Geologic Time & The Rock Record

1. Geologic Time & The Rock Record Chapter 8 Chapter 8

2. Introduction • James Hutton (1726-1797). Proposed that most geologic processes happen very slowly. • Geologists sort Earth’s history into a sequence of events. • Relative Age - Position in that sequence; older or younger than nearby layers • Absolute Age - Numerical age can be determined through analysis of the products of radioactive decay

3. Reading The Layered Rocks • Layered rocks contain clues about past environments at/near surface. • Sequence and relative ages provide basis for reconstructing Earth’s history. • The study of strata is called stratigraphy.

4. Review of Sediment Layers • Most sediment is laid down in the sea, in shallow waters, or in streams. • Each new layer is laid down horizontally over older ones. • Law of original horizontality - sediments are deposited in strata that are horizontal or nearly horizontal.

5. Kinds of Unconformities Unconformity - break or gap in sequence. (3 kinds) 1. Angular unconformity- older strata deformed and cut off by erosion before the younger layers were deposited across them. 2. Disconformity - irregular surface of erosion between parallel strata; no tilting; hard to recognize, because the strata above and below are parallel. 3. Nonconformity - Strata overlie igneous or metamorphic rock.

9. Label the Sections: A B C D Which is the… Angular Unconformity? Conformity? Disconformity? Nonconformity?

10. Significance of Unconformities • Evidence of former seafloors uplifted by tectonic forces and exposed to erosion. • Later tectonic forces depress the surface. • The surface, in turn, becomes a site of deposition of sediment.

11. Stratigraphic Classification The basis of rock stratigraphy is the formation. • - a collection of similar strata that are sufficiently different from adjacent groups • - basis of physical properties they constitute a distinctive, recognizable unit that can be used for geologic mapping over a wide area.

12. Geo Time and Correlation • Correlation – comparing different places to see if they are from the same geo time and processes Index fossil: short-lived but geographically widely distributed. • Determining the relative ages of exposed areas • Use standard geo time scale to establishing ages • Index fossils are useful for this. If a distinctive index fossil is recognizable at an outcrop, a rapid and reliable means of correlation is available.

13. The Geologic Column and the Geologic Time Scale • 19th century – • geologists assembled a geologic column • composite column containing, in chronological order, the succession of known strata, fitted together • based of fossils or other evidence of relative age. • The corresponding column of time is the geologic time scale.

15. Radioactivity • In 1896, the discovery of radioactivity provided the needed method to measure the age of the Earth accurately. • Different kinds of atoms of an element that contain different numbers of neutrons are called isotopes. • Most Isotopes are stable and don’t change.

16. Radioactive Isotopes • A few isotopes, such as 14C, are radioactive. • instability within the nucleus. • transform spontaneously to a nucleus of a more stable isotope of a different chemical element. • The process is called radioactive decay.

17. Half-Lives of Isotopes • Radioactive decay varies among isotopes. • Radioactive decay is measured in half-lifes • amount of time needed for the number of parent atoms to be reduced by one half.

18. Radiocarbon Dating • 14C is useful for dating young samples. • Half-life of 14C is short - 5730 yrs • 14C continuously created in the atmosphere through bombardment of 14C by neutrons created by cosmic radiation.

19. So, How Old Is The Earth? • “Moon dust” brought back by astronauts, is 4.55 billion years old. • The Earth was formed approximately 4.55 billion years ago.

20. How Fossils Are Formed

21. Millions found – but not many • Most living things are quickly recycled upon death. Scavengers and bacteria usually consume all but bones and shells.

22. Freezing (refrigeration) • Best means of preservation of ancient materials. • Rare - continually frozen from death til discovery. • Mammoths and wooly rhinoceros found in ice from Alaska and Siberia. • Specimens with flesh, skin, and hair intact have been found. • suggest that they were flash frozen, with food still in the mouth and stomach.

23. Drying (desiccation) • Mummified bodies • discovered in arid parts • Soft tissues preserved if completely dried.

24. Asphalt • La Brea Tar Pits - 100 pits filled with sticky asphalt or tar. • formed by crude oil seeping through fissures in the earth. • lighter elements evaporated leaving thick sticky asphalt. • pits are famous for Pleistocene fossils • The fossils date between 10 and 40 thousand years old. • Asphalt is an excellent preservative.

25. Amber (Unaltered preservation) • Insects, spiders, and even small lizards have been found, nearly perfectly preserved in amber. • Over millions of years, sap with our fly inside is polymerized and hardened into amber.

26. Carbonization (distillation) • Plant leaves, and some soft body parts of fish, reptiles, and marine invertebrates decompose leaving behind only the carbon. • This carbon creates an impression in the rock outlining the fossil, sometimes with great detail.

27. Casts & Molds • molds and casts of organisms which have dissolved or rotted away, leaving only a trace of their existence. • Casts and molds are types of fossils where the physical characteristics of organisms have been impressed onto rocks.  • buried or trapped in mud, clay, or other materials which hardened around them - leaving molds of the organism. • There are two types of molds:  external and internal. 

28. Permineralization (Petrification) • most common • Minerals fill the cellular spaces and crystallize. • Shape of the original plant or animal is preserved as rock. Sometimes the original material is dissolved away leaving the form and structure but none of the organic material remains.