Australia: The Land Where Time Began
The products of weathering are moved downslope by gravity, whatever the particle size size, fine or coarse. It is usual to distinguish between this type of movement of material and other transporting agents such as rivers, wind and glaciers, though gravity is involved in all types of transport, and the other agents of transport are often also involved with mass movement of materials, often to a significant extent. The authors suggest it is a matter of emphasis which transport agent a movement of material is categorised as.
Regions with steep typography, especially in seismically active zones where the rock masses are fractured, and clay is present, and the instability of the slope is contributed to by heavy rain are areas where mass movements are most easily triggered. Lubrication is provided by clays, whether they are primary or the products of weathering. Mass movements are also contributed to by the clearance of vegetation, tunnelling and mining. The movements come in many forms, from imperceptible to spectacularly obvious. The authors say one or more of criteria, such as the nature of the material, the type of movement, water content, morphological characteristics, movement rate, the climatic setting, the geological setting and the hazard potential, are the basis for classifications that have been suggested, the emphasis depending whether the person doing the classifying is a geologist, an engineer, a geomorphologist or town planner.
Complex processes are involved in mass movements, typically more than 1 process being involved in each event. The authors suggest that for general purposes it is adequate to have a simple classification into movements that mainly involve creep, heaving, slide, slip, slump, flow, fall or collapse.
Individual rock and soil particles are involved in some types of downslope movement of debris, moving episodically, not continuously, under the influence of gravity. Creep is the term used for the type of movement that is so slow as to be imperceptible except by the use of special instruments. According to the authors it is still not clear how angular particles move across a surface of other angular particles if wash is not involved. Short-lived sheets of water involved in downhill wash, slopewash, operating spasmodically, together with creep, can move large amounts of material downslope, the movement often being made visible by the presence of trees or posts that were originally vertical but develop a lean. At Yarra Boulevard, in Studley Park, Melbourne, Australia, a classic exposure can be seen that occurs in a cutting in vertical strata. At this location not only is the uppermost layers of the soil are affected, the upper ends of the strata also display a downslope curvature.
Various forms of patterns of the ground result from buckling, as well as alternating expansion and contraction of unconsolidated debris. Buckling at the edge of plates of polygonal shape as salt continues to crystallise and a problem of space results from the precipitation of halite (salt) on salina beds. Alternate freezing and thawing in cold climates leads to forms such as stone circles and polygons result from heaving. Stones insulate against both heating and drying, and ice needles growing beneath stones cause the stones to be thrust upwards and become unstable causing them to migrate laterally, becoming concentrated in distinct rings. The centres of the polygons that contain a lot of water in the interstices of the fine materials arch when they freeze, thereby continuing the migration laterally and sorting of the coarse debris once the tendency towards sorting has begun.
In gypcrete accumulations in Iraq, there are reports of similar polygonal plates which have raised rims. In the North of Western Australia there are arched calcrete layers. Patterned ground is due to alternating wetting, when hydrophilic clays swell, and drying, when they contract, occurs in regions with either episodic rains or where there is a marked wet and dry season. A number of names have been used in Australia for there microrelief forms, such as melon holes, corduroy soils, crab holes, Bay of Biscay soils, or gilgai, that in an Aboriginal language means a small waterhole. Gilgai form in areas where rainfall is 150 mm/y to areas where annual rainfall is 1500 m, a good indication that the amount of rainfall doesn't determine where they form. see Gilgai
Slide and slip
Failure or the development of instability on some part of a slope lead to movement of large amounts of debris, that is comparatively sudden. Very large volumes of debris are involved in slides and slips, though they are usually shallow, the upper few metres of rock and soil being involved. The mass of rock and soil moves downslope as a coherent mass, displaced along planar surfaces that have become unstable by mechanisms such as saturation with water. Slips occur along pre-existing structural planes. There are instances when large blocks, such as Chimneystack Rock near Yalwal, New South Wales, are believed to have slipped laterally away from the scarp on a cushion of plastic clay.
Slumping occurs by rotational slide along a concave upwards plane, the mass moving essentially coherently. The mass of weathered rock and soil glides over the slip plane, riding up and over the still-stable slope below, and forms a distinct lobe or snout. A tension crack or tension tear is the large gap, of variable dimensions, that forms at the original site of failure.
Flow refers to mass movements in which the movement is fluid, differential movement or flowage being discernable from the crack pattern and disruption of bedding of the material. The mass movements are known as earth flows, rock flows or mud flows, depending on the composition of the material involved, where there is significant differential movement. Lahars are mud and water flows which are of volcanic origin. In the Yankalilla area of South Australia, there are well-developed earth flows on glacigene deposits of Permian age, varying in volume and extent, though movement was restricted to a depth of 5 m in all instances. A steep arcuate head scarp, that bounds a hollow, leads downslope to an elongate lobe that bulges above the general turf level, ripples indicative of differential movement that characterise it.
