Question

What kinds of geologic evidence suggest that continental glaciers spread widely more than 2 billion years ago?

Short answer

Geologic evidence such as the presence of tillites, striations, glacial erratics, and glacial deposits, along with paleomagnetic studies, indicate that continental glaciers were widespread more than 2 billion years ago. These features in ancient rocks show that glaciers were responsible for transporting debris, scratching and grooving bedrock, and disrupting magnetic patterns, proving their extensive occurrence during that time period.

Step-by-step solution

1. Introduction

Continental glaciers are large ice sheets that cover vast areas of land. These glaciers played a significant role in shaping the Earth's surface, leaving behind various geological features that can indicate their presence in the past. To understand how continental glaciers spread widely over 2 billion years ago, we will examine different types of geologic evidence.

2. Tillites

Tillites are sedimentary rocks formed from the deposition of glacial debris. These rocks are often made up of a mixture of different rock fragments and minerals, indicating they were carried by moving glaciers. The presence of tillites in ancient rock formations, dating back to over 2 billion years ago, suggests that continental glaciers were once widespread during this time period.

3. Striations

As glaciers move, they drag along rocks and other debris that can cause scratches and grooves on the underlying bedrock. These markings, called striations, are another type of geologic evidence for ancient continental glaciers. In some areas, striations have been found on rocks that date back more than 2 billion years, suggesting that glaciers once covered these regions.

4. Glacial Erratics

Glacial erratics are large rocks and boulders that have been transported by moving glaciers and deposited in areas far from their original locations. The presence of erratics in regions with rock formations dating back to over 2 billion years ago indicates that glaciers once covered these areas, moving the erratics from other regions and leaving them behind when the ice melted.

5. Glacial Deposits/Drift

Glacial deposits, or drift, are accumulations of sediment and debris that have been transported by glaciers and then deposited as the ice melts. These deposits often consist of a range of rock types and sizes, reflecting the diverse materials carried by glaciers. The presence of ancient glacial deposits in certain regions suggests that continental glaciers were once widespread and responsible for transporting these materials.

6. Paleomagnetism

Lastly, paleomagnetic studies can provide evidence of ancient continental glaciers. Magnetic minerals within rocks record the orientation of Earth's magnetic field at the time they were formed. As glaciers move, they disrupt these magnetic signatures, leading to disorganized magnetic patterns in rocks from the affected areas. The presence of such patterns in rocks over 2 billion years old indicates that glaciers were widespread during this time period. By examining these different types of geologic evidence – tillites, striations, glacial erratics, glacial deposits, and paleomagnetism – we can conclude that continental glaciers were indeed widespread more than 2 billion years ago.