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

history of the Earth, including record of life on Earth as well as physical changes in Earth itself.

that deals with Before ~19 century it was accepted that the Earth was created in a single act, according to the Bible.

Archbishop James world was completed by God on Saturday, October 9, 4004 BC.

Ussher concluded in 1658 that the

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 evolution.

14 C is only 5700 years.

This method is good for dating recent biological

Fossils

Fossils are remains of living Most common fossils are hard parts of animals: bones, shells, teeth.

seas.

organisms buried in rocks.

Best conditions to preserve fossils are floors of shallow 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 .