"Only 12 of Antarctica's 42 seismometers picked up icequakes after the Maule earthquake, but the signals seemed to fit a pattern. The pattern suggests that opening or closing of shallow crevasses generated the tiny tremors. For example, seismic stations near Antarctica's mountain ranges and fast-flowing ice rivers known as ice streams were more likely to see icequakes. These are areas with a lot of crevasses. The high-frequency shaking also fits with cracking of brittle ice." Bold added.
"Antarctica's ice snapped and popped because of a major earthquake in Maule, Chile, halfway around the world [...] Antarctica has been touched by great earthquakes before. In March 2011, Japan's Tohoku tsunami tore off two Manhattan-size icebergs from the Sulzberger Ice Shelf, more than 8,000 miles (13,000 kilometers) south. Sailors also reported a massive Antarctica iceberg-calving event after Chile's 1868 great earthquake."
"Icequakes are seismic tremblings caused by sudden movement within a glacier or ice sheet, such as from a fracturing crevasse. (Anyone who has dropped an ice cube into a glass of water knows ice snaps under stress.)"
"Chile's magnitude-8.8 earthquake on Feb. 27, 2010, set off a flurry of Antarctic icequakes, each lasting from one to 10 seconds, researchers report today (Aug. 10) in the journal Nature Geoscience. The epicenter was 2,900 miles (4,700 km) north of Antarctica."
"We think the crevasses are being activated by the surface waves from this big earthquake coming through, and that's making the icequake."
"Many different kinds of icequakes rumble across Antarctica and Greenland. Known icequake triggers include opening and closing of the fractures called crevasses; glaciers tearing away from sticky bedrock; water runoff; and calving, the breaking off of an iceberg. Spooky underwater sounds from melting, cracking icebergs were once called The Bloop."
At the right is a thin section of Antarctic sea ice. It was taken from an ice core.
At the left is a thin section of half an ice core from Antarctica.
"Thin sections were made in the field to examine crystal sizes and fabrics."
"The GRIP core offers a unique possibility to study the growth, rotation and recrystallization of polar ice at an ideal location, covering a time span of more than 100,000 years. This information is obtained by a comprehensive thin section study of crystal sizes and c-axis orientations along its entire length. The results confirm earlier, basic observations on deep ice cores and have led to new insights. A significant variation of crystal size with climatic parameters is shown to persist to a great depth in the core; the development of a strong crystalline anisotropy in the ice sheet is also demonstrated."
"It is well known that bulk brittle ice has a hexagonal stucture, while brittle ice that forms in pores may be cubic in structure [...]. Adjacent surfaces appear to further alter the dynamics and structure of confined liquids and their crystals, leading in the case of a water/ice system to a state of enhanced rotational motion (plastic ice) just below the confined freezing/melting transitions. This plastic ice layer appears to form at both the ice-silica interface and the ice-vapour surface, and reversibly transforms to brittle ice at lower temperatures."
Systems "with larger dimensions (∼10nm) contain brittle cubic ice and also some hexagonal ice (if a vapour interface is present); even larger systems (> ∼30nm) contain predominately hexagonal ice. It is conjectured that this layer of plastic ice at vapour surfaces may be present at the myriad of such interfaces in macroscopic systems, such as snow-packs, glaciers and icebergs".
Theoretical brittle ice
"For fault zones and tectonic earthquakes, there is a dependence on which direction the wave came from."
Just "one kind of seismic wave, a surface wave, gets the blame for most of Antarctica's icequakes. [...] a Rayleigh wave [...] travels close to the Earth's surface, rolling along like a wave in a lake or the ocean. [...] At some stations, there was also a short icequake burst from a seismic "P wave," which travel through the Earth's interior."
"It's an interesting result. A big earthquake on the other side of the world can shift things in the Earth and make it crack."
"It was possible to count annual layers in the GRIP core to obtain an excellent dating, particularly back to the Younger Dryas period. Parameters used to date the core included ECM, dust, nitrate and ammonium, which all give excellent annual layers, particularly in the Holocene period. Comparison with the previously dated Dye 3 core, using volcanic and other tie-points, provided a starting point. Numerous volcanic eruptions were documented, allowing the possibility to make comparisons with other cores. Deeper ice was dated using ice flow models."
"The [Greenland] Dye 3 1979 core is not completely intact and is not undamaged."
“Below 600 m, the ice became brittle with increasing depth and badly fractured between 800 and 1,200 m. The physical property of the core progressively improved and below ~1,400 m was of excellent quality.”
