologic record
nonconformity

19. what events occur?
angular unconformity

20. what events occur?
nonconformity

21. now that we know all this…what happened?

23. deposition

24. intrusion

25. tilting and
erosion

26. subsidence
and
renewed
deposition

27. missing formation (time)?

28. dike intrusion

29. erosion and exposure

30. subsidence and deposition

31. uplift/sea level fall and river deposition

32. relative ages of the formations

33. relative age: correlation
correlation -- determining time equivalency of rocks within a region, between continents, etc.
how is this done?
physical continuity
physically following a continuous exposure of a rock unit
--most direct; easily done in some locations, not in others
e.g. within the Grand Canyon
lithologic similarity
assuming similar sequences of rocks formed at same time
-- inaccurate if common rocks are involved
e.g. the Grand Canyon and Zion National Parks

34. physical continuity -- Coconino Sandstone in Grand Canyon

35. lithologic similarity -- Coconino and Navajo Sandstones

36. lithologic similarity -- Coconino and Navajo Sandstones
Navajo is much younger!

37. relative age: correlation
how is this done?
faunal succession (correlation by fossils)
fossil species succeed one another through the layers
in a predictable order
index fossil
short-lived organism;
points to narrow range
of geologic time
fossil assemblage
group of fossils
associated
together

40. use of index fossils/fossil assemblages permits global correlation
similar units found in India, Africa, S. America, Australia, Antarctica.

41. established initially as a
relative scale using
sedimentary rocks
and fossils
absolute ages
were determined later
with
radiometric dating

43. absolute time natural clock is necessary -- radiometric dating
(nuclear clock: decay of radioactive isotopes)
-- dendrochrolonology
-- astronomical methods

44. age of the Earth early methods: long debated
• 1625: Archbishop Usher determined Earth was created in 4004 B.C.
by counting generations in the Bible
• Hindus regarded Earth as old: 2000 A.D. is 1.97 million years
according to Hindu calendar
• 1866: Lord Kelvin calculated age by assuming that Earth was
molten and cooled to a solid; age between million years old.
- did not know about radioactive decay (makes heat)
- assumed all heat dissipated by conduction
early isotopic methods (radioactivity known in 1896)
• 1905: first crude estimates yielded 2 billion year age
• meteorites gave dates of 4.5 to 4.6 billion years old
• modern uranium/lead methods yield values of 4.55 billion years

45. radioactive isotopes have nuclei that spontaneously decay
-- emit or capture subatomic particles
parent: decaying radioactive isotope
daughter: decay daughter
parent
daughter
loss or gain
loss or gain of neutron converts parent to daughter of same element
loss or gain of proton changes parent into entirely new daughter

46. electron capture (e- + p+ = n0)
3 primary ways of decay
alpha decay (Z ≥ 58)
particle has 2 neutrons and 2 protons
U238 Th234
92 protons 90 protons
beta decay (n0 = p+ + e-)
breakdown of neutron into an
electron and a proton and loss
of the electron to leave a proton
(result is gain of one proton)
K40 Ca40
19 protons 20 protons
electron capture (e- + p+ = n0)
capture of an electron by a proton
and change of proton to neutron
(result is loss of proton)
K40 Ar40
19 protons 18 protons

47. radiometric dating
uses continuous decay to measure time since rock formed
only possible since late 1890’s -- radioactivity discovered in 1896
as minerals crystallize in magma;
they trap atoms of radioactive isotopes in their crystal structures
radioactive isotopes will decay immediately and continuously
as time passes, rock contains less parent and more daughter

48. half-life amount of time it takes for half the atoms of the
parent isotope to decay
different radioactive isotopes have different and
distinct half-lives
if rock has 12 parents and 12 daughters--ratio of 1:1
…original rock had 24 parents and one half-life has elapsed…
…after another half life, rock will have 6 parents and 18 daughters…
…ratio of 1:3---note that total number (24) remains the same
regardless of isotope, the ratio of parent to daughter atoms
is predictable at each half-life

49. predictable ratios at each half-life
exponential decay (half always remains)

50. exponential decay: never goes to zero
linear

51. (each has its own half-life)
example: Uranium 238 decay to Lead 206 (stable)
several steps
(each has its own half-life)

52. most common dating systems
• uranium-thorium-lead dating (previous example)
U-238, U-235, Th-232
each of these decays through a series of steps to Pb
U-238 to Pb-206 half-life = 4.5 by
U-235 to Pb-207 half-life = 713 my
Th-232 to Pb-208 half-life = 14.1 my
• potassium-argon dating
…argon is a gas--may escape
(ages too young--daughter missing)
K-40 to Ar-40 half-life = 1.3 by
• rubidium-strontium dating
Rb-87 to Sr-87 half-life = 47 by

53. basic geochronological assumptions
decay constants constant through geological time
-- good reasons to believe this is correct from nuclear physics
-- measurements of decay sequences in ancient supernovae
yield the same values as modern lab measurements
system closed to adding or subtracting of parent/daughter
-- isotopic system and type of mineral (rock) are important
-- careful procedure is essential to correct analysis
igneous rocks are most reliable for dating
…metamorphism may cause loss of daughter products…
…sedimentary rocks will give ages of source rocks…

54. Instruments and Techniques
Mass Spectrometry: measure different abundances of specific nuclides based solely on atomic mass.
Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g. 87Sr and 86Sr).
Count individual particles using electronic detectors.
TIMS: thermal ionization mass spectrometry
SIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laser
Sample Preparation: TIMS requires doing chemical separation using chromatographic columns.

55. Clean Lab - Chemical Preparation

56. Thermal Ionization Mass Spectrometer
From:

57. Schematic of Sector MS

58. Zircon Laser Ablation Pit

59. Rate Law for Radioactive Decay
Pt = Po exp - (to –t)
1st order rate law

60. Rb/Sr Age Dating Equation

61. Rb/Sr Isochron Systematics

62. Independent Checks on Radiometric Ages
Correlation of erosion with age on Hawaiian Island Chain: Dates increase in age to the NW as does erosion.
Annual growth bands in Devonian corals: 400/yr yields date that is similar to radiometric date. Consistent with slowing of Earth rotation with time.
Independent determination of Pacific plate motion yields age progression that is consistent with K/Ar dates of the island chains formed by “hotspots”.
Agreement between magnetic “age” from deep marine sediments and radiometric ages of tuffs in East African Rift

63. Other dating methods: dendrochronology
annual growth of trees produces concentric rings
…dating back to 9000 years is possible…
rings need to be calibrated
against C-14 dates to yield
“true” numerical age
other information may also
be obtained from rings,
including rainfall and temperature
can develop composite
chronologies for specific regions
of interest for climate studies
photo © H.D. Grissino-Mayer

64. relative and absolute dates combined
same example
as in
relative age

65. geological time scale eons, eras, periods, epochs
Oldest rocks: Greenland gneisses
Oldest rock fragments: W. Australia detrital zircons

66. earliest life cyanobacteria: primitive single-celled organisms
found in Australia and dated at 3.7 billion years old
modern equivalents in
Shark’s Bay, Australia

67. proportional time scale

68. combine relative and absolute time for geologic time scale