7th Grade Unit 2 Study Guide: Answers

Study Guide Unit 2
1.What is the principle of uniformitarianism?
• The principle of uniformitarianism states that geologic processes that happened in the past, can be explained by current geologic processes.
2. What are two examples of surface changes that occur today that also occurred in the past?
• volcanic eruptions and erosion
3. What are five ways an animal or plant can become a fossil?
• trapped in amber
• trapped in asphalt, as in a tar pit
• buried in rocks
• frozen
• become petrified
4. What are three types of trace fossils and what can they reveal about the past?
• tracks, or footprints left in soft sediment
• burrows, pathways or shelters made by animal
• animal dung or coprolites
• These trace fossils leave clues about how animals behaved in the past.
5. What was Pangaea? What happened to it?
• Pangaea was a single landmass or supercontinent
• It broke up 200 million years ago
6. What are three ways besides fossils that we can learn about Earth’s past climate?
• Tree rings
• Sea floor sediments
• ice cores
7. What is relative dating?
• Determining the order of events, whether an object is older or younger
8. What is the law of superposition?
• Principle that states the younger rocks lie above older rocks if the layers have not been disturbed.
9. Are intrusions older or younger than the rocks that contain them? How do you know?
• Intrusions are younger according to the law of cross-cutting relationships.
10. What do we call a missing layer of rock? What are two ways this can happen?
• Unconformities can happen when rock layers are eroded or sediment is not deposited for a long period of time.
11. What is the difference between absolute and relative dating?
• Absolute dating gives the exact age of a rock or fossil as measured in years
• Relative age tells us the order that events happened in and whether an event or object is younger or older
12. What is radioactive decay?
• The breakdown of an unstable isotope into a stable isotope by shedding particles.
13. What is half life? How do we use half life to calculate absolute age?
• Half life is the amount of time it takes for the unstable parent isotope to break down into the stable daughter isotope.
• We can find out the number of half lives that have occurred in a sample by looking at the ratio of parent to daughter isotopes.
• We then multiply the number of half lives by the number of years per half life to get absolute age.
14. What are three types of radiometric dating and what age range can each be used for?
• Radiocarbon dating: up to 45,000 years old
• Potassium-argon dating: 100,000 years- a few billion years old
• Uranium-lead dating: 100 million years to billions of years old
15. What kind of radiometric dating do we use on fossils?
• Radiocarbon dating or C14 dating
16. How old is the earth? How do we know?
• 4.6 billion years old
• We know from uranium-lead dating performed on meteorites and moon rocks
17. What are the four characteristics of an index fossil?
• Must be present over a wide region
• Must have clearly distinguished features
• Only lived a short period of time
• Found in large numbers within the rock numbers
18. What are the four divisions of the geologic time scale?
• Eon, era, period, epoch
19. What is Precambrian time? How much of Earth’s history does this represent?
• 4.6 billion years-540 million years ago, represents 90% of Earth’s history
20. What eon, era, period, and epoch do we live in?
• Phanerozoic Eon
• Cenozoic Era
• Quaternary Period
• Halocene Epoch
21. During which era did the dinosaurs live? When did they go extinct?
• Mesozoic Era
• 66 million years ago
22. How did oxygen enter the Earth’s atmosphere?
• cyanobacteria released oxygen into the air through photosynthesis
23. What kinds of changes did the Earth experience multiple times over its history?
• mass extinctions, continental drift warming and cooling periods, and life becoming more and more complex