Australia: The Land Where Time Began
Volcanic Activity-Climate Change
In his book, Waking the Giant, Prof. Bill McGuire brings together evidence from several fields to explain how and why the climate is expected to change over coming centuries, including some aspects of climate change that are not mentioned in the public media, such as changes in volcanic and seismic activity. He has included discussion of past climates from periods of the Earth’s history when climatic conditions were similar to those we are facing at the present and into the future if global temperatures continue to rise at the present rate or greater.
Volcanic Activity Associated with Climate Change¹
Volcanoes are not usually spoken of as being associated with climate change, global warming, but the melting of glaciers can increase the rate of volcanic activity substantially.
There are thick permanent ice sheets, especially in high mountain ranges, such as are present on Kamchatka Peninsula in Russia, the Andes in South America and the High Sierras of California, and at high latitudes. According to the author¹ the weight of the ice sheets where they were at their thickest there appears to have been less magma produced deep within the Earth, reducing the amount of magma being supplied to the storage chambers within the volcanoes. The pressure on the inner rocks of the Earth is suggested by the author¹ to be 3.5 million times that of the atmosphere at sea level, a pressure that prevents the rocks beneath the asthenosphere from melting, thereby keeping the Earth mainly solid. High pressures act to suppress melting as most materials, with the exception of water, occupy a greater volume in the liquid form than in the solid form, even though in some places the temperature is high enough to melt the rock. Pressure and temperature both increase with depth, though at different rates from each other, and these also vary from place to place. The asthenosphere is a single thin layer within the mantle of the Earth, in which both temperature and pressure are right for melting to occur.
The asthenosphere is situated about 100-250 km from the surface, near enough to the surface for the tectonic plates to float on it, which allows them to move across the surface of the Earth, as well as providing most magma for volcanoes. The lithosphere, the layer of cool, brittle crust and upper mantle has temperatures that are broadly not hot enough to melt, and below the asthenosphere the pressures are too great to allow melting. It has been estimated that about 10 % of the asthenosphere is molten and the remaining 90 % being so close to melting that it can melt if the temperature increases slightly or the pressure is reduced slightly. This pressure reduction can most easily occur as the result from the removal of a large ice sheet by rapid melting such as the one that covered Iceland at the last glacial maximum. When this happened at the end of the last glacial phase the crust that was now under reduced pressure rose, isostatic rebound, during which the surface was uplifted by several hundred metres, and in the process reduced the pressure being applied to the asthenosphere thereby allowing it to melt. At the start of the present interglacial the volcanoes that had been suppressed by the ice for thousands of years began erupting once more. Among the places where the reawakened volcanoes erupted was Iceland, an island that is composed entirely of solidified magma that has been either erupted or emplaced beneath the surface to fill any gaps the molten lava can find, as the North American tectonic plate moved slowly to the west and the Eurasian plate moved slowly to the east.
An ice sheet that averaged almost a kilometre thick covered the whole of Iceland 20,000 years ago, then global warming triggered rapid melting of the glaciers about 16,000-15,000 years ago with the result that unloading occurred above the volcanoes and their magma source in the asthenosphere. Unloading had progressed enough by 12,000 years ago for the suppressed volcanoes to become active with a vengeance, the eruption rate jumping by 30 to 50 times the previous rate and after 1500 years the rate dropped again to the levels of the present. Part of this volcanic rejuvenation was powered by the magma that was trapped in the magma chambers by the ice loading, but there was also a huge supply of additional magma that was produced from deeper in the asthenosphere when the pressure was released by the unloading, the depressed lithosphere rapidly rebounding by as much as 0.5 km. The rapid, in geological terms, release of pressure trigged a 30-fold increase in magma production.
