A tidewater glacier runs into the sea. The face of the ice fractures and pieces calve off
Rendered wity DAZ Bryce 7.1; a bit of post processing with Photoshop. Click on image for full-size view.
Glaciers occur where the accumulation of snow is greater than the amount that melts during the warmer months. Over time, often decades or centuries, a glacier forms as the snow compacts and turns to ice. A glacier is distinct from sea ice and lake ice that form on the surface of bodies of water.Most glaciers are found in higher latitudes near the poles and high-altitude alpine regions, but are found on all continents save Australia. However, Australia is considered part of Oceania, which includes Papua New Guinea and New Zealand, both of which have glaciers. There are two main types of glaciers: valley glaciers and continental glaciers (known as ice sheets).
Glacial ice is the largest reservoir of freshwater on Earth. Many glaciers store water during one season and release it later as meltwater, a water source that is especially important for plants, animals and human uses when other sources may be scant. The amount of precipitation (whether in the form of snowfall, freezing rain, avalanches, or wind-drifted snow) is important to glacier survival. In areas such as Antarctica, where the low temperatures are ideal for glacier growth, very low annual precipitation causes the glaciers to grow very slowly. Because glacial mass is affected by long-term climate changes, e.g., precipitation, mean temperature, and cloud cover, glacial mass changes are considered among the most sensitive indicators of climate change and are a major source of variations in sea level.
As snow accumulates and compacts underlying snow layers from previous years into solid ice, causing changes in volume, density and crystal structure. The distinctive blue tint of glacial ice is often wrongly attributed to Rayleigh scattering due to bubbles in the ice. The blue color is actually created for the same reason that water is blue, that is, its slight absorption of red light due to an overtone of the infrared OH stretching mode of the water molecule. Glacial ice may also appear white because some ice is highly fractured with air pockets and indiscriminately scatters the visible light spectrum. Rocks and other debris picked up by the glacier add a brown tint to the picture.
Glaciers flow slowly due to stresses induced by their weight. Crevasses and other distinguishing features of a glacier are due to its flow. Another consequence of glacier flow is the transport of rock and debris abraded from the rock beneath it A glacier forms in a location where the accumulation of snow and sleet exceeds the amount of snow that melts. Another type of movement is through basal sliding. In this process, the glacier slides over the terrain on which it sits, lubricated by the presence of liquid water. As the pressure increases toward the base of the glacier, the melting point of water decreases, and the ice melts. Friction between ice and rock and geothermal heat from the Earth’s interior also contribute to melting. This type of movement is dominant in temperate, or warm-based glaciers. The geothermal heat flux becomes more important the thicker a glacier becomes.
Crevasses form due to differences in glacier velocity. As the parts move at different speeds and directions, shear forces cause the two sections to break apart, opening the crack of a crevasse all along the disconnecting faces. Hence, the distance between the two separated parts, while touching and rubbing deep down, frequently widens significantly towards the surface layers, many times creating a wide chasm. Crevasses seldom are more than 150 feet (46 m) deep but in some cases can be 1,000 feet (300 m) or even deeper. Beneath this point, the plastic deformation of the ice under pressure is too great for the differential motion to generate cracks. Transverse crevasses are transverse to flow, as a glacier accelerates where the slope steepens. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form from the edge of the glacier, due to the reduction in speed caused by friction of the valley walls. Marginal crevasses are usually largely transverse to flow.
As the glacier flows over the bedrock’s fractured surface, it softens and lifts blocks of rock that are brought into the ice. This process is known as plucking, and it is produced when sub-glacial water penetrates the fractures and the subsequent freezing expansion separates them from the bedrock. When the ice expands, it acts as a lever that loosens the rock by lifting it. This way, sediments of all sizes become part of the glacier’s load. The rocks frozen into the bottom of the ice then act like grit in sandpaper.
Abrasion occurs when the ice and the load of rock fragments slide over the bedrock and function as sandpaper that smooths and polishes the surface situated below. This pulverized rock is called rock flour. The flour is formed by rock grains of a size between 0.002 and 0.00625 mm. Sometimes the amount of rock flour produced is so high that currents of meltwaters acquire a grayish color. These processes of erosion lead to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides. These further add material to the glacier.
Visible characteristics of glacial abrasion are glacial striations. These are produced when the bottom’s ice contains large chunks of rock that mark scratches in the bedrock. By mapping the direction of the flutes, researchers can determine the direction of the glacier’s movement. Chatter marks are seen as lines of roughly crescent-shape depressions in the rock underlying a glacier, caused by the abrasion where a boulder in the ice catches and is then released repetitively as the glacier drags it over the underlying basal rock.
Tidewater glaciers (as in the image above) are glaciers that terminate in the sea. As the ice reaches the sea pieces break off, or calve, forming icebergs. Most tidewater glaciers calve above sea level, which often results in a tremendous splash as the iceberg strikes the water. If the water is deep, glaciers can calve underwater, causing the iceberg to suddenly leap up out of the water. The Hubbard Glacier is the longest tidewater glacier in Alaska and has a calving face over 10 km (6 mi) long. Yakutat Bay and Glacier Bay are both popular with cruise ship passengers because of the huge glaciers descending hundreds of feet to the water. This glacier type undergoes centuries-long cycles of advance and retreat that are much less affected by the climate changes currently causing the retreat of most other glaciers. Most tidewater glaciers are outlet glaciers of ice caps and ice fields.
Following the Little Ice Age, around 1850, the glaciers of the Earth have retreated substantially through the 1940s. A slight cooling led to the advance of many alpine glaciers from 1950-1985. However, since 1985 glacier retreat and mass balance loss has become increasingly ubiquitous and large.