Lecture 31 Historical Geology Evolution Rock Dating Geologic Time Chapter 15.8  15.12

Earth History 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.Ussher 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.

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).Neptunian 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.fossils James Hutton (1726  1797) adopted that the Earth is very old and uniform changes continuously occurred (Plutonian theory).Plutonian

Rock Formations Analyzing current landscapes and rocks, one can reconstruct history of their formation. Moraines and lakes imply glacier motions.glacier Hot springs and conical mountains tell about previous volcanic activity.springs 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.billions Radiocarbon method is used for smaller time scales, because the half-life of 14 C is only 5700 years. This method is good for dating recent biological evolution.

Fossils Fossils are remains of living organisms buried in rocks.organisms 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.called Later, closer to us, time is divided into 3 major eras.time 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.extinctions