In Iceland of the present the Grimsvötn volcano demonstrates the triggering of volcanic eruptions by unloading. Covering the Grimsvötn volcano is the last remnant of the ice cover that completely shrouded Iceland during the last ice age, the Vatnajökull Ice Cap. This an unusual situation for a volcano and the structure of Grimsvötn is also unusual, the volcano being truncated by a caldera (large crater) 8 km in diameter containing a lake. The geothermal heat typically melts the surrounding ice progressively and this results in a rising lake level, and eventually the pressure of the water bursts the seal with the overlying ice and a glacial flood, called a jökulhlaup in Iceland, ensues. Sometimes the volcano is charged with magma and erupts when the load is removed, as occurred in 2004 and 2011.
The author¹ suggests volcanic activity in the areas surrounding the vast ice sheets of the last ice age, so not buried beneath the ice, appear to have been influenced by the melting of the ice sheets at the end of the glaciation. The volcanoes of the Eifel Mountains in Germany, and in France the Massive Central, appear to have been rejuvenated between about 17,000 and 5,000 years ago. In the surrounding areas more than 50 volcanoes erupted as the ice sheets that covered Scandinavia, the UK and the Alps were retreating rapidly. David Nowell et al. have suggested regional crustal uplift accompanied by a great loss of ice was the probable cause of this bout volcanic activity. The effect of the melting of the ice sheets would have been to release much of the pressure on the asthenosphere and thereby allowing the production of fresh magma as the asthenosphere in the area melted, the magma being vented through the overlying volcanoes. There is also evidence from the Kamchatka Peninsula, the Chilean Andes and the western US for bursts of volcanic activity being associated with the melting of more localised ice fields. In the Sierra Nevada Mountains of eastern California the volcanoes reveal an obvious inverse relationship between volcanic activity and ice sheets over the course of almost 1 million years in which there were a number of glacial advances and retreats. The inhibition of magma from opening fractures, and moving along them, dykes, to feed magma to erupting volcanoes on the surface is believed to be the most likely explanation of such activity as the ice melted. During the interglacial periods when the ice had melted dyke formation was hindered less with the result that the eruption of lava from the volcanoes was easier. This response to higher temperatures and melting ice at the close of the last glacial phase is seen in Iceland, as well as other areas that were extensively glaciated, such as in western Europe, the western US, the easternmost part of Russia and the high Andes, it has been found relatively recently that the volcanic response was in fact much more widespread.
The extensive ice sheet on Greenland, second only to the even vaster ice sheets in Antarctica, over 80 % of the land area, and at its greatest depth reaches up to 3 km in depth, with a total volume of nearly 3 million km³. This ice has been accumulating for more than 100,000 years, and since the advent of ice coring it has been realised that this frozen archive stores a treasure trove of information on the changing climate, and the tiny gas bubbles trapped in the ice give a window into the atmosphere with its component gases over the period the ice was being deposited, providing information on the temperature, sea level and solar activity at the time the bubbles of atmosphere were trapped, since before the last ice age. Among the information recorded in the ice are the events happening around the world that added large volumes of material to the atmosphere such as dust storms, wild fires and volcanic eruptions. Among the eruptions recorded as layers of volcanic dust or more often as sulphate films as the sulphuric acid aerosol veils that resulted from eruptions such as Lakagigar, Tambora, Krakatoa and Pinatubo.
Analysis of this ice core by Greg Zielinski et al. has revealed 2 periods of enhanced volcanic activity, one between 35,000 and 22,000 years ago at a time when the global temperature was dropping and ice sheets were expanding, and one in post-glacial times, between 17,000 and 6,000 years ago when the ice sheets were melting. It has been suggested by Zielinski et al., based on the ice core analysis, that these global eruption clusters 'might reflect a global volcanic response to the influence of the changing climate at the time' proposing that crustal stress changes resulting from the loss of ice mass during deglaciation and ice loading during the expansion of glaciers expanded in the build-up to the Last Glacial Maximum (LGM).
