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 errare ("to wander"), 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 (16,500 metric 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:
The term "erratic" is commonly used to refer to erratic blocks, which geologist Archibald 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 including:
Evidence supports another possibility for the 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]
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]
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]
Sediments from the late Pleistocene period lying on the floor of the North Atlantic show a series of layers (referred to as Heinrich 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 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.
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 1.5-metre-diameter (5 ft) boulder can be carried by a 3-metre-high (10 ft) iceberg and could be found stranded at higher elevations than a 2-metre (7 ft) boulder, which requires a 4-metre-high (13 ft) iceberg. [9]
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 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 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]
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]
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, [12] 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 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) [13] 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. [14] 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, [15] Charpentier as well as Schimper [14] 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. [16]
If glacial ice is "rafted" by a flood such as that created when the ice dam broke during the Missoula floods, then 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.
A glacier is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as crevasses and seracs, as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques, moraines, or fjords. Although a glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and greenhouse periods during which there are no glaciers on the planet. Earth is currently in the ice age called Quaternary glaciation. Individual pulses of cold climate within an ice age are termed glacial periods, and intermittent warm periods within an ice age are called interglacials or interstadials.
A moraine is any accumulation of unconsolidated debris, sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sheet. It may consist of partly rounded particles ranging in size from boulders down to gravel and sand, in a groundmass of finely-divided clayey material sometimes called glacial flour. Lateral moraines are those formed at the side of the ice flow, and terminal moraines were formed at the foot, marking the maximum advance of the glacier. Other types of moraine include ground moraines and medial moraines.
Till or glacial till is unsorted glacial sediment.
Glaciology is the scientific study of glaciers, or, more generally, ice and natural phenomena that involve ice.
A jökulhlaup is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst floods from Vatnajökull, Iceland, which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption, but it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir.
A cirque is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie and cwm. A cirque may also be a similarly shaped landform arising from fluvial erosion.
Glacial landforms are landforms created by the action of glaciers. Most of today's glacial landforms were created by the movement of large ice sheets during the Quaternary glaciations. Some areas, like Fennoscandia and the southern Andes, have extensive occurrences of glacial landforms; other areas, such as the Sahara, display rare and very old fossil glacial landforms.
Parent material is the underlying geological material in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering and the mode by which the materials were most recently transported.
Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the weathering and erosion of pieces of bedrock, especially large "joint blocks". This occurs in a type of glacier called a "valley glacier". As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock. The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice/water phase transition, gradually loosening the rock between the joints. This produces large chunks of rock called joint blocks. Eventually these joint blocks come loose and become trapped in the glacier.
A tunnel valley is a U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. They can be as long as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep.
Dropstones are isolated fragments of rock found within finer-grained water-deposited sedimentary rocks or pyroclastic beds. They range in size from small pebbles to boulders. The critical distinguishing feature is that there is evidence that they were not transported by normal water currents, but rather dropped in vertically through the air or water column. Such depositions can occur during a volcanic eruption, e.g.
Fluvioglacial landforms or glaciofluvial landforms are those that result from the associated erosion and deposition of sediments caused by glacial meltwater. Glaciers contain suspended sediment loads, much of which is initially picked up from the underlying landmass. Landforms are shaped by glacial erosion through processes such as glacial quarrying, abrasion, and meltwater. Glacial meltwater contributes to the erosion of bedrock through both mechanical and chemical processes. Fluvio-glacial processes can occur on the surface and within the glacier. The deposits that happen within the glacier are revealed after the entire glacier melts or partially retreats. Fluvio-glacial landforms and erosional surfaces include: outwash plains, kames, kame terraces, kettle holes, eskers, varves, and proglacial lakes.
The Foothills Erratics Train is a 580 miles (930 km) long, narrow, linear scatter of thousands of typically angular boulders of distinctive quartzite and pebbly quartzite that lie on the surface of a generally north-south strip of the Canadian Prairies. These boulders, which are between 1 foot (0.30 m) and 135 feet (41 m) in length, are glacial erratics that lie upon a surficial blanket of Late Wisconsin glacial till. The largest glacial erratic within the Foothills Erratics Train is Big Rock.
Boulder Park National Natural Landmark, of Douglas County, Washington, along with the nearby McNeil Canyon Haystack Rocks and Sims Corner Eskers and Kames natural landmarks, illustrate well-preserved examples of classic Pleistocene ice stagnation landforms that are found in Washington. These landforms include numerous glacial erratics and haystack rocks that occur near and on the Withrow Moraine, which is the terminal moraine of the Okanogan ice lobe.
A balancing rock, also called a balanced rock or precarious boulder, is a naturally occurring geological formation featuring a large rock or boulder, sometimes of substantial size, resting on other rocks, bedrock, or on glacial till. Some formations known by this name only appear to be balancing, but are in fact firmly connected to a base rock by a pedestal or stem.
New England is a region in the North Eastern United States consisting of the states Rhode Island, Connecticut, Massachusetts, New Hampshire, Vermont, and Maine. Most of New England consists geologically of volcanic island arcs that accreted onto the eastern edge of the Laurentian Craton in prehistoric times. Much of the bedrock found in New England is heavily metamorphosed due to the numerous mountain building events that occurred in the region. These events culminated in the formation of Pangaea; the coastline as it exists today was created by rifting during the Jurassic and Cretaceous periods. The most recent rock layers are glacial conglomerates.
This is a list of erratics on and around Rügen – the largest island off the Baltic coast of Germany. An erratic is usually defined as an individual block of rock lying on the surface of the land which has a volume of at least one cubic metre and which was transported by a glacier to its present site during the ice age.
Glacial erratic boulders of Estonia are large boulders of rock which have been formed and moved into Estonia by glacial action during previous ice ages. They are now registered and protected by the government.
The Edaga Arbi Glacials are a Palaeozoic geological formation in Tigray and in Eritrea. The matrix is composed of grey, black and purple clays, that contains rock fragments up to 6 metres across. Pollen dating yields a Late Carboniferous to Early Permian age.
erratic glacier.
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