Download Historical Geology.

Lecture 31
Historical Geology
Chapter 15.8  15.12
• Evolution
• Rock Dating
• Geologic Time
Earth History
Two lines of changes can be traced in the Earth’s crust:
changes in inorganic objects and living organisms.
They allow us to reconstruct the Earth’s 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.
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).
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
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.
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.
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.