In cycles of glaciation-deglaciation associated with rapid changes of climate, loading and unloading by ice are not the only factors involved, especially as the global climate warms at the beginning of an interglacial phase there are many more environmental changes, as well as a number of ways in which volcanic activity can be triggered to erupt at these times. The glaciers at high altitudes on ice-capped volcanoes thin dramatically, possibly even disappear. These volcanoes, unlike those in Iceland when 1 km of ice was lost from above them at the close of the glacial phase, are unlikely to lose enough ice, even if the entire ice cap melts, to induce the production of more magma from the asthenosphere, though they can respond with more explosive eruptions as the gases in the magma are able to expand more easily with the result that they form a disrupted magma froth that is expelled violently. An additional factor is that when the buttressing effect of the ice is lost a volcano can be more prone to collapse, with an eruption being triggered by failure and removal as parts of the flanks slide, thus reducing the pressure on any magma that is present in the volcano.
Meltwater from the melting cover of snow and ice, or increased precipitation resulting from changes to the weather pattern in the area can also destabilise a volcano when the flanks are saturated. In these circumstances the likelihood of water and magma coming into contact at shallow depths is increased. According to the author¹ this is always a volatile mix that can trigger an explosive blast that can be direct or by collapse of the flank.
At times when the climate is in large and rapid transition there are also large redistributions of water on the Earth, there are also major variations in the amount of ice, as well as global sea level changes that are comparable. At the close of the last ice age there was a rise in the sea level of 130 m that would have added an enormous amount of load-related forces on the continental margins and island chains, which are the location of most volcanoes. The author¹ asks if the distinct bursts of volcanic activity that are recorded in the Greenland ice could be a volcanic response to the changing global sea levels, added to the reaction to variations in the ice load.
In 1990 a team of European researchers led by the author¹ examined the timing of eruptions in the Mediterranean region, studying the record in mud cores from the sea floor looking for volcanic ash layers indicating eruptions. They found no relationship between volcanic activity in the area and absolute sea level, though there was a very clear correlation with the rate of sea level change and the occurrence of eruptions. The warming at the close of the last glacial phase was the time of most clustering of eruptions, as is also seen in the ice cores from Greenland. Eruptions in the Mediterranean region that were large enough to leave layers of ash on the sea floor occurred every 350 years, on average, between 15,000 and 8,000 years ago, though when averaged over the past 80,000 years the average separation of eruptions is more like every 1050 years. There were also 2 other clusters of enhanced volcanic activity, one between 61,000 and 55,000 years ago and another 38,000-35,000 years ago, of similar age to the one recognised by Zielinski et al.
According to the author¹ 'it is sometimes hard to credit just how rapidly the sea levels rose, and how relentlessly huge tracts of land were inundated, following the end of the last ice age'. This is particularly the case during the latest part of the Pleistocene and the Early to Middle Holocene. The sea level has risen on average about 6-7 cm/year since the Last Glacial Maximum, while the sea level rose about 20 cm during the last century. The rise of the seas following the close of the last ice age occurred in fits and starts that reflect the rate of addition to the oceans of glacial meltwater from the retreating glaciers. The oceans underwent 3 so-called catastrophic rise events between 15,000 and 8,000 years ago, the sea levels rising as much as several metres per century as huge volumes of freshwater flooded into the oceans from the giant meltwater lakes accumulating along the fronts of diminishing ice fields in North America, Europe and Asia. Low-lying land was swamped and land bridges were cut and the oceans transgressed further inland. At this time the oceans rapidly rose up the flanks of the about 800 island and coastal volcanoes, encroaching on the balance of the 1500 active volcanoes in the world, most of which are situated within 250 km o the coastline. This enormous weight of water appears to have loaded the crust in such a manner that is suggested to have favoured the outpouring of any stored magma in the volcanoes affected contributing significantly to the burst of volcanic activity that is a characteristic of the world immediately following the close of the last glacial period. The author¹ suggests the amount and rate of sea level rise required to trigger an eruption in a volcano that is 'primed' to erupt seems to be not very great. This effect can be seen at the present at a small volcano in Alaska.
