Historical Geology

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Lecture 31

• Historical Geology

Chapter 15.8 ? 15.12

• Evolution
• Rock Dating
• Geologic Time

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Earth History
Two lines of changes can be traced in the Earths
crust changes in inorganic objects and living
organisms. They allow us to reconstruct the
Earths history.
Historical geology Branch of geology that deals
with history of the Earth, including record of
life on Earth as well as physical changes in
Earth itself.
Before 19 century it was accepted that the Earth
was created in a single act, according to the
Bible.
Archbishop James Ussher concluded in 1658 that
the world was completed by God on Saturday,
October 9, 4004 BC.
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First Scientific Ideas
Abraham Werner (1750?1817) proposed a theory of
rock formation in 3 events due to sudden
precipitations from a universal ocean (Neptunian
view of Earth).
Georges Cuvier (1769?1832) began working with
fossils and suggested that different groups of
animals appeared as a result of special creations
followed by catastrophes.
James Hutton (1726?1797) adopted that the Earth
is very old and uniform changes continuously
occurred (Plutonian theory).
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Rock Formations
Analyzing current landscapes and rocks, one can
reconstruct history of their formation.
Moraines and lakes imply glacier motions. Hot
springs and conical mountains tell about previous
volcanic activity. The presence of coal suggests
that there was a swamp at this place long ago.
Two fundamental problems in reconstructing the
crust evolution To arrange events occurred in a
small region in order To correlate events in
different regions
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Radiometric Dating
Decay of radioactive elements proceeds at the
same rate. The ratio of a nuclide to its daughter
changes with time.
Common reactions used for dating involve elements
with half-life of billions of years and give
results with an accuracy of ?2500 years per
million years.
Radiocarbon method is used for smaller time
scales, because the half-life of 14C is only 5700
years. This method is good for dating recent
biological evolution.
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Fossils
Fossils are remains of living organisms buried in
rocks. Most common fossils are hard parts of
animals bones, shells, teeth. Best conditions to
preserve fossils are floors of shallow seas.
Fossil records show progressive changes in groups
of organisms with time. This allows to date rocks
in which fossils are found. Fossils also help to
reconstruct the environment in which the
organisms lived.
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Geologic Time
The records of events earlier than 570 million
years ago are very sparse. The entire period from
the creation of Earth to 570 million years ago is
called Precambrian time.
Later, closer to us, time is divided into 3 major
eras.
Paleozoic 570?225 million years ago (ancient
life) Mesozoic 225?65 million years ago
(intermediate life) Cenozoic 65 million years
ago ? present time (recent life)
The division is based on records of mass
extinctions.