Glacial erratic

Big Rock in Okotoks, Alberta
Indian Rock in the Village of Montebello, New York
Indian Rock in the Village of Montebello, New York
The Giebichenstein in Stöckse, Germany
Doane Rock, at Cape Cod National Seashore
Glacial erratics from Norway on Schokland in Holland

A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and 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 (15,000 tonnes or 17,000 short tons) 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:

Formation of erratics

Multiple erratics on the terminal moraine of the Okanagon Lobe. Cascade mountains in the background.

The term "erratic" is commonly used to refer to erratic blocks, which Geikie describes as: "large masses of rock, often as big as a house, that have been transported by glacier-ice, and have been lodged in a prominent position in the glacier valleys or have been scattered over hills and plains. And examination of their mineralogical character leads the identification of their sources…".[2] In geology, an erratic is material moved by geologic forces from one location to another, usually by a glacier.

Erratics are formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes: abrasion/scouring, plucking, ice thrusting and glacially-induced spalling. Glaciers crack pieces of bedrock off in the process of plucking, producing the larger erratics. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. Glacially-induced spalling occurs when ice lens formation with the rocks below the glacier spall off layers of rock, providing smaller debris which is ground into the glacial basal material to become till.[3][4]

Evidence supports another option for creation of erratics as well, rock avalanches onto the upper surface of the glacier (supraglacial). 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:[5]

  • Monolithologic composition - a cluster of boulders of similar composition are frequently found in close proximity. Commingling of the multiple lithologies normally present throughout the glaciated basin, has not occurred.[5]
  • Angularity - the supraglacially transported rocks tend to be rough and irregular, with no sign of subglacial abrasion. The sides of boulders are roughly planar, suggesting that some surfaces may be original fracture planes.[5]
  • Great size - the size distribution of the boulders tends to be skewed toward larger boulders than those produced subglacially.[5]
  • Surficial positioning of the boulders - the boulders are positioned on the surface of glacial deposits, as opposed to partially or totally buried.[5]
  • Restricted areal extents - the boulder fields tend to have limited areal extent; the boulders cluster together, consistent with the boulders landing on the surface of the glacier and subsequently deposited on top of the glacial drift.[5]
  • Orientations - the boulders may be close enough that original fracture planes can be matched.[5]
  • Locations of the boulder trains - the boulders appear in rows, trains or clusters along the lateral moraines as opposed to being located on the terminal moraine or in the general glacial field.[5]

Glacier-borne erratic

Two icebergs clearly retain fragments of the moraine (rock debris) that forms a dark line along the upper surface of the glacier. The inclusion of the moraine illustrates how land-based rocks and sediment are carried by ice.

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.[6]

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.[7][8]

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 2 times the length of the car (i.e., ~ 9 meters) in one direction and 5 times the height of the car in the other direction (i.e., ~ 9 meters). 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 centimeter, this puts the mass of the rock at about 400 to 500 metric tons (consistent with the references).
Yeager Rock, a 400-metric-ton (440-short-ton) erratic on the Waterville Plateau, Washington. 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 (referred to at Heinrich layers) which contain ice-rafted debris. They were formed between 14,000 & 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 3,000 kilometres (1,900 mi) distant from the point at which the ice floes originally broke free.[7]

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 (e.g., Lake Musselshell in central Montana) and temporary lakes (e.g., Lake Lewis in Washington state. 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.

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, 1.5-metre (4.9 ft) diameter boulder can be carried by a 3-metre (9.8 ft) high iceberg and could be found stranded at higher elevations than a 2-metre (6.6 ft) boulder which requires a 4-metre (13 ft) high iceberg.[9]

Large erratics

Glacial erratic Ehalkivi with overground volume 930 cubic metres (1,220 cu yd) (weight approximately 2,500 metric tons or 2,800 short tons) in Estonia
Area exposed by the retreat of Alaska's Steller Glacier in August 1996, the westernmost part of Bering Glacier's piedmont lobe. The ground surface is covered by glacial sediment deposited as lodgement and ablation till. The erratic is an angular, 20-foot (6.1 m)-high piece of gneiss. Bering Glacier, Alaska flows through Wrangell-Saint Elias National Park

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 (schollen) 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 allochthonus, 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 1 square kilometre (250 acres) in area and 30 metres (98 ft) in thickness can be found on the Canadian prairies, Poland, England, Denmark and Sweden. One erratic megablock located in Saskatchewan is 30 by 38 kilometres (19 mi × 24 mi) (and up to 100 metres or 330 feet thick). 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 300 kilometres (190 mi) from their source.[10]

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 40 centimetres (16 in) in diameter, rocks entangled in the roots of drifting logs, and even in transport of stones accumulated in the stomachs of pinnipeds during foraging.[11]

History

During the 18th century, erratics were deemed a major geological paradox. Erratics were once considered evidence of a vast flood approximately 10,000 years ago, similar to the legendary floods described in the texts of ancient civilizations throughout the world. Ancient legends of an epic flood come from many cultures including Mesoamerican, Sumerian (Epic of Gilgamesh), Hebrew (Old Testament) and Indian culture. Among others, the Swiss politician, jurist and theologican Bernhard Friedrich Kuhn 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 (1788 — 1859), 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 Last Glacial Maximum (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 (v. 1, 1830)[12] 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.[13] 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,[14] Charpentier as well as Schimper[13] 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 HMS Beagle, 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.[15]

Examples

Glacier-borne erratics

Example of mixed erratics. The boulder in the foreground is basalt. The boulder across the fence is granite.