St Paul's Volcano
Pavlov is a 2500 m-high volcano that has the appearance of a typical volcano, cone-shaped with a snow-capped summit, situated on the flat plains of the Alaska Peninsula. It was originally named 'Pavlofskoi Volcan' by Captain Lutke in 1836, which the Alaskan Volcano Observatory roughly translates to Paul's or St Paul's Volcano. There are 2 other volcanoes nearby Pavlof Sister and Little Pavlof on the southwestern flank of Pavlof. Pavlof is 1 of 41 active volcanoes in Alaska. It has erupted more than 40 times over about the last 200 years making it the most active volcano in Alaska, and one of the volcanoes that erupt most frequently in the US and it is fascinating to scientists because of the timing of its eruptions.
What makes it unusual is that it only erupts in autumn and winter. Between 1973 and 1998 of the 16 times it erupted 13 times were between September and December, statistically not random times. The activity pattern of this volcano suggests there may be something either inherent in the volcano or it's local environment that determines the timing of its eruptions. The timing of Pavlof's eruptions were compared with a number of different phenomena that the researchers, Steve McNutt and John Beaven, US seismologists, thought may have been responsible for the unusual timing of the eruptions. The only phenomena that correlated with the eruptions turned out to be annual sea level variations that result from wind patterns changes during September to December that cause the Pacific Ocean to creep upwards along the Alaska Peninsula. They found a sea level rise of 17 cm, after correcting for seasonal variations in atmospheric pressure, appears to be sufficient to control the timing of the eruptions. McNutt and Beaven point out that correlation does not necessarily imply cause and effect.
As the author¹ points out it seems counterintuitive that a rise of 17 cm of water over the Alaska Peninsula, even though it would involve a great deal of water accumulating around the many square kilometres of the Peninsula, totaling a bit under 2 kPa at 17 cm depth, would have more effect than variations that occur with sea level pressure changes occurring with everyday changes of the weather, that can be 5 times as great or more. And the daily tides involving changes of metres rather than centimetre, also don't appear to have any effect. He says the actual cause of the difference in effect of the 17 cm rise is straightforward, involving the rate of application of the stress and the response of the material to the stress applied to it. In the case of the crust of the Earth the loading stress changes happen too quickly for the crust to deform in response to stress applied by daily tides and changes of atmospheric pressure, as when a storm passes over the peninsula. There is however sufficient time for the crust to deform when the sea level is elevated for several months, and in the case of Pavlof long enough to encourage in some manner the volcano to erupt. To show how material can respond differently depending on how the stress is applied the author¹ uses silly putty as an example. If this silicon polymer is struck with a sharp blow it breaks, but if it is left at rest for long enough it subsides into a puddle.
Once the suggestion was accepted it proved to be not difficult to establish a possible mechanism. A small amount of crustal bending close to Pavlof resulting from the small applied pressure would slightly compress the feeder system that supplies magma to the volcano and this would result in an eruption.
The author¹ suggests that if the small sea level increase can cause a single volcano to erupt, it seems reasonable that the same mechanism, scaled up sufficiently and applied to the hundreds of coastal volcanoes around the margins of the continents it might explain the outburst of volcanic activity that was characteristic of the post-glacial times.
Another mechanism has been suggested by computer modelling carried out by Andy Pullen, Imperial College London that has shown that ocean loading exerts forces acting to make volcano flanks more unstable and more likely to collapse, at the same time that stresses within affected volcanoes are altered by rising sea levels so as to promote magma expulsion. The eruption of Mount St Helens displayed that when part of the flank of a volcano is removed suddenly by a giant landslide any magma in the volcano is decompressed explosively, resulting in an enormous volcanic detonation, the previously compressed gases suddenly expanding tear the magma apart violently as they blast it up and out. The author suggests this may have been a mechanism that causes the eruption of costal and island volcanoes.
The last straw effect
According to the author¹ the mechanism that has been proposed for the eruptions of Pavlof is convincing, and could be a mechanism that operated in the coastal and island volcanoes at the beginning of the present interglacial, and he suggest the evidence is accumulating that there are other examples of volcanoes in which very small changes in their external environment trigger an eruption of a volcano that is primed with magma, the change being viewed as the 'last straw' that finally triggers an eruption. Stromboli, 'the beacon of the Mediterranean', is another example of the operation of such a mechanism. It always has a good supply of magma but appears to become more active at times of high barometric pressure in its vicinity that apparently provides the small nudge that it needs to trigger an eruption.
