Australia: The Land Where Time Began
Volcanoes - Volcanic landforms
As well as being a very destructive force, volcanoes also have a beneficial side, bringing material from deep in the lithosphere and asthenosphere the surface where it forms new crust, as well as possibly the hydrosphere and atmosphere. Fertile new soils are also produced from volcanic ash, that weathers rapidly, on a geological time scale. This part of volcanic process is what is missing from the Australian continent, what little volcanic activity that has taken place in Australia in the more recent past is concentrated along the east coast, the majority of the land surface of Australia having had no renewal of soils by volcanic activity for many millions of years. Geomorphologically volcanic provinces are characterised by distinctive landforms, most being constructional, produced from lava and ash, though those that result from explosive activity and erosion are destructional.
Lava commonly extrudes from structures other than the top of a central cone of a mountain, as is usually visualised, also issuing from vents on the flanks of volcanoes or from long fissures, as occurs at the mid-ocean ridges. What looks like smoke is actually hot ash and dust, the apparent flames being reflections of the ash and dust in the air and of the molten rock that is emerging.
The distribution of volcanoes on the surface of the Earth is not random, the areas of volcanic activity occurring in distinct zones, such as the Pacific ring of fire (girdle of fire), in the mid-latitudes, the Alpine Fold Belt that extends through the Antilles on the Mediterranean, and in into the Indonesian Archipelago, the African Rift Zone, and the system of mountain ranges on the ocean floor. In many cases the vents are aligned or occur in arcs, that are consistent with the structure of the country rock, indicating that magma has risen along fractures, even at the local scale. Volcanoes and volcanic activity can be classified according to the composition of their lava, and on the continuum of types, the Hawaiian and Peléean types being at the opposite extremes, all volcanoes fitting somewhere between these 2 extremes, though each volcano, as well as every eruption, are unique, having a wide range of vents, timing and forms that result. The types of volcanoes are named from specific volcanoes that are well known.
Acid or sodic lavas with high levels of free silica, alkalis, fluids and gases characterise Peléean volcanoes, the lava emerging at temperatures of 700-900o C. This lava type is viscous (sticky) and doesn't flow. In the early stages of an eruption low domes and acicular forms (needle-shaped) develop, but are soon destroyed by internal explosive blasts. The authors describe cooled lava of this type as looking like toothpaste that has been squeezed from a tube, columnar joints indicating its volcanic origin. Many volcanic rocks display columnar joints, regardless of their composition. These structures, frequently called organ pipes, resulting from cooling and contraction, that are often hexagonal in cross section, are elongated perpendicular to the cooling surface.
In this type of silicic volcano the vents are blocked by the viscous lava allowing pressure to build just below the blockage until the plugs are blasted out by a typically explosive eruption. The fragmentation of rocks produced pyroclastic materials such as volcanic bombs (scoria, lapilli) and volcanic dust that is thrown high into the atmosphere where it is distributed for very large distances by the wind. Clouds of incandescent gas and dust, nuées ardentes (glowing clouds) move at high speed down the valleys, incinerating everything in the path. It was a nuées ardentes that destroyed the town of St Pierre on the island of Martinique during the eruption of Mt Pelée in 1902, the only survivor being a man who was in prison, the strong walls of the building and the small windows protecting him from the blast of scorching dust and gas.
This type of eruption spread ash, formed by fragmentation of volcanic rock, over enormous distances. An example is Quisapu in the south of Chile, that erupted in 1932, spreading ash as far as Rio de Janeiro, on the Atlantic coast of Brazil. Ash from the 1902 eruption of Soufriere, in the Antilles, was deposited on a ship that was 1500 km away. This type of ash forms deposits with distinct layers. All volcanic rocks that are fragmented (clastic) are called tephra, the layers of tephra are useful for dating rocks and surfaces by physical methods, because they form marker horizons that are easily recognised. In places such as Japan they can be dated by historical records. Very deep ash deposits form close to the eruptive centres, about 6 m being deposited near Soufriere during an eruption that occurred in the Late Pleistocene. Volcanic activity in the Late Cainozoic inundated plains, valleys and basins with ash in the Lake Taupo area of North Island, New Zealand, as well as the southwestern US. According to the authors, such deposits of tephra cover the ground in a similar manner to water.
Volcanic ash can have disastrous effects whether it is associated with Peléean type volcanoes, Hawaiian type volcanoes or basaltic eruptions. In 1783 ash from the basaltic Laki fissure in Iceland killed pasture, eventually leading to the loss of half of the cattle on the island, the famine that resulted killing about 1/5 of the human population.
