Geology - the study of the earth: "geo" + "ology"
Why be concerned with geology? It's important in many ways:
its materials (e.g., rocks) and . . .
its processes (e.g., plate tectonics, earthquakes, etc.)
. . . and the history of the planet and its life forms from its origin
to the present.
'It is the job of the geologist to unravel this history and write the
biography of the earth.'
relies heavily on multiple working hypotheses;
there are few "right" answers, . . . . . although this doesn't apply
Will Durant, "Civilization exists by geological consent, subject to change
materials-minerals (gold, uranium, coal); rocks for building materials.
processes-earthquakes (can we predict them?); LA's big one, safer buildings;
volcanic eruptions (Pinatubo, Mt. St. Helen's; landslides).
-where to drill a well for water;
-where to site a landfill, nuclear power plant;
-flood control of rivers.
-can I build on this wetland (delineation); should I live on this barrier
There are two major subdivisions of geology:
1) Physical geology-materials and processes; what's the world
made of? What makes mountains? Subdisciplines:
a) structural b) petrology c) geochemistry d)
2) Historical geology-origin of the earth; a chronological history
of the record of life; fossils, dinosaurs, etc. One subdiscipline is paleontology.
A Brief History of Geology
1) Aristotle-the first 'great' geologist; a philosopher who
inhibited later research through the middle ages.
17th and 18th centuries-theory of Catastrophism prevailed; major proponent
was Georges Cuvier
Nature of Scientific Inquiry-Scientific method
proclaimed that earth features (mountains, valleys, etc.) were produced
by cataclysmic events which no longer occur.
earth was thought to be quite young; Anglican Bishop James Ussher of
Ireland traced all the 'begats' in the Bible and proclaimed the creation
of earth to have occurred at 9:00 am, October 26, 4004 BC. This
makes the world about 6000 years old!
late 18th century-James Hutton, 'Father of Modern Geology'; a Scottish
physician; proposed the Doctrine of Uniformitarianism.
. . " The present is the key to the past."
A corollary of this is that the 'laws of nature are unchanging' throughout
time and space.
we don't see such cataclysms; erosion is (and was) a slow process; this
says that the world must be old to have produced the features we see.
An example of this logic: calculate the uplift rate of some mountains
by fossils found at 10,000 feet which were dated to 15 million years. This
gave a minimum rate of 0.2 mm/year.
Erosion is lowering the North American continent at an average rate
of 3 mm/1000 years.
Charles Lyell-early 1800's.
-discuss hypothesis vs. theory vs. law
A hypothesis is not a guess.
A theory is a widely held view which is the best explanation of
scientific facts; no major inconsistencies.
A law is a generalization for which we have no contradictory
How old is the Earth? 4.6 billion years (4,600,000,000
years). The source of this number is radiometric dating using the
Uranium-Thorium system. See Chapter 10 (Geologic Time).
Early ideas of the age of the Earth:
1. 1654 Archbishop
Ussher (Ireland), traced genealogy in Bible, using the 'Begats'.
He concluded that Earth was created October 26, 4004 BC, 9:00 am.
This makes the Earth 6000 years old. This led to the Doctrine of
Catastrophism which held that the Earth was shaped by series of giant disasters,
requiring the fitting of many processes into a short time scale.
2. 1770's, 1780's "Revolution"
Hutton, Father of Geology (Scotland) 1726-1797. Published Theory
of the Earth in 1785. Noting that Hadrian's
Wall, built as a defense by Romans in England in the 122 A.D. and named for
Emperor Hadrian, showed no significant
change after 1500 years, he suspected that Earth was much older than Ussher's
estimate. He suggested that slow processes shape earth. Mountains
arise continuously as a balance against erosion and weathering. He
proposed the Doctrine of Uniformitarianism: "The present is key to the
past". The physical and chemical laws that govern nature are uniform.
Unconformity at Siccar Point, Scotland was a major factor in his logic.
He suggested, "No vestige of a beginning, no prospect of an end."
3. How old is Earth? Quantitative scientific methods.
a. In 1897, Lord Kelvin assumed that the Earth
was originally molten and calculated a date based on cooling through conduction
and radiation. Age of Earth was calculated to be about 24-40 million
Problem: Earth has an internal heat source (radioactive
b. In 1899 - 1901, John Joly (Irish) calculated the rate
of delivery of salt to the ocean. River water has only a small concentration
of salts (~0 ppt). Rivers flow to the sea. Evaporative concentration of
salts. Salinity of the ocean averages 35 ppt.
Age of Ocean
= Total salt in oceans (in grams) divided by rate of salt added (grams
Age of Earth
was calculated to be 90-100 million years.
Problems: no way to account for recycled salt, salt incorporated
into clay minerals, salt deposits.
c. Thickness of total sedimentary record divided by average
sedimentation rates (in mm/yr). In 1860, calculated to be about 3 million
years old. In 1910, calculated to be about 1.6 billion years old.
Early measurements of maximum thickness of sediment ranged from 25,000
m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks
is at least 150,000 m. Sedimentation rates average about 0.3 m/1000
years. At this rate, the age of the first fossiliferous rocks is
about 500 million years.
