All volcanic eruptions, whether they involve arc, rift or hotspot volcanoes, clearly deliver material from the interior to the surface. The extruded rocks build up mountains, islands, and lava flows, some of which have added to the size of continents. The gases and volatiles brought up to the surface have built up the atmosphere and oceans. From the amount of certain elements in the current atmosphere, such as Neon, we are confident that the Earth that accreted had no initial atmosphere, which must have boiled off during the heavy bombardments and magma ocean phase. Thus, all of the present atmosphere and oceans have come from the interior (apart from minor additions from infalling comets). Looking at how volatiles come out of the ground today, we are convinced that volcanoes are now, and presumably have always been the main sources of gas transfer from the interior to the surface. The current atmosphere is 79% nitrogen and 21% oxygen, with traces of water and other materials. Nitrogen is being released at many volcanoes today, and fortunately the Earth is warm enough that nitrogen has not combined with hydrogen in the Earth's atmosphere to condense out as ammonia (as appears to be the case on the major gas-planets such as Jupiter). While free oxygen is not coming out of volcanoes, carbon dioxide does, and plant respiration is responsible for the conversion to an oxygen atmosphere, as we noted before.
We can test the gasses coming out of volcanoes today to see if it is consistent with what is found in the atmosphere. Volcanic gas samples reveal large amounts of water (H2O), carbon dioxide (CO2) and Nitrogen (N2) coming out today, along with smaller amounts of sulfur dioxide (SO2), Hydrogen sulfide (H2S), carbon monoxide (CO), Hydrogen (H2), hydrochloric acid (HCl), and Methane (CH4). If we take the current rate at which theses gases are emerging, and calculate the cumulative volumes over the history of the planet, we can only account for about 25% of the total water, chlorine, and nitrogen at the surface. This implies that if volcanism has always been the main source of atmospheric gases, there must have been more intensive volcanism, and perhaps larger gas fluxes in the past. Some materials, such as sulfur are less abundant on the surface than would be expected based on the sulfur expulsion rate. This implies that there is some mechanism for eliminating sulfur from the atmosphere. Sulfur is a very reactive material, and can combine with other elements to produce minerals that are efficiently subducted, thus cleansing the surface of much of the sulfur that emerges.
In addition to the gases that come out of volcanoes, there are large amounts of solid materials ejected, typically the smaller particles of dust reach the highest levels in the atmosphere. For strong vertical eruptions, dust can be propelled up into the stratosphere, above 17 kilometers. Once there, the suspended dust particles can block solar radiation, effectively heating up the stratosphere while the lower troposphere cools. This eruptive cooling was first noted by Ben Franklin in 1783 while he was in Europe, and he attributed the cooling to eruption of Laki on Iceland that year. The most important eruptions for influencing climate are massive vertical eruptions, which propel material into the stratosphere. Examples include the 1815 eruption of Tambora, which gave out 30-150 cubic kilometers of material and the 1883 eruption of Krakatoa. The eruption of Mt. Mazama which was the volcano preceding the collapsed caldera of Crater Lake, Oregon erupted in about 4000 B.C., sending out 5 cubic miles of material, more than 100 times that produced by the 1980 eruption of Mt. St. Helens.
Around the turn of the century, there was a spate of particularly large eruptions on Earth, with the 1883 eruption of Krakatoa, the 1902 eruptions of Pelee, Soufriere and Santa Maria, and the 1912 eruption of Katmai. Tracking of the average temperature around the Earth over the past 100 years, shows that this time was relatively cool, by about half of a degree compared to the period from 1920-1945 which had few eruptions. From 1945 on there have been more eruptions, which is slowly causing the temperature to decrease. The long-term oscillations of temperature associated with volcanism must be understood when investigating global warming or global cooling phenomena.
