Rivers of Ice: Glaciers
A glacier is a large, persistent body of dense ice that is constantly moving under its own weight. Glaciers are powerful agents of erosion and are responsible for carving many dramatic landscapes.
Types of Glaciers
1.Continental Glaciers (or Ice Sheets): Immense sheets of ice that cover vast areas of a continent (e.g., Greenland, Antarctica). They flow outwards from a central zone of accumulation.
2.Alpine Glaciers (or Valley Glaciers): Glaciers that form in mountainous areas and are confined to valleys. They flow downslope like very slow rivers.
Glacial Features
Erosional Features:
U-shaped Valley: Glaciers carve wide, flat-bottomed valleys, in contrast to the V-shaped valleys carved by rivers.
Cirque: A bowl-shaped, amphitheater-like depression at the head of a glacial valley.
Arête: A sharp, knife-like ridge formed between two adjacent glacial valleys.
Horn: A sharp, pyramid-like peak formed where several cirques erode back to back (e.g., the Matterhorn).
Depositional Features:
Till: An unsorted mixture of sediment (clay, sand, gravel, boulders) deposited directly by melting ice.
Moraine: A ridge of till deposited at the edges or end of a glacier. (Lateral, medial, and terminal moraines).
Drumlin: An elongated, teardrop-shaped hill of till, with the steep side facing the direction from which the ice advanced.
The Cause of Ice Ages: Milankovitch Cycles
Ice ages are periods of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets. The primary trigger for these cycles is variations in Earth's orbit, known as Milankovitch cycles.
1.Eccentricity: The shape of Earth's orbit around the Sun varies from nearly circular to more elliptical over a cycle of about 100,000 years.
2.Axial Tilt (Obliquity): The angle of Earth's axis of rotation varies between 22.1° and 24.5° over a cycle of about 41,000 years. A greater tilt leads to more extreme seasons.
3.Precession: The slow 'wobble' of Earth's axis, like a spinning top, over a cycle of about 26,000 years.
These cycles alter the amount and distribution of solar radiation reaching the Earth's surface, especially at high latitudes. When they align to produce cool summers in the Northern Hemisphere, snow can persist year-round, allowing ice sheets to grow and initiating an ice age.