Large blocks topple or fall downslope when fractures form or are widened throughout a mass of rock, or the rock mass is undermined. An accumulation of debris at the slope foot in the form of scree or talus cones, or a layered regolith, can form when other fragments are dislodged as a block tumbles downslope. Sudden slope failure can also cause the formation of cones.
Rapid movement is involved in slides, flows and falls, the results being easy to detect. When a whole sector of a hill slope can become unstable, that may be the result of the access of water, flowage can occur in which different parts of the mass move at different speeds. As there is less friction in the centre of the mass, the centre moves more rapidly than the base or side, where there is more friction. Very large masses of rock waste may slide over the adjacent surface. Flowing and sliding are involved in most mass movements, slickensides, striations or scratches, found beneath even flows that may be quite fluid, indicate slipping along a gliding plane. Slumping and block falls are involved in many movements, before, but especially after the main movement.
Subsidence or collapse
Subsidence may occur wherever there is substantial removal of material, whether by humans or natural, as in solution. The formation of shafts, dolines or sinkholes can result from weathering by solution that leads to local subsidence or collapse, most commonly occurring in limestone terrains, though it also occurs in lateritised surfaces where sinkholes result from the removal in solution of silica and iron. In a number of places, such as parts of Africa and South America, where dolines are present, and are also well represented in laterite formed on peridotite (harzburgite) in New Caledonia. In Australia, there are hundreds of such sinkholes in lateritised sediments dating to the Cretaceous, that continue developing between Newcastle Waters and Katherine on the Sturt Plateau in the Northern Territory. According to the authors, it has been suggested that in East Africa, on a wider scale, the lowering of land surfaces has occurred by silica solution by processes of volume decrease, settling and compaction.
About 80 km to the southwest of Sydney, at Nattai North in the Burrangorang Valley, where subsidence has been caused by coal and oil shale mining, has subsequently led to rock falls and landslides. It is one of the largest mass movements known in Australia, though it is small by world standards. The rock fall avalanche that occurred at Nattai North involved the collapse of part of a 200 m high sandstone escarpment of horizontally bedded sandstone with subvertical joints. After about 15 years of mining beneath and behind the escarpment for about 15 years before the collapse in 1965, with further collapse occurring spasmodically since that time. The authors suggest the natural stress distribution within the face has been altered by removal of material from beneath and behind the escarpment, leading to further collapse as the underlying beds collapse into the extracted seam. Water was allowed to enter the rock by vertical tension cracks that developed behind the scarp. At the base of the pillars claystone bands that were weaker, were further stressed by subsidence as well as being softened by weathering. Failure at the base of these detached pillars, together with backward toppling, led eventually to collapse. At the Demira and Wongawilli collieries near Wollongong, similar subsidence has occurred, that ranges up to 2408 mm, accompanied by cracking.
The authors note that some debris mass movements, where there is no evidence of lubrication by water, appear to generate strong winds locally at their margins, the resulting high pressure air being the agent of lubrication, in some cases being attributed to the collapse of cliffs on a large scale, though they say there are some who question this mechanism. The debris is believed to ride downslope on a cushion of air that has been trapped by the falling debris. During this process air is squeezed out laterally, but as more air is trapped by the tumbling front the movement continues. Some of the largest and most destructive landslides that are known of have been attributed to this type of fluidisation flow. Some examples suggested occur in Canada, the Frank Slide, Alberta, that occurred in 1902. In Switzerland, the Elm Landslide in 1888. In California, the Blackhawk Landslide.
Spread over a wide area, the debris associated with the mass movement is thin, with no sorting taking place and the constituent fragments show little sign of abrasion. As well as extending downslope these flows have been known to continue up to 600 m up a slope, that is gentle to moderate, under their own momentum. According to the authors, observers of such flows have reported very rapid movement of the debris, at speeds of more than 160 km/h, and one case is known where speeds are believed to have reached about 480 km/h for a short period. The authors suggest these observations and other evidence points to lubrication or fluidisation of flow by air that has been trapped beneath and within the rapidly moving debris, as the rolling front envelopes more air while squeezing it out at the sides as it moves in the manner of a hovercraft, the debris being carried along on the cushion of air.