“The deep ice core drilling terminated in August 1981. The ice core is 2035 m long and has a diameter of 10 cm. It was drilled with less than 6° deviation from vertical, and less than 2 m is missing. The deepest 22 m consists of silty ice with an increasing concentration of pebbles downward. In the depth interval 800 to 1400 m the ice was extremely brittle, and even careful handling unavoidably damaged this part of the core, but the rest of the core is in good to excellent condition.”
The depth interval 800 to 1400 m would be a period approximately from about two thousand years ago to about five or six thousand years ago.
"Melting has been commonplace throughout the Holocene. Summer melting is usually the rule at Dye 3, and there is occasional melting even in north Greenland. All of these meltings disturb the clarity of the annual record to some degree."
“An exceptionally warm spell can produce features which extend downwards by percolation, along isolated channels, into the snow of several previous years. This can happen in regions which generally have little or no melting at the snow surface as exemplified during mid July 1954 in north-west Greenland4. Such an event could lead to the conclusion that two or three successive years had abnormally warm summers, whereas all the icing formed during a single period which lasted for several days. The location where melt features will have the greatest climactic significance is high in the percolation facies where summer melting is common but deep percolation is minimal4. Dye 3 in southern Greenland (65°11’N; 43°50’W) is such a location.”
"The brittle zone mentioned above [...] corresponds in Dye 3 1979 with the steady state grain size (crystal size) from ~637 - ~1737 m depth range. This is also the Holocene climatic optimum period."
"Continuous measurements made in the field included dielectric profiling and electrical conductivity (related to the concentrations of neutral salts and acid)."
"The morphology of chaotic terrain forms a continuum from areas consisting of densely packed blocks with fractures and narrow lanes of matrix between them ([first image at the right]), to areas consisting of almost all matrix and no blocks ([first image at the left]). Conamara Chaos, the most intensely studied chaos area ([second image at the right]), lies near the middle of this continuum, with -60% of its area consisting of matrix and the remainder consisting of blocks [Spaunet al., 1998]. In addition to these large chaos areas, chaotic terrain also occurs in the interiors of some small (-10 km diameter) features [Spaun et al., 1999] known as "lenticulae.""
"In Conamara Chaos, where data with spatial resolution of up to ten meters per pixel were obtained, the hummocky matrix appears to be a jumbled collection of ice chunks of all sizes, from a kilometer to tens of meters across ([second image on the left])."
"Nuclear Magnetic Resonance and Neutron Scattering of the dynamics and phase-fractions of water/ice systems in templated porous silicas (SBA-15) indicate that what was believed to be a non-frozen surface water layer is actually plastic ice, the quantity varying (continuously and reversibly) with temperature, and converting to a brittle (mainly cubic) ice at lower temperatures."
"Chemical measurements made continuously in the field were ammonium, nitrate, hydrogen peroxide, formaldehyde, calcium and dust, while discontinuous measurements of other anions and cations were made by ion-chromatography."
"Samples were cut in the field for oxygen isotope and deuterium analysis (with a resolution of about 3 cm in parts of the core), for trace gas analysis, for measurements of 10Be, and mechanical properties of the ice, among others."
"Measurements of borehole temperatures have allowed a re-calibration of the oxygen isotope-temperature relation for the GRIP ice core. This work indicates that the temperature chnage at the end of the last glacial period was more than 20 degrees, a result found independently in the GISP2 borehole. These increased temperature changes provide a renewed challenge to those seeking mechanisms for the transitions."
At right in green is a plot of carbon dioxide concentration with temperature and dust concentration.
"The ice loads on marine structures are affected by the failure process of ice. Brittle failure is one of the important failure modes. Ice fails in a brittle manner when the loading rate is high or the temperature is low."
"Among numerous other findings, new insights using markers of biological material have proved particularly exciting. Methane has been found to change in time with many rapid climate changes. Spikes of ammonium and organic acids have been found to be markers for biomass burning, while background concentrations of these species indicate the advances of vegetation in North America."
Earth has ice caps, ice sheets, ice fields, and glaciers.
In the image at the right, "Aureum Chaos is a large crater that was filled with sediment after its formation. After the infilling of sediment, something occurred that caused the sediment to be broken up into large, slumped blocks and smaller knobs. Currently, it is believed that the blocks and knobs form when material is removed from the subsurface, creating void space. Subsurface ice was problably heated, and the water burst out to the surface, maybe forming a temporary lake."
