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Chapter 22: Problem 13

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How was the standard four-stage chronology of North American glaciations developed? Why is it so incomplete? How was it modified with the study of marine sediment? (D)

Short Answer

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The standard four-stage chronology of North American glaciations was developed by observing and comparing glacial deposits across the continent, classifying them into Nebraskan, Kansan, Illinoian, and Wisconsinan glaciations. It was incomplete due to gaps in the geologic record and the regional nature of the study. The study of marine sediment, specifically deep-sea sediment cores and oxygen isotope ratios, helped modify the chronology by providing more accurate identification and correlation of glaciations and offering additional information on interglacial periods.

Step by step solution

01

Introduction

The four-stage chronology of North American glaciations refers to the classification of ice ages that occurred in North America throughout the Quaternary period. It was originally developed by observing the glacial deposits in North America and is based on specific intervals within the geologic record. However, the initial model was incomplete, and it was later refined with the study of marine sediment.
02

Developing the four-stage chronology of North American glaciations

The four-stage chronology was developed by observing and comparing the evidence of glacial deposits across North America. Geologists studied a variety of features, such as moraines, outwash plains, and glacial erratics, to determine the relative ages of these glaciations. Based on these observations, the glacial history of North America was divided into four main timeframes, known as Nebraskan, Kansan, Illinoian, and the most recent, Wisconsinan glaciations.
03

Why the original four-stage chronology is incomplete

The original four-stage chronology is incomplete because it only accounts for the glaciations that occurred during specific intervals in the geological record. These glaciations left distinct deposits, which allowed for the differentiation between the four stages. However, there are gaps in the geologic record, suggesting that the full extent of glaciations in North America has not been captured. Moreover, the regional nature of the study limited the understanding of the glaciations and their global implications.
04

Modifying the chronology with the study of marine sediment

The study of marine sediment has helped provide a more complete understanding of the glaciations in North America. Researchers have analyzed deep-sea sediment cores to measure the relative abundance of oxygen isotopes, which can indicate global ice volume and temperature changes. This data has allowed geologists to more accurately identify and correlate different glaciations and provide additional information on interglacial periods between the four main glaciations.
05

Conclusion

In conclusion, the standard four-stage chronology of North American glaciations was developed by studying glacial deposits across North America, but it was incomplete due to gaps in the geological record and the regional nature of the study. The analysis of marine sediment has helped refine this chronology, providing a better understanding of the timing, extent, and global implications of glaciations in North America.

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