Uncovering Earth’s Past

1. Uncovering Earth’s Past Date: 4/3/13 p. 111 in ILL

2. Fossils • Fossils– traces or remains of living things from a long time ago and exist in a variety of formshttp://www.fossilmuseum.net/FossilGalleries.htm • Original Remains– fossils that are actual bodies or body parts of an organism that have been trapped in ICE, AMBER, or TAR http://www.fossilmuseum.net/Fossil_Galleries/FossilAmber.htm

3. Original Remains – 1 month old mammoth found in Tar in a swamp

4. Fossils • Rock Fossils– rock can contain very detailed shapes, outlines, or imprints of just about every living thing (even as small as bacteria); these organisms get buried in sediment and over time the organic parts are replaced by minerals, causing them to turn to stone

5. Rock Fossils Types • molds and casts 2. petrified wood Animation of a cast and a mold forming

6. Rock Fossils • carbon films—created when an organism dies, sinks into Earth’s layers, and pressure destroys all of the materials that make up the organism except the carbon, which forms a very thin layer on the rock 4. trace fossils—evidence of an organism’s activities ex. footprints

7. Index Fossils • The fossilized remains of organisms that lived for only a short period of geologic time and spread out over large areas, therefore can be used to indicate the age of the rock layer that contains them. ex. ammonites were common during the Mesozoic Era (245 to 65 mya) and went extinct during the K-T extinction, so if ammonites are found, the rock layer containing them must have formed between 245 and 65 mya

8. Other ways to determine earth’s past—Tree Rings • As a tree grows, it produces new layers of wood around the trunk just under the bark. • In general, one layer of wood grows each year. • These layers are visible as concentric circles in a cross section when the tree is cut down. • Tree rings show how the tree’s environment changed from year to year ex. forest fires, droughts, etc.

9. Other ways to determine earth’s past—Tree Rings

10. Other ways to determine earth’s past—Ice Cores • Air, dust, and ash trapped in ice can provide evidence about how the atmosphere and climate have changed and help predict how it might change in the future. • Scientists drill into the ice and take tubular samples to examine. What kind of information can we get from ice core samples?

11. Other ways to determine earth’s past—Ice Cores

12. Other ways to determine earth’s past—Ice Cores

13. Ways to Date Earth’s history—Relative Age • Relative Age—the age of an event or object in relation to other events. ex. give your age in relative terms

14. Ways to Date Earth’s history—Relative Age • Sedimentary rock—formed when sediment builds up over time to form horizontal layers trapping any decaying organisms and leaving evidence of major events

15. Ways to Date Earth’s history—Relative Age

16. Ways to Date Earth’s history—Relative Age • The Law of Superposition states that under normal circumstances, younger rock layers will be deposited on top of older rock layers.

17. Ways to Date Earth’s history—Relative Age The Law of Superposition demonstrated at the Grand Canyon

18. Ways to Date Earth’s history—Relative Age • Igneous rock– formed when magma (melted rock) cools • Magma can cut through sedimentary layers, so it must be younger than the layers it cuts through.

19. Ways to Date Earth’s history—Relative Age Igneous rock cutting through sedimentary rock layers.

20. Ways to Date Earth’s history—Relative Age • An unconformity represents a gap in geological time. • It occurs when either erosion wears away rocks or rock deposits never form; therefore, a gap exists between when the rocks below the unconformity formed and when those above it formed. Animation of an unconformity

21. Ways to Date Earth’s history—Relative Age Which rock layer is the oldest? Which rock layer is the youngest? Where is an unconformity?

22. Ways to Date Earth’s history—Absolute Age • The actual age of an event or fossil is known as the absolute age. • Scientists use a variety of technologies to determine absolute age and it is measured as accurately as possible.

23. Ways to Date Earth’s history—Absolute Age • One way that scientists measure absolute age is using radioactive dating(also known as radiometric dating). • This method uses isotopes (unstable forms of atoms that break down in predictable ways—radioactivity)

24. Ways to Date Earth’s history—Absolute Age • The rate of change for the radioactive atom is constant and is not affected by temperature or other environmental factors. • The rate of change is measured in half-lives.

25. Ways to Date Earth’s history—Absolute Age • A half-life is the length of time it takes for half (50%) of the atoms of a radioactive element to change from its unstable form to its stable form. • The original form (radioactive) is referred to as parent atoms and it turns into daughter atoms (stable) with each half-life.

26. Video Clip! • Video

27. Ways to Date Earth’s history—Absolute Age • Different isotopes have different half-lives. ex. carbon-14 has a half-life of 5730 years uranium-235 has a half-life of about 704 million years

28. Ways to Date Earth’s history—Absolute Age You do the math! A parent isotope has a half-life of 100 million years. How old is a rock sample that contains ¼ of its original parent isotope?

29. Sedimentary rock layers show the order in which rocks formed. The Law of Superposition is used to determine the relative ages of the rock and/or fossils found in the rock. Radioactive dating can be used to determine the absolute age of igneous rock. Scientists combine information about the relative and absolute ages of rocks and fossils to construct a timeline of Earth.

30. Questions???