Examples of well-known erratics include:

Flood-borne erratics

In the event that glacial ice is "rafted" by a flood such as that created when the ice dam broke during the Missoula Floods, the erratics are deposited where the ice finally releases its debris load. One of the more unusual examples is found far from its origin in Idaho at Erratic Rock State Natural Site just outside McMinnville, Oregon. The park includes a 40-short-ton (36 t) specimen, the largest erratic found in the Willamette Valley.

See also

Wikimedia Commons has media related to Glacial erratic.

References

  1. ↑ Bard, Edouard (June 2004). "Effet de serre et glaciations, une perspective historique (Greenhouse effect and ice ages: historical perspective)". Comptes Rendus Geosciences -- External Geophysics, Climate and Environment (Climate) (Copyright © 2004 Académie des sciences. Published by Éditions scientifiques et médicales Elsevier SAS.) 336 (7–8): 603–638. Bibcode:2004CRGeo.336..603B. doi:10.1016/j.crte.2004.02.005. Retrieved 14 December 2009.
  2. ↑ Geikie, Sir Archibald (1882). "Textbook of geology". Retrieved 12 December 2009.
  3. ↑ Bell, Robin E. (27 April 2008). "The role of subglacial water in ice-sheet mass balance". Nature Geoscience 1 (5802): 297–304. Bibcode:2008NatGe...1..297B. doi:10.1038/ngeo186.
  4. ↑ Rempel, A. W. (2008). "A theory for ice-till interactions and sediment entrainment beneath glaciers". Journal of Geophysical Research (American Geophysical Union.) 113 (113=): F01013. Bibcode:2008JGRF..11301013R. doi:10.1029/2007JF000870.
  5. 1 2 3 4 5 6 7 8 Evenson, Edward B.; Patrick A. Burkhart; John C. Gosse; Gregory S. Baker; Dan Jackofsky; Andres Meglioli; Ian Dalziel; Stefan Kraus; Richard B. Alley; Claudio Berti (December 2009). "Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's boulders," Tierra del Fuego". GSA Today (The Geological Society of America) 19 (12): 4–10. doi:10.1130/GSATG72A.1. Retrieved 14 December 2009.
  6. ↑ Evans, David J.A; Chris D. Clark; Wishart A. Mitchell (May 2005). "The last British Ice Sheet: A review of the evidence utilised in the compilation of the Glacial Map of Britain". Earth-Science Reviews (Copyright © 2005 Elsevier B.V) 70 (3–4): 253. Bibcode:2005ESRv...70..253E. doi:10.1016/j.earscirev.2005.01.001. Retrieved 10 September 2013.
  7. 1 2 Bond, Gerard; Hartmut Heinrich, Wallace Broecker, Laurent Labeyrie ,Jerry McManus, John Andrews, Sylvain Huon, Ruediger Jantschik, Silke Clasen ,Christine Simet, Kathy Tedesco, Mieczyslawa Klas, Georges Bonani, & Susan Ivy (1992). "Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period". Nature 360 (6401): 245–249. doi:10.1038/360245a0. Cite uses deprecated parameter |coauthors= (help)
  8. ↑ Tripatia, Aradhna K.; Robert A. Eagleb; Andrew Mortonc; Julian A. Dowdeswelld; Katie L. Atkinsone; Yannick Bahéf; Caroline F. Dawbera; Emma Khadung; Ruth M.H. Shawa; Oliver Shorttleh; Lavaniya Thanabalasundarami (2007). "Evidence for glaciation in the Northern Hemisphere back to 44 Ma from ice-rafted debris in the Greenland Sea". Earth and Planetary Science Letters (Elsevier B.V.) 265 (1–2): 112–122. Bibcode:2008E&PSL.265..112T. doi:10.1016/j.epsl.2007.09.045.
  9. ↑ Davisa, Niole K.,; William W. Locke IIIa, Kenneth L. Piercea, b and Robert C. Finkelc (May 2006). "Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA". Geomorphology (Elsevier B.V.) 75 (3–4): 330–345. Bibcode:2006Geomo..75..330D. doi:10.1016/j.geomorph.2005.07.021. Cite uses deprecated parameter |coauthors= (help)
  10. ↑ Talbot, Christopher J. (April 1999). "Ice ages and nuclear waste isolation". Engineering Geology (Elsevier Science) 52 (3–4): 177–192. doi:10.1016/S0013-7952(99)00005-8.
  11. ↑ Paduan, Jennifer B.; David A. Clague; Alicé S. Davis (28 November 2007). "Erratic continental rocks on volcanic seamounts off the US west coast". Marine Geology 246 (1): 1–8. doi:10.1016/j.margeo.2007.07.007.
  12. ↑ Principles of Geology, Volume 1; Sir Charles Lyell - Geology - 1830
  13. 1 2 E.P. Evans: "The Authorship of the Glacial Theory", North American review Volume 145, Issue 368, July 1887. Accessed on February 25, 2008.
  14. ↑ Cameron, Dorothy (1964). Early discoverers XXII, Goethe-Discoverer of the ice age. Journal of glaciology (PDF).
  15. ↑ Edward B. Evenson, Patrick A. Burkhart, John C. Gosse, Gregory S. Baker, Dan Jackofsky, Andres Meglioli, Ian Dalziel, Stefan Kraus, Richard B. Alley, Claudio Berti; "Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's boulders", Tierra del Fuego; GSA Today; Volume 19 Issue 12 (December 2009); pp. 4-10
  16. ↑ Bleasdell, Rev. William (1872). "On Modern Glacial Action in Canada". Quarterly Journal of the Geological Society 28: 392–396. doi:10.1144/GSL.JGS.1872.028.01-02.45.
  17. ↑ "Glacial erratic - Coronation Park, Crosby". Geograph UK.
  18. ↑ http://www.bbc.co.uk/news/uk-england-beds-bucks-herts-36024009. Missing or empty |title= (help)

External links

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