Glacial erratic
A glacial erratic is a glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word , are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock in Alberta.
Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:
- They can be transported by glaciers, and are thereby one of a series of indicators which mark the path of prehistoric glacier movement. Their lithographic origin can be traced to the parent bedrock, allowing for confirmation of the ice flow route.
- They can be transported by ice rafting, which allows quantification of the extent of glacial flooding resulting from ice dam failures which release the waters stored in proglacial lakes such as Lake Missoula. Erratics released by ice rafts that were stranded and subsequently melted, dropping their load, allow characterization of the high-water marks for transient floods in areas like temporary Lake Lewis.
- Erratics dropped by icebergs melting in the ocean can be used to track Antarctic and Arctic-region glacial movements for periods prior to record retention. Also known as dropstones, these can be correlated with ocean temperatures and levels to better understand and calibrate models of the global climate.
Formation of erratics
Erratics are formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes including:
- Abrasion/Scouring – debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till.
- Plucking – pieces of bedrock are cracked off by glaciers, producing larger erratics.
- Ice thrusting – the glacier freezes to its bed, moving large sheets of frozen sediment at its base along with it.
- Glacially induced spalling – layers of rock are spalled off the rocks below the glacier during ice lens formation. This provides smaller debris, which is ground into the glacial basal material, to become till.
Evidence supports another possibility for the creation of erratics as well: rock avalanches onto the upper surface of the glacier. Rock avalanche–supraglacial transport occurs when the glacier undercuts a rock face, which fails by avalanche onto the upper surface of the glacier. The characteristics of rock avalanche–supraglacial transport includes:
Glacier-borne erratic
Erratics provide an important tool in characterizing the directions of glacier flows, which are routinely reconstructed used on a combination of moraines, eskers, drumlins, meltwater channels and similar data. Erratic distributions and glacial till properties allow for identification of the source rock from which they derive, which confirms the flow direction, particularly when the erratic source outcrop is unique to a limited locality. Erratic materials may be transported by multiple glacier flows prior to their deposition, which can complicate the reconstruction of the glacial flow.Ice-rafted erratic
Glacial ice entrains debris of varying sizes from small particles to extremely large masses of rock. This debris is transported to the coast by glacier ice and released during the production, drift and melting of icebergs. The rate of debris release by ice depends upon the size of the ice mass in which it is carried as well as the temperature of the ocean through which the ice floe passes.Image:Yeager-Rock-Erractic-PB110039.JPG|thumb|alt=This photo shows an automobile passing in front of a rock which is essentially fully exposed. The rock has a rough, dark surface indicating it is weathered basalt and is roughly circular in exposed cross-section. The rock is immediately adjacent to a roadway—the road cut removed much of the earth from one side of it exposing it—from the excavation it is evident that the rock sits on a mound of glacial till. The rock is approximately two times the length of the car in one direction and five times the height of the car in the other direction. Since the rock has not tipped onto the road and no structural support is provided, it must be approximately as deep as it is wide and high. Since the density of basalt is 3 grams per cubic centimetre, this puts the mass of the rock at about 400 to 500 metric tons. |Yeager Rock, a boulder on the Waterville Plateau, Washington. Although transported by a glacier, this boulder is not a true erratic because it is of the same lithology as the underlying, till-blanketed bedrock. Note the glacial till below the rock.
Sediments from the late Pleistocene period lying on the floor of the North Atlantic show a series of layers which contain ice-rafted debris. They were formed between 14,000 and 70,000 years before the present. The deposited debris can be traced back to the origin by both the nature of the materials released and the continuous path of debris release. Some paths extend more than distant from the point at which the ice floes originally broke free.
The location and altitude of ice-rafted boulders relative to the modern landscape has been used to identify the highest level of water in proglacial lakes and temporary lakes. Ice-rafted debris is deposited when the iceberg strands on the shore and subsequently melts, or drops out of the ice floe as it melts. Hence all erratic deposits are deposited below the actual high water level of the lake; however, the measured altitude of ice-rafted debris can be used to estimate the lake surface elevation.
Image:Angular glacial erratic on Lambert Dome-750px.jpg|thumb|right|Angular glacial erratic on Lembert Dome
This is accomplished by recognizing that on a fresh-water lake, the iceberg floats until the volume of its ice-rafted debris exceeds 5% of the volume of the iceberg. Therefore, a correlation between the iceberg size and the boulder size can be established. For example, a boulder can be carried by a iceberg and could be found stranded at higher elevations than a boulder, which requires a iceberg.
Large erratics
Large erratics consisting of slabs of bedrock that have been lifted and transported by glacier ice to subsequently be stranded above thin glacial or fluvioglacial deposits are referred to as glacial floes, rafts or erratic megablocks. Erratic megablocks have typical length-to-thickness ratios on the order of 100 to 1. These megablocks may be found partially exposed or completely buried by till and are clearly allochthonous, since they overlay glacial till. Megablocks can be so large that they are mistaken for bedrock until underlying glacial or fluvial sediments are identified by drilling or excavation. Such erratic megablocks greater than in area and in thickness can be found on the Canadian Prairies, Poland, England, Denmark and Sweden. One erratic megablock located in Saskatchewan is . Their sources can be identified by locating the bedrock from which they were separated; several rafts from Poland and Alberta were determined to have been transported over from their source.Nonglacial erratics
In geology an erratic is any material which is not native to the immediate locale but has been transported from elsewhere. The most common examples of erratics are associated with glacial transport, either by direct glacier-borne transport or by ice rafting. However, other erratics have been identified as the result of kelp holdfasts, which have been documented to transport rocks up to in diameter, rocks entangled in the roots of drifting logs, and even in transport of stones accumulated in the stomachs of pinnipeds during foraging.History
During the 18th century, erratics were deemed a major geological paradox. Geologists identify erratics by studying the rocks surrounding the position of the erratic and the rock of the erratic itself. Erratics were once considered evidence of a biblical flood, but in the 19th century scientists gradually came to accept that erratics pointed to an ice age in Earth's past. Among others, the Swiss politician, jurist and theologian saw glaciers as a possible solution as early as 1788. However, the idea of ice ages and glaciation as a geological force took a while to be accepted. Ignaz Venetz, a Swiss engineer, naturalist and glaciologist was one of the first scientists to recognize glaciers as a major force in shaping the earth.In the 19th century, many scientists came to favor erratics as evidence for the end of the ice age 10,000 years ago, rather than a flood. Geologists have suggested that landslides or rockfalls initially dropped the rocks on top of glacial ice. The glaciers continued to move, carrying the rocks with them. When the ice melted, the erratics were left in their present locations.
Charles Lyell's Principles of Geology provided an early description of the erratic which is consistent with the modern understanding. Louis Agassiz was the first to scientifically propose that the Earth had been subject to a past ice age. In the same year, he was elected a foreign member of the Royal Swedish Academy of Sciences. Prior to this proposal, Goethe, de Saussure, Venetz, Jean de Charpentier, Karl Friedrich Schimper and others had made the glaciers of the Alps the subjects of special study, and Goethe, Charpentier as well as Schimper had even arrived at the conclusion that the erratic blocks of alpine rocks scattered over the slopes and summits of the Jura Mountains had been moved there by glaciers.
Charles Darwin published extensively on geologic phenomena including the distribution of erratic boulders. In his accounts written during the voyage of, Darwin observed a number of large erratic boulders of notable size south of the Strait of Magellan, Tierra del Fuego and attributed them to ice rafting from Antarctica. Recent research suggests that they are more likely the result of glacial ice flows carrying the boulders to their current locations.