Problems: did not account for past erosion or differences in
sedimentation rates; also ancient sedimentary rocks are metamorphosed or
d. Charles Lyell 1800's compared amount of evolution
shown by marine mollusks in the various series of the Tertiary System with
the amount that had occurred since the beginning of the Pleistocene. Estimated
80 million years for the Cenozoic alone.
e. Discovery of radioactivity by Henri Becquerel in 1896.
In 1905, Rutherford and Boltwood used radioactive decay to measure the
age of rocks and minerals. Uranium decay produces He, leading to a date
of 500 million years. In 1907, Boltwood suspected that lead
was the stable end product of the decay of uranium. He published the age
of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion
years. So far, oldest dated Earth rocks are 3.96 billion years. Older
rocks include meteorites and moon rocks with dates on the order of 4.6
billion years. Moon rocks, highland ~ 4.5 by, mare basalt ~ 3.2 -
3.8 by. Meteorites - older than 4.5 by
was used after WWI (1918). Led to the discovery of over 200 isotopes.
Many radioactive elements can be used as geologic clocks. Each element
decays at its own nearly constant rate. Once this decay rate is known,
geologists can estimate the length of time over which decay has been occurring
by measuring the amount of radioactive parent and the amount of stable
daughter elements. Example: Potassium-Argon dating.
Why is the Earth younger than the moon and meteorites?
Earth is geologically active.
Has a hot, molten interior.
Rocks are remelted and their internal clocks are reset.
Also, rocks on Earth's surface are acted on by erosion and weathering.
Rocks on Earth surface are not as old as the Earth, they are "recycled"
rock materials. Rocks broken down into sediment (gravel, sand, silt,
clay). Sediment will turn into sedimentary rock over time. Older
rocks are buried deeply under younger rocks.
Where do we find the oldest rocks on Earth?
Canadian Shield. (NW Territories near Great Slave Lake, 3.96 by). Gneiss.
Narrows the gap between origin of Earth and first rocks to 640 million
years. (Geotimes 12/1989). Before this, oldest rocks known were from
Isukasia region of Greenland (3.8 by).
Glaciers 2 miles thick scraped off young recycled rocks.
Land rose 250 ft since ice was removed => more erosion.
Very old rocks are at the surface in the Canadian Shield area.
Up to about 3.8 or 3.96 billion years old.
Multicellular life did not appear until about 1 billion years ago. Before
this, 3 billion years ago single celled life only. Hard parts like shells
don't appear until 600 million years ago. (Trilobites)
How do we know that these things are millions or even billions of years
Absolute dating tells exactly (+ or -) how old something
e.g., radiometric dating allows us to age the earth at
billion yrs (4.6 x 109); would take 31.7 years to count
to 1 billion at one count/second; 146 years to get to 4.6 billion. One
billion beer bottles would circle the earth > 17x.
Compress, for example, the entire 4.6 billion
years of geologic time into a single year. On that scale, the oldest
rocks we know date from about mid-March. Living things first appeared
in the sea in May. Land plants and animals emerged in late November
and the widespread swamps that formed the Pennsylvanian coal deposits flourished
for about four days in early December. Dinosaurs became dominant
in mid-December, but disappeared on the 26th, at about the time the Rocky
Mountains were first uplifted. Manlike creatures appeared sometime
during the evening of December 31st, and the most recent continental ice
sheets began to recede from the Great Lakes area and from northern Europe
about 1 minute and 15 seconds before midnight on the 31st. Rome ruled the
Western world for 5 seconds from 11:59"45 to 11:59:50. Columbus discovered
America 3 seconds before midnight, and the science of geology was born
with the writings of James Hutton just slightly more than one second before
the end of our eventful year of years.
Click here for a brief
chronology of what's happened on earth in the last 4.6 billion years!
*Don L. Eicher, Geologic Time, 2nd ed.
(Englewood Cliffs, NJ: Prentice Hall, 1978), pp 18-19.
Relative dating compares one object to another w/o knowing the
a) law of superposition -the stratum on top is younger
than the one below it (if strata are undeformed); but . . .
See Geologic Time Scale in lab book and text book; create a mnemonic, a
device to remember these; quiz next time.
b) principle of faunal succession -extensive fossil studies
allow us to date an object by the fossils they contain.
(a) Q uick (quaternary)
D uring (a)
Structure of the Earth
Crust (5-40 km)
Mantle (2885 km)
km = .61 mile
Outer Core (2270 km)
Inner Core (1216 km)
The Earth is like a jellybean, only smoother: relief from trench to
mountain top is ~12 mi.
lithosphere-solid; composed of crust and upper mantle.
Surface is continents and basins.
asthenosphere -plastic; lower part of upper mantle.
boundary is the about 1/2 way down the continental shelf due to
changing sea levels.
|High levels of
||K, Na, Si
||Fe, Mg, Ca
Origin of the Earth-excellent film (KC#195) The Third Planet,
of Planet Earth series.
1st half-geological origin of earth by accretion. 2nd half-mass
extinctions by meteorites.
meteor Toutatis passed at 2.2 million mi in Dec. 1992
smaller earth-crossing asteroid passed w/in 106,000 mi. in 1990. (in between
us and the moon!!)
apparently on 26 million year cycle.