(a) What kinds of evidence do glaciers leave
behind?
(b) Explain how the direction of motion of
ancient glaciers can be determined.
(c) What can explain evidence of ancient
glaciers near the equator?
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(a) Glaciers leave behind various types of evidence that can help in understanding their past presence. Some of the key evidences include:
1. Glacial Striations: These are scratches and grooves left on the underlying bedrock due to the movement of glacial ice. They indicate the direction of glacier motion.
2. Erratics: Glaciers can transport large boulders and rocks from one location to another. These rocks, called erratics, can be found in areas where they don't naturally occur, providing evidence of glacial activity.
3. Moraines: Glaciers deposit rocks, sediment, and debris along their edges. These deposits are known as moraines and can be categorized into lateral, medial, and terminal moraines depending on their location within the glacier.
4. U-shaped Valleys: Glaciers reshape the landscapes they move through, often resulting in U-shaped valleys. These valleys have distinct characteristics like steep sides and flat bottoms, indicating the presence of past glaciation.
5. Glacial Till: As glaciers retreat, they leave behind a mixture of unsorted sediment known as glacial till. This can include a wide range of particle sizes and types, providing evidence of past glacier activity.
(b) Determining the direction of motion of ancient glaciers can be done using several indicators:
1. Striations on Bedrock: Glacial ice contains rocks and debris that act as natural grinding tools. As the ice moves, these materials scratch and groove the underlying bedrock, leaving behind striations. By analyzing the orientation of these striations, one can determine the direction in which the glacier was moving.
2. Drumlin Formation: A drumlin is an elongated hill composed of till that has a streamlined shape with a blunt end facing the direction of ice movement. By examining the long axes of drumlins, it is possible to deduce the direction of glacial motion.
3. Roche Moutonnée: A roche moutonnée is a bedrock outcrop that has been sculpted by glaciation. One side of the feature is gently sloping and smoothed due to the abrasive action of the glacial ice, whereas the other side is steep and fractured. The steep side indicates the direction from which the glacier advanced.
4. Riegel: When glaciers retreat, they leave behind ridges called riegels. These can help determine the direction of glacial movement.
(c) The presence of evidence of ancient glaciers near the equator can be explained by a few factors:
1. Plate Tectonics: Over geological timescales, the movement of tectonic plates can cause changes in the Earth's climate. Although the equator is generally associated with a warm and tropical climate, plate movements can lead to changes in atmospheric circulation patterns, resulting in cooler temperatures conducive to glacial formation.
2. Elevation: Glaciers require certain elevation and topographic conditions to form. Even near the equator, high mountain ranges with sufficient elevation can support the development of glaciers.
3. Climate Change: Throughout Earth's history, there have been periods of significant climate change, including glaciations. Fluctuations in atmospheric carbon dioxide levels, variations in solar radiation, and other factors can cause regional cooling and the formation of glaciated regions even near the equator.
It is important to note that while evidence of ancient glaciers near the equator exists, it does not imply that glaciers of the same scale are necessarily present in those regions today. Climate conditions have undoubtedly changed over time, leading to the retreat or disappearance of glaciers in various locations.