GEOLOGIC TIME (abbreviated versions of overheads)
Geochronology - study and measurement of time as it relates to the history of the Earth.
Relative time scales - defined by sedimentary rock sequences and their included fossils; arranged events in order of occurrence.
Absolute time scales - defined as the actual number of years = old.
Time scales of geological processes
Event | Time Unit | Time in yrs |
Earthquake | 1 sec - several min | 10-8-10-5 |
Floods | days | 10-2 |
Measurable erosion | 1-100s years | 100-101 |
Opening of the Atlantic Ocean 1 km | 25,000 years | 104 |
Uplift of mtn range to 3000m at a rate of 0.2 mm/year | 15,000,000 years | 107 |
Age of the Earth | 4,500,000,000 years | 4.5*109 |
(1) geologic processes are
(2) Parts of the geologic record may be
PRINCIPLES USED FOR RELATIVE AGE
(0) Priciple of Uniformitariansim - The Present is the
Key to the Past.
(1) Principle of superposition - in a sequence of rocks that are not overturned the oldest rocks
(2) Principle of original = horizontality
- sediments are deposited in
(3) Principle of original = continuity - a sedimentary layer forms (at the time of deposition) a :
(b) changing to a bed of a different composition (e.g. sandstone to shale)
(c) hitting a barrier
(5) Principle of inclusions - a rock containing fragments of another rock must be
KINDS OF UNCONFORMITIES - erosional markers
(1) Angular = unconformity - angular discontinuity between
(2) Disconformity - irregular surface of
(3) Nonconformity - strata lie on
STRATIGRAPHIC = CLASSIFICATION
Strata reflect the physical and biological = character of a part of the Earth at some past time.
Units of stratigraphic classification:
Formation - = distinctive rock unit that can be mapped (e.g. Navaho sandstone)
System - primary = time-stratigraphic unit involving time interval necessary for deposition of a = time-stratigraphic unit.
(2) Use key marker = beds - thin distinctive units such as ash falls.
(3) Use index = fossils - common widespread fossils of restricted age.
ABSOLUTE GEOLOGIC TIME
Philosophically, there is a contrast between = eastern and western cultural views of time.
Eastern example
Hindu philosophers believed that one cycle of the universe = equaled 1 day in the life of Brahma, the creator god, or =BB 4.3 = billion years.
Hindu scriptures postulate the Earth was about half through a = cycle; or about 2 b.y. old.
Herodotus (450 BC) - Greek historian concluded the = Nile Delta was built by countless flood deposits over 1000s of years.
St. Augustine - followed these ideas to explain = natural phenomena through deductive reasoning.
During the late Middle Ages there was a mix of theology with = scientific thought (very repressive) e.g. In mid 1600s Archbishop Ussher of = Ireland counted up the ages of the various patriarchs in the Bible and concluded the Earth = was created at 9 am, October 26, 4004 BC.
In the mid 18th century Comte de Buffon calculated = the age of the Earth as 75,000 years based on cooling of a metal ball from the molten = state.
In the 19th century Lord Kelvin used estimated = cooling rates of the Earth/sun to give 20-40 millions years.
Breakthrough: discovery of radioactivity by = Becquerel and Madame Curie in the 1890s.
Three types of radioactive decay
(1) Alpha (a ) decay ( 42He+2 nucleus)
corresponds to loss of 2 neutrons, 2 protons per each decay)
e.g. 238U decay
1st step: 23892U radioactive due to instability in the nucleus - i.e. will spontaneously transform to a more stable isotope of the same element or an isotope of a = different element.
Parent - atomic nucleus undergoing decay
Daughters - product of the radioactive decay
Because the decay involves the nucleus and not the electrons (define the nature of the chemical bonds), the rate of decay is
The rate of radioactive decay is measured by
The constant rate of decay of different radioactive isotopes forms
MEASURING LONG HALF-LIVES OF ISOTOPES
Count the radioactive disintegrations of individual atoms (e.g. by counting the scintillations produced when alpha-particles from the decaying atoms strike a ZnS screen).
Divide the number of disintegrations per second for a given sample of radioactive element by the number of atoms in the sample (determined by multiplying Avogadro's number (6.023*1023) and by the number of grams divided by the atomic number of the element.
Obtain the probability that an individual atom will undergo a radioactive disintegration in 1 second.
The half-life is then the time required for these probabilities to build up to 50%.
RADIOMETRIC DATING
(1) There is a
(2) Once the mineral or rock crystallizes (or organism dies),
(4) Using isotopes with known half-lives (or decay constants) it is possible to
Date | Half-lives | Parent atoms | Daughter atoms | % parent remaining | parent/ daughter ratio |
1998 | 0 |
l
l l l
l l ll
|
100 | 0 | |
2998 | 1 |
l
l l l
|
|
50 | 1:1 |
3998 | 2 |
l
l
|
|
25 | 1:3 |
4998 | 3 |
l
|
10 m.y - 4.5 b.y.
|
zircon | |
40K | 40Ar | 1.3 b.y. | 50,000 y - 4.5 b.y. | muscovite
biotite hornblende volc. rocks |
|
87Rb | 87Sr | 47 b.y. | 10 m.y. - 4.5 m.y. | muscovite
biotite K-feldspar Ig./Met. rx |
|
14C | 14N | 5730 y. | 100 - 70,000 y. | bone
wood cloth glacier ice |
Other geochronologic techniques
Magnetic Polarity Time Scale
- particularly effective in lavas
- weaker magnetism in sedimentary rocks
record of reversal goes back
Toilet Tissue Time Scale
Age (Ma) | Event | Sheet # |
4,200 | Oldest material dated - Australian zircons | 87 |
3,996 | Oldest dated rock - NW Territories | 131 |
2,500 | Archean/Proterozoic boundary - appearance of multi-celled animals | 456 |
570 | Proterozoic/Phanerozoic - appearance of marine invertebrates | 876 |
505 | Appearance of first fish | 890 |
438 | First land plants | 905 |
320 | First reptiles | 930 |
245 | Paleozoic/Mesozoic boundary - beginning of age of dinosaurs and conifers | 947 |
66 | Mesozoic/Tertiary (K-T) boundary - disappearance of dinosaurs and rise of mammals and flowering plants | 985 |
23 | Grasses become abundant | 995 |
2 | First appearance of humans | last 2" of last sheet |