One of the most obvious aspects of volcanism is that it involves heat, and this energy source is inviting to tap. In the past few decades there have been numerous attempts to exploit Geothermal energy, in both volcanic and other areas. Several approaches have been tried, including hydrothermal energy, which involves bringing to the surface water heated by interaction with hot rocks at depth, and then using the steam energy for power. The U.S. potential energy from this source is twice the total energy in the world's oil supply. While the U.S. does not have that many volcanic areas with large hydrothermal systems, there is heat in all of the rock beneath our feet, and efforts to tap that heat are being explored. This U.S. abundance of so-called hot dry rock could provide 6000 times the world's supply of oil, if we can perfect technologies to extract the heat. Finally, magma reservoirs are sometimes shallow enough that we might be able to directly tap the heat in the molten rock. For the U.S. this would supply about 80 times the world's oil supply. So, with this bounty of ready energy below us, why do we still use fossil fuels? In part, it has proven difficult to efficiently extract much of the heat energy. For example, geothermal systems can quickly be drained of water, so that new water has to be input by recharge systems that require lots of energy themselves. While the water may heat up as it penetrates down to near a 900-1200 degree magma body, thus rising as steam with temperatures of 100-350 degrees C, the hot water is very reactive and often cements up the porous rock with minerals leached from the rock itself. Thus, there tends to be a finite lifetime to the circulation patterns in the ground, and it can be hard to sustain the productivity of the hydrothermal system.
Nonetheless, a few areas around the world are producing significant power from hydrothermal systems. Iceland is one of the most advanced nations in this respect, with 70% or so of the power for the capital city of Reykjavik being produced by hydrothermal power. The Geysers in northern California produces 500 megawatts of power per year, with 240 degree steam. This is enough power to meet the needs of San Francisco.
Another benefit of volcanic systems is that minerals and ores are concentrated near them. Water that emerges from or interacts with a magma body tends to be enriched in materials such as Fl, S, Zn, Cu, Pb, U, Au, Ag, Hg. These materials can precipitate out as the water circulates through the crust and cools in hydrothermal veins. This has given rise to major ore deposits. Some forms of volcanic up-wellings rise extremely rapidly through continental lithosphere, bringing high pressure minerals to the surface. This is how diamonds reach the surface. Diamond is a high pressure form of pure carbon, relatively unstable at the surface, where the common form of carbon called graphite is most stable. Diamonds are rapidly brought up from depths of 100-300 km in upwellings called Kimberlites. Thus, some of the glory of diamond rich rock in the interior are shared with us at the surface due to volcanism.
Eruption Prediction and Hazard Mitigation
As for earthquakes, the societal response to natural hazards posed by volcanoes is couched in terms of the specific hazards that they present, as well as the viable options for dealing with the phenomena. We'll consider some of the specific hazards associated with each, and discuss the mitigation strategies that have evolved.
Some of the major volcanic hazards are:
|
1) Primary |
Lava |
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Ash |
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|
Nuee Ardentes |
|
|
Gas |
|
|
2) Secondary |
Lahars |
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Tsunami |
|
|
Agricultural |
Lava flows are usually not very dangerous, as they move rather slowly. But, there are exceptions. For example, the 1977 Nyiragongo (Zaire) volcano had a side fissure drain the lava lake in the main crater very rapidly, with lava squirting out at 60 miles per hour. This caused 1000 fatalities. Hawaii has had relatively rapid flows run into developed areas, but usually evacuation is possible.
Ash falls are typified by the 79 AD eruption of Mt. Vesuvius, which blanketed Pompeii and Herculaneum with mud ash and gas, causing about 20,000 fatalities. The main U.S. concern is over the downwind (easterly) deposit of ash from Cascade volcanoes, as occurred in 1980 for Mt. St. Helens.
Nuee Ardente are fast moving flows of gas and magma, in a volcanic avalanche. These are very deadly, as they flow fast and with utter destruction. Examples include the 1902 eruption of La Soufriere, St. Vincent, which killed 1500, and the 1902 eruption of Mt. Pelee, Martinique, when the town of St. Pierre was overrun, killing 30,000 (leaving only 2 survivors in jail). The latter event was particularly sad, as evacuation had been encouraged based on activity of the volcano and massive migrations of snakes through town away from the volcano.
Gas emissions accompany every volcano, but in some cases there are special conditions that allow the gasses to build up. The 1783 eruption of Mt. Laki, Iceland had a massive flux of sulfuric and fluorine gas, which killed great numbers of livestock. The ensuing famine led to 10,000 fatalities. The 1986 Lake Nios, Cameroon event involved an overturn of CO2 that had accumulated at the base of a lake in the crater. Over 1500 people were asphyxiated by the gas cloud that bubbled forth.