The Soufriere Hills volcano on the island of Montserrat in the Caribbean, in its 17th year, it is apparently rainfall that triggers it to erupt. With this volcano eruptions are related to a giant dome extruding lava that erupts violently when part of the flank collapses or from explosions that blast through it. An eruption that occurred in 2001 following several months of largely dry weather as the dome grew steadily when an episode of torrential rain triggered the collapse of the dome leading to a pyroclastic flow, fast flowing torrents of hot rock, dust and gas. It has been suggested that in the case of this volcano during heavy rain water doesn't vaporise on the outside of the rock, rather it finds its was down through cracks before it is converted to high pressure steam that is capable of destabilising the flank of the dome that is already oversteepened. A similar trigger has been recognised for eruptions of Mount St Helens where in the 1980s and early 1990s there were half a dozen times when the lava dome exploded within days of storms passing, heavy rain being suggested as the trigger of the eruptions. In this case 2 possible mechanisms have been suggested, heavier rain either triggering the instability leading to its collapse or accelerating the growth of cooling fractures, both suggestions presenting as possible ways of triggering eruptions by explosive gas release.
Mt Etna in Sicily erupts preferentially in summer and spring. A mechanism that has been proposed for this is that it is responding to small stress changes acting on or beneath the structure in a cyclical manner, such as velocity variations of the Earth in its orbit around the Sun. According to this proposal fractionally small stress reductions at certain times in the cycle may result in fractures opening or existing fractures widening, and if there is sufficient magma near the surface this may lead to an eruption. In 1947 Thomas Jagger of the Hawaii Volcano Observatory speculated on a possible relationship between the phases of the moon and the activity of the Hawaiian volcanoes. According to the author¹ in the 19th and 20th centuries there appeared to be a connection between gravitational effects of the Sun and the Moon and eruptions of the Hawaiian volcanoes. It is known the same gravitational effects that cause the tides also pull on the solid crust of the Earth. These earth tides result at the equator in the rocks being squeezed and stretched by more than half a metre.
The author suggests that weaknesses within the structure of Kilauea in Hawaii may be affected by stresses in the volcano in such a way that magma can move more easily at times of highest Earth tides occurring every 2 weeks so it tends to erupt at these times. It has been suggested that Earth tides may also have been responsible for the patterns seen in other volcanoes such as Stromboli, Mt Lamington, PNG, Soufriere, St Vincent, and Pelée, Martinique, the latter 2 in the Caribbean. The eruption of Pelée in 1902 killed 29,000 people in St Pierre, the only survivor of the population of the town was a man who was in prison at the time of the eruption. The author¹ suggests Earth tides may be an effective and widespread driver of volcanic eruptions. 700 volcanic eruptions that occurred since 1900 were examined by Fred Mauk and Malcolm Johnston in 1973, their conclusion being that there was a link between the timing of eruptions and the solid Earth tide maximum, that happened every 2 weeks, that was statistically significant, the volcanoes in Japan in particular demonstrating the link. As the Earth tide minimums also appear to be associated with the timing of eruptions of some volcanoes it appears the overall pattern does not seem to be straightforward. It has proven difficult to determine a cause and effect link between Earth tides and volcanic eruptions, as volcanoes are complex structures that may not be capable of reacting instantaneously to exerted tidal stresses, stresses that may make it easier for magma to move through pre-existing fractures by reducing the pressure sufficiently, and by the time the eruption occurs the cause of the change in pressure may be no longer present in the vicinity of the volcano. The author¹ suggests it is surprising there are so many volcanoes demonstrating a link between the timing of their eruptions and the Earth tides, given the potential lags and other complications, and he suggests the global pattern of global eruptions may have a completely different explanation from the one that has been proposed.