Volcanoes of the Hawaiian type have lavas that are more fluid than that of the Peléean type volcanoes, their lavas being high in ferromagnesian minerals but no silica, the cones they form being built with gently sloping flanks and plateaux that cover large areas. The lavas that exude from such volcanoes are at about 1100o C, from vents that are essentially circular, flowing at speeds of up to 32 km/h, spreading over huge areas, with long tongues that extend down the valleys. In north Queensland, some extend for 80 km from their vents, such as those of the Einasleigh River valley, the old Flinders River and the former Galah Creek. High-volume and continuous extrusion and flow has been attributed to these unusually long formations, flows travelling along narrow valleys.
When broad areas of basalt lava flows are observed the surface appears even, forming plains or plateaux above which are low cones or vents, some of which have smooth slopes, while others, scoria cones formed by the deposition of scoria (jagged fragments of rock. Broad areas of level ground make up the plains, that are separated by escarpments or low, steep rises. Rivers have formed some of the escarpments by erosion, though others mark the margins of separate lava flows. Stepped basalt plains are referred to as traps (Swedish trappa, and German Treppe - step), such as the Deccan Traps of peninsula India. On closer observation the surface is rough. There are 2 types of surface forms - ropy or pillowy, that are angular or blocky, the Hawaiian terms pahoehoe and aa respectively. Where the rocks have been exposed for some time to the atmosphere, soil that has formed has then washed into depressions, though recently formed areas of lava have no soil, being little weathered, The result is that stony rises and depressions floored with soil are typical of many Australian basalt plains and plateaux. The well-fractured consolidated rock has prominent columnar joints that result from contraction as the lava cooled, there are few rivers and the surface is pervious. Caverns are common, some of which have lava stalactites and drips. The Undara Lava Tubes or caves, located in the McBride province in north Queensland, have an average height of 10 m and an average width of 15 m.
Other volcanic activity
In places such as Iceland, basaltic lava emerges from long linear features, referred to as Iceland-type volcanoes, where there are few cones or craters that can be recognised as volcanoes. This is another case in which the lava spreads over huge areas forming layers, that are essentially flat, underlying plains and plateaux that cover extensive areas, some fields being of such a large extent they are called flood basalts.
Types that are intermediate between the 2 extremes of volcano type are Strombolian and Vulcanean type, the Vulcanean being closer to the Peléean type and the Strombolean type closer to the Hawaiian extreme. There are also many volcanoes that have changed the type of lava they produce over time, accompanied by a change of activity. Another well-known volcanic phenomenon is the emission of sulphurous gases from a volcanic vent, Solfataric activity, named for a town, Solfatara near Naples in Italy. Other volcanic activity is represented by such features as geysers, boiling mud, sulphur and silica springs, steam and gas emissions represent the dying phase of volcanic activity. Such hot springs have been used for their therapeutic value and for the production of thermal energy, as in New Zealand and Iceland.
Calderas and craters
When lava is extruded from a central cone a conical cone builds up by the addition of multiple layers of ash and scoria. The central part of cones sink into the vent to form a crater, when there eruption has finished. Retreat by erosion may take place rapidly, as the steep walls are often formed of unconsolidated material, the crater becoming enlarged to several times the diameter of the vent. Crater lakes sometimes occupy volcanic craters. Where craters form as a result of shallow explosive activity, that lack rims that are sharply defined, the lakes that form in them are called maar lakes. Calderas are the steep-walled depressions, that are roughly circular, over a volcanic vent. When the magma chambers, that are several kilometres below the surface, partially empty of magma following an eruption, the unsupported roof of the magma chamber collapsing to form the caldera.
Plateaux and deep gorges, that often have stepped sidewalls, as a result of several flows being exposed that have varying levels of resistance to erosion, are formed by dissection of volcanic plains. Triangular facets, separated by the valleys of streams, result from the dissection of cones. In English they are called planezes. This term originally referred to plateaux that are lava-capped, and in France the original meaning of the term has been retained. Sommas, named after a feature with that name at Vesuvius in southern Italy, are the most dramatic erosional forms resulting from the the erosion of volcanic cones and craters, in the form of arcuate rims that are remnants of old craters. In Australia, Mt Warning in northeastern New South Wales is of this type. It is what remains after the core of the Tweed Shield Volcano, with an elliptical ring structure of 5.5-8.0 km in diameter, has been eroded. Ring dikes that were emplaced 23.1-22.4 Ma cut the whole complex. The central crater, that the authors say may have possibly reached elevations of 1830-1980 m, about 915 m higher than the summit of the present Mt Warning, has been eroded by streams, with the result that the flanks of the old dome of basalt has been eroded to form planezes of low angle separated by deep gorges. The Tweed and McPherson Ranges, to the west and northwest of Mt Warning, preserve the circular plan form of the volcano and the planezes.
