Problem 3
This scenic image is from Monument Valley in the northeastern corner of Arizona. The bedrock in this region consists of layers of sedimentary rocks. Although the prominent rock exposures ("monuments") in this photo are widely separated, we can infer that they represent a oncecontinuous layer. Discuss the principle that allows us to make this inference.
Problem 6
These polished stones are called gastroliths. Explain how such objects can be considered fossils. What category of fossil are they? Name another example of a fossil in this category.
Problem 7
If a radioactive isotope of thorium (atomic number 90 , mass number 232) emits 6 alpha particles and 4 beta particles during the course of radioactive decay, what are the atomic number and mass number of the stable daughter product?
Problem 8
A hypothetical radioactive isotope has a half-life of 10,000 years. If the ratio of radioactive parent to stable daughter product is \(1: 3\), how old is the rock that contains the radioactive material?
Problem 9
Solve the problems below that relate to the magnitude of Earth history. To make calculations easier, round Earth's age to 5 billion years. a. What percentage of geologic time is represented by recorded history? (Assume 5000 years for the length of recorded history.) b. Humans and their close relatives (hominins) have been around for roughly 5 million years. What percentage of geologic time is represented by the history of this group? c. The first abundant fossil evidence does not appear until the beginning of the Cambrian period, about 540 million years ago. What percentage of geologic time is represented by abundant fossil evidence?
Problem 10
A portion of a popular college text in historical geology includes 10 chapters (281 pages) in a unit titled "The Story of Earth." Two chapters (49 pages) are devoted to Precambrian time. By contrast, the last two chapters ( 67 pages) focus on the most recent 23 million years, with 25 of those pages devoted to the Holocene Epoch, which began 10,000 years ago. a. Compare the percentage of pages devoted to the Precambrian to the actual percentage of geologic time that this span represents. b. How does the number of pages about the Holocene compare to its actual percentage of geologic time? c. Suggest some reasons why the text seems to have such an unequal treatment of Earth history.