"Galileo spacecraft observations of Europa suggest the existence of a brittle ice crust (or lithosphere) at most -2 km thick, and maybe thinner locally, overlying a liquid water or ductile ice layer [Carr et al., 1998; Pappalardo et al., 1998, 1999]. Elastic and viscous models of buckling based on the spacing between possible folds in the Astypalaea Linea region give a thickness for the buckling layer of -2 km [Prockter and Pappalardo, 2000]. Evidence derived from the width troughs (interpreted as possible grabens) in the surroundings of Callanish, a possible impact structure, might denote a brittle-ductile transition locally as shallow as 0.5 km [Moore et al., 1998]. Besides this, study of ice flexion induced by a dome-type structure located close to Conamara Chaos suggests an elastic lithosphere thickness of only -0.1-0.5 km [Williams and Greeley, 1998]."
The "odd surface terrain patterns [of Europa] likely come about due to convection. [...] The ice shell of Jupiter’s moon Europa is marked by regions of disrupted ice known as chaos terrains that cover up to 40% of the satellite’s surface, most commonly occurring within 40° of the equator. Concurrence with salt deposits implies a coupling between the geologically active ice shell and the underlying liquid water ocean at lower latitudes. Europa’s ocean dynamics have been assumed to adopt a two-dimensional pattern, which channels the moon’s internal heat to higher latitudes. [...] heterogeneous heating promotes the formation of chaos features through increased melting of the ice shell and subsequent deposition of marine ice at low latitudes."
"This rendering [at the second right] of Europa shows the temperature field in a simulation of the icy Jupiter moon's global ocean dynamics, where hot plumes (red) rise from the seafloor and cool fluid (blue) sinks down from the ice-ocean border. More heat is delivered to the ice shell near the equator, consistent with the distribution of chaos terrains on Europa."
"This rendering [at the second left] of Jupiter's icy moon Europa shows so-called isosurfaces of warmer (red) and cooler (blue) temperatures in a simulation of Europa’s global ocean dynamics. More heat is delivered to the ice shell near the equator where convection is more vigorous, consistent with the distribution of chaos terrains on Europa."
The fourth image at the right is a "view of the Conamara Chaos region on Jupiter's moon Europa taken by NASA's Galileo spacecraft shows an area where the icy surface has been broken into many separate plates that have moved laterally and rotated. These plates are surrounded by a topographically lower matrix. This matrix material may have been emplaced as water, slush, or warm flowing ice, which rose up from below the surface. One of the plates is seen as a flat, lineated area in the upper portion of the image. Below this plate, a tall twin-peaked mountain of ice rises from the matrix to a height of more than 250 meters (800 feet). The matrix in this area appears to consist of a jumble of many different sized chunks of ice. Though the matrix may have consisted of a loose jumble of ice blocks while it was forming, the large fracture running vertically along the left side of the image shows that the matrix later became a hardened crust, and is frozen today. The Brooklyn Bridge in New York City would be just large enough to span this fracture."
"North is to the top right of the picture, and the sun illuminates the surface from the east. This image, centered at approximately 8 degrees north latitude and 274 degrees west longitude, covers an area approximately 4 kilometers by 7 kilometers (2.5 miles by 4 miles). The resolution is 9 meters (30 feet) per picture element. This image was taken on December 16, 1997 at a range of 900 kilometers (540 miles) by Galileo's solid state imaging system."
"Chaotic terrain on Europa is interpreted to be the result of the breakup of brittle surface materials over a mobile substrate."
At the third left, "the mottled appearance results from areas of the bright, icy crust that have been broken apart (known as "chaos" terrain), exposing a darker underlying material. This terrain is typified by the area in the upper right-hand part of the image. The mottled terrain represents some of the most recent geologic activity on Europa. Also shown in this image is a smooth, gray band (lower part of image) representing a zone where the Europan crust has been fractured, separated, and filled in with material derived from the interior. The chaos terrain and the gray band show that this satellite has been subjected to intense geological deformation."
At the right a researcher saws the GRIP core on site for analysis.
"A huge number of analyses were made on the core in the field, while other samples were prepared in the field for shipment to laboratories around Europe. measurements in the field helped scientists to select samples for special and urgen analyses, and to exclude the contamination risk from chemicals such as organic acids."
- Most brittle ice before it is shattered may have the largest grain sizes.
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