Chapter 13 Clues to Earth’s Past

1. How do we know they existed. 2. Where can we find fossils. 3 1. How do we know they existed? 2. Where can we find fossils? 3. What process eventually exposes the fossil at the surface?

FOSSILS Nearly all fossils are found in sedimentary rock

4. What are fossils?

Fossils Remains, imprints, or traces of prehistoric organisms Prehistoric – before history was written Tells when, where, and how organisms lived

5. What is the name for scientists who study fossils?

Paleontologist Scientists who study fossils

6. What conditions are needed for fossils to form?

Conditions Needed For Fossils to Form: Organisms must quickly be covered by sediment to keep scavengers away & bacteria from causing decay (bananas) Organisms with hard parts have a better chance of fossilization b/c they decay slower

7. What are the 7 types of preservation of fossils?

Answer to 7: permineralized remains, carbon films,coal, molds and casts, original remains, trace fossils, trails and burrows

8. What are permineralized remains?

7 Types of Preservation/Fossils: Permineralized Remains 1. when minerals fill small holes in hard parts like bone, teeth, & shells Bones have small holes that are usually filled w/ blood cells, nerves, etc. When org. die, soft tissue decays leaving holes for minerals to fill In some permineralized remains (fossils in which spaces are filled w/ minerals) can encase original genetic material 2. water flows over buried fossil, dissolves fossil and fills space w/ minerals in water

9. What are carbon films?

Types of Preservation Carbon Films Thin film of carbon residue that forms a silhouette of the organism Sediment buries the dead org., as sediment piles up heat and pressure force liquids out leaving carbon behind

10. What type of fossil preservation has accumulated plant matter that is buried and carbonized?

Types of Preservation Coal Plant matter accumulated & buried then carbonized forming coal Since plant structure is usually lost there is not much revealed about the plant

11. What type of fossil preservation has rock with pore space which allows water to flow through decaying the organism and leaving behind a cavity in the shape of the organism? 12. Mineral-rich water flows through answer to #11 and fills cavity producing a copy of the organism is called _____________.

Types of Preservation Molds and Casts Hard parts of organisms are covered by sediment sediment is compacted, cemented and turns to rock Rock has pore space & allows water to flow through decaying the organism & leaving behind a cavity in the shape of the org. = mold Mineral-rich water flows through mold & fills cavity producing a copy of the org. = cast

13.What type of fossil preservation has organisms original remains preserved in amber, ice, or tar?

Type of Preservation Original Remains Original soft parts can be preserved in – Amber – sticky tree resin Ice tar

14. What type of fossil preservation has fossilized tracks and other evidence of organisms activity?

Types of Preservation Trace Fossils Fossilized tracks & other evidence of org. activity Can tell size, weight, age, social behavior Ex. Texas tracks of Sauropod, small prints inside large on outside – What des this suggest about social behavior?

15. What type of fossil preservation has tunnels left behind by burrowing organism?

Types of Preservation Trails and Burrows Tunnels left behind by burrowing org.

16. ______ fossils are the remains of species that existed on Earth for relatively short periods of time, were abundant, and were widespread geographically.

Index Fossils Existed for short periods of time Were abundant (lots of them) Widespread geographically (covered a broad area) Used to identify ages of other rock layers Fossil Range Chart 367

Fossils determine past climates Organisms that are limited to certain habitats can be found in completely different habitats Ex: Fossils of sea organisms found in deserts tells us that desert was once covered by water Fossils in VA are found mainly in Coastal Plain, Valley & Ridge, and Appalachian Plateau Most fossils found in VA are marine org. Proves that large area of state covered by seawater

Relative Ages of Rocks Section 2

Principle of Superposition Older rocks on bottom, younger on top Sediments accumulate in horizontal layers

Relative age Determine the age of a layer of rock in comparison to the layers around it. Ex: layers of sedimentary rock that are offset by a fault Which is older the fault or the sedimentary rock? D B C A

“Structures” Intrusions and faults

Igneous intrusion – where magma has filled a gap between rock layers

Fossils can be used to determine the age of disturbed rock layers What conclusion can you draw if a younger fossil is found on the bottom and older on top?

Unconformities When layers of rock are missing creating a gap in geologic time Develop when agents of erosion (water, glaciers) remove layers by washing or scraping away

Types of Unconformities Angular unconformity Horizontal layers of rock are tilted and uplifted Erosion & weathering erode the tilted rock layers Younger sediment is deposited on top

Types of Unconformities Disconformity layer of rock that is exposed to erosion w/ no new deposition Top layer as well as others can erode Deposition begins again which leaves a gap in time F GAP IN GEOLOGIC TIME D EROSION DEPOSITION E C B B B A A A

Types of Unconformities Nonconformity Igneous or metamorphic rock are uplifted and eroded Sedimentary rocks are deposited on top Sediment/sedimentary rock Igneous/ Metamorphic rock Igneous/ Metamorphic rock

How can you match rock layers Finding the same types of fossils in the same type of rock will prove the rock layers are the same age

Absolute Ages of Rocks Section 3

Absolute Age The age in years of a rock or other object Determined using atoms that make up the material Gives a numerical age to an event

Difference between Relative Age and Absolute Age Relative age is a comparison with surrounding rock layers (no definite year) Absolute age tells age in years

Radioactive Decay The process of isotopes breaking down & giving off energy Used to determine the absolute age of a rock # of neutron determines form of element or isotope Protons (p+) – in nucleus Neutrons (n˚) – in nucleus Electrons (e-) – cloud surrounding nucleus # of p+ determines the identity # of n˚ determines form of the element or isotope Some isotopes are unstable & break down Unstable atom changes into a new atom (parent material/daughter material) Cut paper demo

How atoms breakdown: Alpha Decay – isotope gives off 2 protons & 2 neutrons = alpha particle Beta Decay – n˚ breaks down to a p+ & an e-, e- leaves atom = beta particle Nucleus has lost a n˚ but gained a p+, # of p+ has changed & new element has formed

Half-life Parent isotopes undergo radioactive decay either alpha or beta producing a daughter product Each element has a given amount of time for half of the parent isotope to decay into daughter material That amount of time is an isotope’s half-life After each time period only half of the remaining parent isotope has decayed

Radiometric dating – process used to calculate the absolute age Using ratio of parent isotope to daughter product in a mineral & knowing the half-life of the parent w/ older rock scientist have to pick elements w/ longer half-lives Ex. 50 yr old rock Would you use an isotope w/ a half-life of 2 months. Radiometric Ages

Radiocarbon Dating Used to date objects 75,000 years old and younger Half-life of 5,730 years Compare the amount of carbon-14 from object to the carbon-14 in the environment

Can only use absolute dating with metamorphic and igneous rock. Why not sedimentary?

Age of Earth Oldest rocks on Earth: 4.28 billion years old Estimated age of Earth: 4.6 billion years old

Uniformitarianism Earth processes occurring today are similar to those in the past – James Hutton Hutton used this principle to support his idea that the Earth was more than a few thousand years old Takes long periods of time for rock layers to form and mountains to erode, therefore Earth is older than thousands of years