Praxis Review for Earth Science

1. Praxis Review for Earth Science By Frank H. Osborne, Ph. D. 4: History of the Earth and its Life Forms

2. Uniformitarianism There is an assumption that the geological process that we see taking place today have always been at work and were at work in the past.

3. Stratigraphic Correlation Original horizontality The principle of original horizontality is based on the observation that when sedimentary particles settle out of the water, they are under the influence of gravity. As a result, they form horizontal beds.

4. Stratigraphic Correlation Superposition The principle of superposition is that in undisturbed layers of rock, the oldest layer will be found at the bottom and the youngest layer will be found on the top.

5. Stratigraphic Correlation Cross-cutting relationships The principle of cross-cutting relationships is used to determine the relative ages of events. An igneous intrusion or a fault must be younger that the rocks it intrudes or cuts.

6. Stratigraphic Correlation Lateral Continuity The principle of lateral continuity states that sediment extends laterally in all directions until it thins out or terminates against the edge of the depositional basin.

7. Index Fossils Correlation using index fossils Index fossils are used to indicate rock layers in different places that are part of the same formation. Rock layers of a given age will generally contain the same forms of index fossils.

8. Index Fossils Grand Canyon The sedimentary rocks of the Grand Canyon were originally deposited horizontally under the water or in a coastal environment (principle of original horizontality). The oldest rocks are at the bottom and the youngest are at the top (principle of superposition)

9. Index Fossils Correlating rock layers in the Grand Canyon The exposed rocks extend in all directions and can be shown to be in the north and south walls (principle of lateral continuity).

10. Unconformities An unconformity is a point in the geological record where strata are missing. It is a discontinuity consisting of an erosion surface between younger strata and older rocks.

11. The Fossil Record Types of fossils Fossils may be the remains of organisms deposited in the sediments. The remains may create a depression or mold which later fills in to become a cast. Some fossils are made by mineral substitution in the sediments. Imprints are impressions made in the sediments by the object.

12. Composition of the Earth Fossils A fossil is the remains of something that was alive or was made by something that was alive. The object becomes a fossil as a result of a sequence of steps.

13. How a fossil is formed An animal dies and decays. Only the bones remain.

14. How a fossil is formed The bones are covered by sediments under water.

15. How a fossil is formed Over many years the layers of sediments above the bones increase. The sediments become sedimentary rock.

16. How a fossil is formed The molecules of the bones are slowly replaced by minerals. Bone turns to stone.

17. How a fossil is formed After some time, uplift causes the rocks to be lifted above sea level.

18. How a fossil is formed Erosion washes the rocks away and exposes the fossil.

19. Fossils Fossils and the age of rocks Before the discovery of radioactivity, geologists did not know the age of rocks. So they used index fossils. Certain kinds of fossils can be used to correlate layers of rocks in different locations. Radioactive dating of rocks involves Uranium-238. It has a very long half-life which is 4.5 x 109 (4.5 billion) years.

20. Geologic Time Scale The geologic time scale The geologic time scale is a representation of the history of the Earth. Events older than 600 million years are in the Pre-Cambrian era. Since then are the Paleozoic, Mesozoic and Cenozoic eras.

21. Geologic Time Scale Geologic Time Scale Geologic time is divided into eras, periods and epochs. Mainly the distinctions are based on fossil evidence.

22. Age and Dating Absolute age tells how old something is as compared with the age of the Earth. In some cases rocks must be compared with each other so only the relative age is known. Radioactive dating employs measurements of radioactive nuclei that decay at known rates. For example, U-238 decays into Pb-206 at a known rate that can be measured in rocks, thereby providing their ages.

23. Paleogeography The continents have not always been in the same locations. Plate tectonic forces have been at work throughout most of the history of the Earth. There is evidence that North America used to be located on the Equator and Africa used to be located on the South Pole. In this case, North America would have been tropical and Africa, frozen.

24. Paleogeography Fitting the continents together Try cutting the map to fit the Americas and Africa together. We think it looked like this 200 million years ago.

25. Paleontology Paleontology is the study of fossils. Using fossils, geological time sequences and fossil positions in the geological record, it is possible to construct a sequence of the development of life on Earth. Mass extinctions have occurred at several important points in the fossil record. The most recent is the K-T boundary at 65 million years ago.

26. The End 4: History of the Earth and its Life Forms