It has been demonstrated by a study carried out of the start dates of more than 3,000 volcanic eruptions that had been recorded in the catalogue of volcanic activity at the Smithsonian Institute, Washington, that occurred over the past 300 years that was evidence that was clear and demonstrable that on a worldwide scale there was a definite volcano season (Mason & Pyle, 2004). Significantly more volcanoes erupt between November and April than between April and October.
The author¹ suggests that as there has been a steady accumulation of evidence that volcanic systems are sensitive to changes in the environment that are very small the idea of a volcano season should not be surprising. A volcano must be primed, it feeder system ready with a magma supply at a critical point needing only a small nudge to burst out of the volcano, if a tiny physical change is to be translated into an eruption, and the fact that such a significant number of the active volcanoes of the world respond to such tiny seasonal variations in their environment suggests that many are in this state for much of the time. No new magma is involved over the course of the volcano season, and there is no overall increase of the number of eruptions, as there was at the end of the last ice age. The volcanoes are being 'forced' to erupt within a particular time frame by external environmental conditions.
The underlying cause of the seasonality of the eruptions proved be more complicated than the speculations that had been considered suggested, with variations of a number of environmental parameters being involved.. According to the author¹ the primary cause of the seasonal pattern appears to be the deformation of the surface of the Earth that occurs annually that accompanies the wholesale redistribution of water during the operation of the water cycle, that is the movement around the surface of the Earth and between the surface of the land and the ocean and the atmosphere that is a continuous process.
It has been very difficult to measure the very small deformations that have been suggested to be driving the volcano season, requiring GPS technology to accomplish the measurement. A new model for the wholesale deformation of the Earth was published in the journal science (Geoff Blewitt et al., 2001). The team involved scientists from the University of Nevada in the US and Newcastle University in the UK recognised a seasonal cycle of changes in the shape of the Earth over the course of a year. The study revealed that the shape of the Earth changed, in a similar manner to a beating heart, the changes in the shape of the Earth taking place systematically and repeatedly, each 'Earthbeat' taking 1 year. Over the course of a single 'beat' the Northern Hemisphere contracts with the peak being reached in February and March, and simultaneously the Southern Hemisphere expands. With the next 'beat' the Northern Hemisphere expands, the peak occurring in August and September, and the Southern Hemisphere contracts simultaneously. The changes detected are miniscule compared with the diameter of the Earth. The North Pole moves downwards by 3 mm at the peak of the compression and points near the equator move to the north by 1.5 mm. A vast wave of ephemeral weight moves across the Earth, the changes involving snow cover, moisture in the soil and the mass of the atmosphere, taking 1 year to complete. When it is winter in the Northern Hemisphere snow, groundwater that has been recharged and the cooler, denser atmosphere exert extra load on the crust pushing it down, the same occurring in the Southern Hemisphere as winter moves south of the equator. Over the course of a single cycle the total mass of water moved between hemispheres is enormous, totalling about 1016 kg, more than 500 times greater than the total weight of coal mined over the entire Earth in 1 year.
Mason et al., suggest that it is the movement of surface water over the course of a year driving the annual deformation of the Earth's surface that causes the seasonal response of volcanoes around the world. The author¹ suggests that the picture is more complex than this as individual volcanoes and specific volcanic regions behave in different ways. Volcanoes erupting along the 'Ring of Fire' around the Pacific, and locally in the case of some individual volcanoes, define the overall volcano season, and the author¹ also suggests the volcano season mainly reflects the timing of small explosive events that make up most of the catalogue that was used in the study. He points out that about half the eruptions in some places appear to be controlled by effects on a seasonal basis, though in other places the eruptions due to these changes can be as low as 20 %. And then there are regions in which there appears to be no seasonality of eruptions, such as in the Mediterranean region. In general volcanic response to sea level changes that occur seasonally appear to be ambivalent, unlike in the case of Pavlof. Peaks in rate of eruptions that are seasonal seem to coincide with sea levels that are dropping, as occurs in Central America, the Kamchatka Peninsula and Alaska, excluding Pavlof. In Melanesia in the southwest Pacific, island volcanoes tend to erupt at times of high sea levels in the region. According to Mason's model, though a volcano also responds to other factors, erupting at other times of the year, the tendency is for its eruptions to be concentrated, to a greater or lesser degree, when the deformation wave reaches its vicinity.