Lahars are volcanic mudflows, typically resulting from eruption under ice and snow at the summit, which mixes with the ash to make fast flowing muds. The 1985 Nevado del Ruiz event involved a 12 foot mudflow that descended on Armero, Colombia, killing 23,000. There had been six hours to evacuate, but this was not enough. Such mudflows occur again and again, in predictable paths, so some planning can be done. The main U.S. concern is for Mt. Rainier, where past mud flows run into Seattle.
Tsunamis occur when eruptions displace ocean water. The 1883 Krakatoa, Java eruption ejected 5 cubic miles of material, producing a 100 foot high wave which spread out and destroyed 300 towns, taking 36000 lives. The 1450 B.C. eruption of Santorini ejected almost 25 cubic miles of rock, producing the huge sea waves that swept away Minoan civilization.
Agricultural catastrophes are secondary consequences of the widespread devastation of volcanic eruptions. The 1815 Tambora eruption covered 1 million square miles with ash, with 30-150 cubic kilometers of volume. While this took 10,000 lives directly, about 80,000 perished due to famine. In 1902, Santa Maria volcano in Guatemala had a 5.5 cubic kilometer eruption. The ash killed many birds, leading to flies and mosquitoes flourishing. Malaria outbreaks ensued and are blamed for 3000 deaths.
These hazards are varied and clearly not easily controlled. Indeed it is a basic fact that we cannot do much to limit damage from volcanic hazards by better construction methods and the like. The emphasis is on prediction of the event so that evacuation can save lives.
Volcanic prediction is difficult, but in many ways it is more viable than for earthquakes. The major difficulty is that every volcano has distinctive behavior, which must be characterized case by case. This is challenging because of the long period of repose between explosions.
Most volcanic predictions are based on various phenomena:
For some volcanoes, the statistical behavior can be characterized if there is a history of 10-20 eruptive sequences. This is an empirical approach, as there is no simple physical theory for the eruptive cycle. Volcanoes are observed to have either random eruption sequences, or sequences in which the probability of an eruption increases with time since the last eruption, or in which the probability decreases with time since the last eruption. The latter case corresponds to bursts of activity followed by repose, while the middle case is gradual accumulation of pressure during periods of repose. Individual behavior may be stable over fairly short periods of time, even though it may change through the lifetime of the volcano.
Statistical methods give gross probabilities, but not short-time prediction. Earthquake activity near volcanoes is due to stresses in the crust, temperature changes, magma movements, and gas explosions. Earthquake swarms are often precursors to eruptions, especially when the depth of events decreases with time, reflecting ascent of magma. Prolonged ground vibrations called harmonic tremor are associated with resonances in the fluid filled plumbing of the volcano. On average when an increase in earthquake activity is observed, 58% of the time it precedes an eruption, 38% of the time there is no eruption, and 4% of the time there is an eruption with no earthquakes. Individual volcanoes differ in the reliability of earthquake precursors, and again each volcano must be characterized.
Inflation of the magma chamber below the volcano causes tilting and uplift of the surface which can be measured. This is the direct result of ascent of magma and build-up of gas pressure. For Kilauea, the probability of eruption increases with increasing tilt of the surface. But, the problem is often that tilting occurs, but magma may not reach the surface. How to anticipate actual outpourings is a problem.
There are many instances of successful prediction of major eruptions. We will see a case in the movie about Mt. Pinatubo. But, a sobering case history is offered by Soufriere of Guadelupe in 1976. This volcano had come under observation due to small steam explosions and earthquake tremors. Numerous scientific teams arrived, and bickered over what to make of the activity, and the media played up the arguments. Ultimately, a town of 74,000 was evacuated for 4 months, at a cost of $500 million dollars. But, no eruption happened. The agonizing over this choice may have played a conservative role in the reluctance to evacuate Armero in 1985 when the Nevado del Ruiz lahar destroyed the city and killed 23,000.
The U.S. has set up a hierarchy of volcano warning levels:
The 1980 eruption of Mt. St. Helens came during a Hazard Warning based on seismic activity, inflation of the summit and history of the mountain, which has had many lateral explosions and violent eruptions.
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