The most common causes of mass movement are unusually high precipitation, especially in the humid tropics, seismic shaking and human activities, the movement involving the loss of support for the debris mass. In the humid tropics the vegetation cover tends to bind the surface soil, but this binding is overcome by gravity, huge areas of soil and trees sliding downslope with the vegetation still alive. The instability of the slopes is contributed to by the deep soil cover, the production of non-crystalline colloidal clays is another factor, as the soil lacks the cohesion of solid rock while the clays are lubricants. In the humid tropics the most important factor in instability is consistently heavy rain. The density of the soil is increased by the percolation of water into it. When present in large volumes in soil, water increases pore pressure, pushing the constituent particles further from each other, allowing them to move relative to one another under gravity, though in small quantities soil water can retard differential motion between the particles by increasing friction. In the summer of 1976, many disastrous landslides occurred in mountainous regions of Bali as a result of heavy rains.
Mass movements also occur in temperate as well as arid and semiarid land, that are the result of heavy rain. The mountainous areas of Japan, considered cool temperate, also experienced severe landslides in 1976. Mass movement of debris in sub-Arctic regions, that have been suggested to possibly be the most important single form of transport of weathered material in such regions, occur in summer when the uppermost part of the permafrost thaws, leading to the presence of large volumes of water above the still frozen permafrost.
These environmental factors are often sufficient to trigger mass movements, but there are other factors that can contribute to such movements. Horizontal shaking and slope failure can be caused by earthquakes, initiating mass movements. More than 400 relatively recent landslides have been identified in Torricelli Mountains and the Bewani Mountains in the northern part of Papua New Guinea, that occurred in the uplands with steep slopes, mostly in the eastern portion of the region. According to the authors, there is no known asymmetric variation in either slope or rock type between the eastern and western parts of the region, the distribution of landslides being attributed to a period of earthquake activity that occurred in the east in 1935, some of the slides being dated to this time. There are also some older scars that have since been revegetated are suggested to possibly be related to earthquakes in 1915 and 1917(?).
Slope failure and landslides are related to tectonic instability, landslides being a common, and according to the authors, even an inevitable corollary of earthquakes. In 1971 more than 1000 landslides in an area of 250 km2 were attributed to the San Fernando Earthquake in California. Landslides have been known to block rivers, causing the formation of temporary lakes, in mountainous areas. In Iran, the prehistoric Saidmarreh Landslip may be the result of undermining by the Saidmarreh River of the valley side, that are high and steep, but the authors suggest an earthquake also contributed to it. During this landslide a very large mass of limestone fell into the valley from a height of about 1500 m, travelling across the valley, possibly on a cushion of air, the rock fracturing into a mass of angular blocks, some up 20 m in diameter, and covering an area of 166 km2, 100-130 m thick, and with a volume of possibly 20 km3. The mass movement carried on about 14 km across the floor of the valley and 500-600 m up the opposite slope. The valley was flooded by the blocking of the river, 3 lakes forming, a small one and 2 large lakes. When such blockages occur, the resulting dams are fragile, being unconsolidated, and when they eventually burst the resulting floods are catastrophic downstream from the burst dam.
The separation, rotation and lateral translocation of large blocks, that have been difficult to explain by conventional means, have been suggested to have resulted from seismic shaking in addition to mass movement initiation. The authors suggest A-tents and wedges may be formed by such dislodged blocks, such as at Wudinna Hill on the Eyre Peninsula, South Australia. In the Gawler Ranges, toppled dacite columns have been suggested to have resulted from seismic shaking. Such an explanation may be a widespread cause of dislodged blocks, as well as other minor forms, though it is difficult to prove. It has also been suggested that undermining and collapse could have been caused by slope erosion below the columns.
Vegetation removal, excavation or erosion of the slope base, or animals such as rabbits and wombats burrowing, can lead to collapse. Another cause of collapse can be the drying of dune sand in the sun. Observers of such dune collapses report a bang like guns being fired. There were reports by the explorers Mitchell and Sturt of 'sounds like distant artillery'. Wills and King, of the Burke & Wills expedition of 1861 heard the 'noise of an explosion, as of a gun', that they attributed to Burke, but found when they returned to camp that he was not responsible. The authors suggest it was most likely to have been the result of the sun drying sand exposed on the slope of a dune causing it to collapse, the sound heard being the thud of the collapse.
One stabilising factor can be removed when woodland or forest is cleared for agriculture, and by logging, especially in the example cited for the humid tropics. In the Maleny Plateau in southeast Queensland, vegetation clearance has been implicated in recent mass movements, though other factors also involved are basaltic soils rich in clay, steep slopes and storm events. In the Yankalilla area of South Australia, multiple causes are also involved in the earth flows of fluvioglacial deposits from the Permian. All the above have formed after European settlement, and all have been attributed to a combination of factors that include climate, geology, soil, and human activity.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|