The importance of the volcano season
According to the author¹ the identification of the a volcanic season draws the previous ideas about the way volcanoes respond to external perturbations together into a coherent whole. The discovery of the tendency of volcanic activity to be regulated on seasonal timescales in the modern era is suggested by Mason et al. to provide a contemporary analogue for phenomena that were previously believed to have operated in the postglacial period, and earlier, thousands of years ago. It is also suggested by the recognition of a seasonal pattern that responses of individual volcanoes to storms, heavy rainfall of atmospheric pressure changes are part of a much larger-scale relationship between volcanic activity and global climate. The suggestion that past global sea level change episodes were associated with vigorous bouts of volcanic activity is supported by the results of Mason's study. It has been suggested by Mason et al. that falling sea levels might be more effective, though Pavlof and the widespread volcanic activity associated with the melting of the vast ice sheets suggest it is rising sea levels that are more effective. McGuire et al. suggest the rate of change is the important factor, volcanic eruptions being triggered by sea level changes that are large enough and fast enough, whether the level is rising or falling, will both promote volcanic activity, though the mechanism involved will differ.
Pavlof demonstrates that when the sea level rises the stresses within and beneath the volcano may change as a result of crustal loading that results in an eruption. The collapse of a volcano's flank can also trigger an eruption in response to changes within the structure, marine erosion, or as a consequence of rising water tables increasing the instability, promoting the violent interaction between water and magma at shallow depths. Resident magma may exploit widening and opening fractures to get to the surface when tensional conditions develop higher up in a volcano as its lower parts and the crust beneath are bent and compressed. The rise of magma may be promoted when the weight acting on the flanks in contact with the sea is reduced as a result of falling sea levels. When the buttressing effect of seawater is removed it can increase the instability of the flank and encourage collapse that triggers an eruption.
According to the author¹ to gain a better understanding of how climate interacts with volcanic activity it is essential to determine how volcanoes respond to changing sea levels. He says that if climate can be changed by volcanic activity and simultaneously volcanic activity can trigger changes of climate, then it is the the proverbial chicken and egg situation fraught with many types of feedbacks and loops. One example is when ice sheet formation caused sea levels to drop as the world cools, the falling sea levels could trigger explosive volcanic activity that puts sulphuric acid droplets into the stratosphere that increasingly rapidly pushes the world further into cooler conditions that lead to an ice age. The alternative scenario involves rising sea levels in a warming world with the retreat of glaciers and the rising levels of the sea that could be expected to cause volcanoes to erupt that again puts sulphuric acid droplets into the stratosphere and contributes to a cooling effect that could hamper the transition from hothouse to icehouse conditions.
The sulphuric acid production is a positive feedback in the first example where a cooling world in which volcanoes are triggered to erupt tends to push the climate further towards icehouse conditions. In this case the question that needs to be asked is whether in a cooling Earth can the volcanic activity be sufficient to add to the cooling trend? If there are enough volcanoes erupting adding enough sulphur to the atmosphere, or if a single volcano erupts on the scale of Toba 74,000 years ago maybe this could happen. Many have speculated about the possibility that the last glaciation could have been accelerated towards the icehouse world that prevailed at the time by the extremely large eruption of Toba that produced the largest volcanic crater in the world. It has also been speculated that the eruption of Toba might have been triggered by climate change in the cooling period that led to the last glaciation (Mike Rampino, 1979; and others). It has since been suggested (Rampino & Self, 1990s) that the Toba eruption, the largest in the last 100,000 years, may have been a pivotal role in the major climate transition at a time when the ice sheets had reached about 1/4 of their final extent at the last glacial maximum. The possibility has been considered by Rampino and Self that it may have been crustal stress changes resulting from the rapid growth of ice sheets, with the associated sea level drop, as much as 40 m over a time span of 7,000 years, that was responsible for the timing of the Toba eruption.
