Name _____KEY___________________________________ Date _______________ Period ______

Fossils, Relative and Absolute Age & Geo Time Study Guide

1. Explain how a layer of marine fossils is discovered in a desert environment.

The area was once covered by water.

2. Explain how fossils of warm-weathered plants were found on an island in the Arctic Ocean.	The island drifted from a tropical region to its present location.
3. What evidence would a scientist use when examining an ice core to support a volcanic eruption caused a change in global temperatures?	Amounts of atmospheric dust

4. Index fossils are fossils of species that lived for only a short period of time and lived on many parts of the planet. How are index fossils useful to scientists?	Index fossils are useful because they tell the relative ages of the rock layers in which they occur. For example, if you found an ammonite fossil in a sedimentary rock layer, you would know that rock layer was from the Mesozoic Era since ammonites existed 245 to 65 mya.
5. Provide two reasons the fossil record is studied to understand how life has changed on Earth.	1. fossils can build up a picture of Earth's past environments. The fossils found in an area tell whether the area was a shallow bay, an ocean bottom, or a freshwater swamp. fossils also provide evidence about the past climate of a region. Scientists can use fossils to learn more about changes in Earth's surface.

2. the fossil record reveals a surprising fact: fossils occur in a particular order. Older rocks contain fossils of simpler organisms. Younger rocks contain fossils of more complex organisms. In other words, the fossil record shows that life on Earth has evolved or changed over time.

6. How do fossil records show that

organisms become extinct? Give a

specific example from the picture on the right.

A type of organism is extinct if it no

longer exists and will never again live on

Earth. In the picture, both the ammonite and and dinosaur show extinction. They both

show up in a couple of layers but then

disappear and are not shown again.

7. Why are there no fossils in layers C and E in the picture in #6?	They are igneous rock layers (made out of magma). See question #10.
8. a) In the picture in #6, which layer is oldest? b) which layer is youngest? c) How do you know?	a) A b) G

c) law of superposition
9. A wasp has been trapped in amber. What type of fossil would this be?	Original remains

10. Fossils are almost never found in igneous rock because the organisms	Are destroyed by heat before they can form fossils.
11. If an index fossil is found in two rock beds that are separated by an ocean, what do you know about the relationship between the two rock beds?	1) The rock beds used to be in the same area

2) the rock beds are the same age

3) the rock beds are made of the same type of rock

12. Which kinds of animal parts are most likely to be preserved as fossils?	teeth and bones

13. a. What type of dating uses layered sedimentary rock to tell which events on Earth occurred before other events? b. Provide two laws used to determine the age of rock layers.

a. relative age

b. Law of superposition: in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it

Law of cross-cutting relationships: If a piece of magma pushes into a body of rock, then the intrusion (magma) must be younger than the rock layers it cuts through.
14. Explain how ice cores show the history of Earth’s climate?

They contain evidence showing changes in the atmospheric composition over time. Snowfall that collects on glaciers each year captures atmospheric concentrations of dust, sea-salts, ash, gas bubbles and human pollutants.
15. Provide two examples of a trace fossil.

Fossilized poop, burrows, footprints, trails
16. The soft parts of an organism often do not fossilize because they undergo	decomposition
17. What is the difference between an original remain and a fossil preserved in rock?	**Fossils in rock were created when minerals replaced the parts of an organism and original remains are preserved when whole bodies are encased in ice or amber.

**Fossils in rocks show shape of body and original remains include soft body parts.
18. Describe the necessary steps for rock fossils to form?

1) an animal dies and sinks into shallow water

2) many sediment layers cover the animal

3) through pressure & time, the sediment becomes rock, preserving parts (usually hard body parts) of the animal

4) weathering and erosion eventually expose the fossil at the surface
19. In what type of environment(s) would you most likely find petrified (mineralized) fossils?

A dry lake bed

20.

a) Put the layers in order from oldest to youngest.

P, K, M, S, X, R, A, B, J, F

b) Which letter is showing a unconformity? How do you know?

A, because it is showing the erosion of layer X

c) Which layer could we find its absolute age? How do you know?

R, because it is an igneous rock layer

21. Describe the process of radioactive decay

During radioactive decay, the atoms of an unstable element break down to form atoms of a stable element
22. Explain how scientists use absolute age to date rocks.	Scientists date rocks using absolute age by comparing the percentage of an unstable element to the percentage of the stable element.

23. Radioactive dating shows that the Earth is about _______ years old.	4.6 billion years old

24. Explain why half-lives are reliable tools for dating igneous rock.	Radioactive elements occur naturally in igneous rocks. Scientists use the rate at which these elements decay to calculate the rock's age. The rate of decay of each radioactive element is constant – it never changes. The half-life of a radioactive element is the time it takes for half of the radioactive atoms to decay.
25. Explain why Carbon-14 is useful for dating objects no more than 50,000 years old.	Carbon-14 has a short half-life and too little is left after 50,000 years
26. An isotope of cesium (cesium-137) has a half life of 30 years. If 1.0 mg of cesium-137 disintegrates over a period of 90 years, how many mg of cesium-137 would remain?	90 years / 30 years = 3 half lives

1/2 = 0.5; 0.5/2 =0.25; 0.25/2 = 0.125

0.125 mg
27. The Earth’s geologic time scale is a record of Earth’s history based on	Major Events

28. What makes up almost 90% (first 4 billion years) of Earth’s history?	Precambrian Time
29. Why are there no plant or animal fossils from Precambrian Time?	this part of geologic history occurred before plants and animals appeared

30. The correct sequence of geologic eras from earliest to most recent is	Precambrian, Paleozoic, Mesozoic, Cenozoic
31. The first forms of life were __________and what time period did this event occur?	Single-celled bacteria; Precambrian

31. What Era is known as the Age of Reptiles?	Mesozoic
32. What Era did modern humans evolve in?	Cenozoic
33. What Era is known as the Age of Invertebrates (Fish)?	Paleozoic
34. Around 65 million years ago, the dinosaurs

and many other life forms underwent a mass

extinction. What do most scientists agree was the

cause of this mass extinction?	global climate change caused by an asteroid hitting Earth. When the asteroid hit Earth, the impact threw huge amounts of dust and water vapor into atmosphere. Blocked sunlight. Increased volcanic activity.

Fossils, Relative Age & Geo Time Study Guide

CHCCS

2012-03-18