Isolated remnants with steep sides have been formed from volcanic plugs. These plugs, that are formed from material that has a different composition from that of the cone, block the vent during the later stages of an eruption. The rocks of the core and crater are eventually weathered and eroded leaving the steep-sided remnant of vent material that often has columnar joints. In southeastern Queensland, the Glasshouse Mountains are a series of domes and peaks, of trachytic material, each of which represents an eruption centre that dates to 28-24 Ma. Plugs and high vertical walls have been formed from fissure fillings in the Warrumbungles of northeastern New South Wales. Volcanic remnants from the Middle Miocene (about 15 Ma), are present at Belougeny Spire, Crater Bluff and Breadknife.
Volcanoes were previously belied to be related to mountain building, and the differences between acid and basaltic volcanoes was believed to be the result of acid volcanoes producing simatic material that had been contaminated as it passed through the continental or sialic rafts. But there are known to be many basaltic volcanic fields on the continents, such as the Deccan Traps, the Columbia River basalts, eastern Australia, etc., with many of the acid eruptions that are most destructive occurring in ocean settings, such as the volcanic island of Krakatau (Krakatoa) between Sumatra and Java in Indonesia.
According to the authors, an explanation of the distribution of volcanoes has been provided by plate tectonics that is more rational. Divergent plate junctions have been found to be associated with many basaltic volcanoes, the molten material welling up from the asthenosphere at very high temperatures, as occurs in the basaltic eruptions on the mid-ocean ridges. Subduction zones or convergent junctions are associated with acid volcanicity, the crust being recycled, pushed up below an adjacent plate to depths where the rock melts partially, thus explaining the fact that acid lavas are cooler than basic (basaltic) ones, originating in an immediate sense from depths that are relatively shallow, the geothermal gradient averaging 25o C/km. The authors suggest that the melting point may be lowered by the presence of water within the descending plate, molten rock being produced more quickly than would otherwise be the case. The acid fraction is boiled off, ascending through the upper crust, whatever the mechanism, probably along fractures, emerging at distances of 100-150 km from the subduction zone that is marked by a deep ocean trench.
There are many old volcanic and old basalt fields that are not related to plate junctions. Located in the middle of the Pacific Plate, Hawaii is a long way from the nearest plate junction. In Tanzania, Nyiragongo and Nymalagria are active almost constantly, though they are not near plate margins. The formation of the long chain of the Hawaiian Islands, that extends southeast to northwest in the middle of the Pacific Plate that is moving to the northwest, provides clues to the origin of such volcanoes that are well away from plate margins. The active youngest volcanoes are on the island that is on the extreme southeast of the Hawaiian chain, the islands becoming older along the chain in a northwestwardly direction. It has been suggested that the Hawaiian group have resulted from being atop a hotspot, where a mantle plume punches through the crust from deep in the asthenosphere, to erupt as a basaltic volcano. Situated southeast of the main island, Loihi Island is a recent eruption. As the plate continues its movement to the northwest, the older volcanoes are carried along with it. This explanation has been supported by radiometric dating.
In the case of the Deccan Traps and other large basaltic lava fields, the concept has been developed and extended in scale to explain them. The authors say there has been a suggestion that following the fragmentation of Pangaea, about 200 Ma, the several fragments drifted in different directions, passing over various hotspots in the Late Mesozoic and Cainozoic. At this time the Drakensberg, the Columbia Plateau, the Deccan Traps, the Parana Basin, as well as other areas, were inundated by huge amounts of lava that flooded out over these landscapes to form the features that are so prominent at the present. Each of these extrusions, in some areas involving millions of cubic kilometres of basalt, this process took no more than 1 My to complete. The hotspots remained in the same position as the continents passed over them, the volcanic activity ceasing before the continents arrived at their present positions. Various linkages have been proposed between the hotspots and the regions of flood basalt, some of which have been supported by evidence, the remainder awaiting proof. One suggestion is that when the landmass that is now Peninsula India was situated over the hotspot that is now beneath Reunion Island, the lavas of the Deccan Traps were extruded. The Namibian basalt lava fields and the Parana Basin Fields formed when the relevant areas were situated above the hotspot that is now beneath Tristan de Cunha, on the Mid-Atlantic Ridge. In the Drakensberg, the Stormberg basalt sequence formed when the region was passing over the present position of Crozet (Marion) Island, that is now to the southeast of the continent.
Tectonic and structural forms
Most of the volcanoes discussed in Source 1, volcanic cones, craters and associated forms, that result by internal activities of the Earth, are of the tectonic type of landform, their morphology deriving from the properties of the lava that is brought to the surface from deep within the Earth. The viscosity of the lava, that is determined by its composition, determines the form taken by a volcano, whether a shield volcano or steep-sided domes and needles.
Structural landforms are formed by differential erosion of volcanoes to produce structures such as volcanic plugs, that result from preferential weathering and erosion of a part of the volcanic complex over another. Most of the flanks of the Tweed Volcano have been removed by erosion to leave Mt Warning. Most of the volcanic plugs remain, as with the Warrumbungles, where only the more resistant materials remain in the form of plugs and veins.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|