According to the author¹ almost 3,000 km³ of volcanic debris was blasted out of the crater of Toba by an eruption that was about 1,000 times greater than the 1980 eruption of Mount St Helens, and up to about 40 km into the atmosphere. It has been estimated that it would have added about 5,000 million tonnes of sulphur dioxide to the atmosphere, the sulphate signature being present in the Greenland ice cores. These cores also record what happened to the climate over the years immediately after the eruption. According to the cores there was a global temperature drop of 10° C as a result of massive loading of the stratosphere with sulphuric acid aerosols, remaining at the lower temperature for several years. Following this period of lower global temperatures there was an extended period of cooling with the advance of glaciers that lasted for about 1,000 years, that Rampino & Self say is the long-term effect of the eruption. The author¹ suggests the impressive temperature drop immediately following the eruption is now accepted as convincing evidence of a volcanic winter, though he is not convinced that the extended colder phase with advancing glaciers that occurred in the following 1,000 years is a lingering effect of the eruption. It has been demonstrated with a computer model by Gareth Jones et al. that though snow and ice cover increased to cover more than 1/3 of the Earth the initiation of a longer-lasting episode of glaciation didn't seem likely. This result was confirmed by Alan Robock et al. in the USA, reaffirming that the global temperature drop immediately after the eruption was indeed a result of the eruption, and they suggested that it may have been colder and longer than was previously believed. They suggest that there was no hint in their simulation of an impact on the global climate that was long enough to have promoted a glacial phase and a cold snap that lasted another 1,000 years. It has since been suggested by Rampino and Self that there are other example in the past 2 million years where large individual eruptions or clusters of eruptions at times when the climate was in a phase of major transition, pointing to their possible significance in accelerating cooling or slowing warming, depending on the circumstances.
When major volcanic activity occurs during a period of global warming it is believed to possibly initiate a negative feedback, such as the warming being slowed down by the injection of sulphuric acid aerosols into the stratosphere, in the same way that in times when the climate is cooling bouts of massive volcanic activity can provide positive feedback. One result of this could be a transition from a glacial to an interglacial that could become more difficult than it would otherwise be. The author¹ suggests it is tempting to speculate that a large eruption or eruption cluster during the revival of volcanic activity at the close of the last glacial phase may have had a role in sudden cooling that is known as the Younger Dryas, possibly in South America where the Younger Dryas appears to have begun, though it has also been attributed to a very large influx of glacial meltwater into the North Atlantic.
It has recently been proposed by Peter Huybers and Charles Langmuir, of Harvard University, that a warming effect at a time of major changes of climate may result from bursts of volcanic activity, arguing for a big increase in the amount of carbon dioxide released into the atmosphere as a result of bursts of volcanic activity that occur when climates are changing dramatically, and overwhelming any cooling effect that may be provided by sulphuric acid aerosols in the stratosphere. The author¹ suggests that if this proves to be the true situation then volcanoes accelerate warming at the transition from a glacial phase to an interglacial phase, though he says the vast majority of current evidence points to the cooling due to the outgassing of sulphur dioxide far outweighing any warming effect resulting from any carbon dioxide that is emitted. At least 2 studies have indicated that volcanic activity is closely correlated with cooling episodes accompanying the general warming progressive trend in post-glacial times.
The author¹ concludes about volcanoes and their activity in connection with changing climate is that, most crucially, when the active volcanoes of the world are considered collectively, they have been found to be primed systems that are always on the edge of stability which makes them highly sensitive to extremely small changes in their external environment. At the present this allows the hydrological cycle to regulate the annual variations in their activity, and in the past has made them sensitive to periods of massive, sudden climate change, especially at times of major transitions of the climate. As temperatures rise as a result of anthropogenic climate change there will be increasing loss of ice mass at high latitudes and altitudes, and the associated increased mass in the oceans, permafrost melts, and there are changes in the weather patterns and precipitation there will be major changes to the hydrological cycle.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|