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For me, there is something strangely captivating about magma and lava.
Rock so hot it flows like water, and when it meets the ocean the explosive quench constructs new land.
THE GEOLOGY OF SABA
Saba is the northernmost volcanic island in the Active Arc of the Lesser Antilles with an area of only 13 sq. km and a population of around 1200. It is rhomb shaped and is a single volcano measuring 4.6 km east to west and 4.0 km north to south rising to a central peak of Mt. Scenery at 887m. The island has a permanent population of around 1,100 but the presence of a medical college and tourists can double this number. The appearance of the island is that of a deceptively simple stratovolcano, but this is not the case as it has been built up of a large number of Pelean domes with their aprons of coarse pyroclastic deposits that form a distinctive shoulder on the island at about 450 to 500 m a.s.l. Mt. Scenery is a younger cone perched somewhat eccentrically to the north on this foundation of Pelean domes. A prominent sector collapse scar exists on the south-western flanks of the volcano in which the island’s administrative capital sits. The upper part of this scar has been buried by the younger deposits of Mt. Scenery and the lower part by the Pelean dome and pyroclastic aprons of Great, Bunker and Paris Hills. The submarine flanks of the island slope away uniformly in all directions, except the west where about 1.3km offshore there is a single parasitic conical submarine Pelean dome rising from depths of 300m to only 23 m below sea level.
Of course, the same forces that create inviting environs for people to inhabit can abruptly destroy them:
Blast from the Past
The eruption of Mount Tambora killed thousands, plunged much of the world into a frightful chill and offers lessons for today
Robert Evans/SMITHSONIAN MAGAZINE
The most destructive explosion on earth in the past 10,000 years was the eruption of an obscure volcano in Indonesia called Mount Tambora. More than 13,000 feet high, Tambora blew up in 1815 and blasted 12 cubic miles of gases, dust and rock into the atmosphere and onto the island of Sumbawa and the surrounding area. Rivers of incandescent ash poured down the mountain’s flanks and burned grasslands and forests. The ground shook, sending tsunamis racing across the JavaSea. An estimated 10,000 of the island’s inhabitants died instantly.
A volcano can be quiet across the lifespan of generations of humans, but it can be as unsettled and tumultuous as a cranky toddler:
Society Islands, French Polynesia
17.87 S, 148.07 W
summit elevation 435 m
Mehetia Island is the summit of a volcano which rises 4000 m from the sea floor. The volcano is located 113 km east of Tahiti and forms a 1.5 km wide island. The summit contains a 150 m wide and 80 m deep crater.
There are no historic eruptions at Mehetia Island despite young volcanic landforms such as a well-preserved crater and lava flows. There is no fumarolic activity on the island, but the volcano is considered active with undersea eruptions occurring at a depth of 600 m.
1981 Earthquake swarms
Between 6th March and December 1981 Mehetia Island experienced 3500 earthquakes over magnitude 1.4. The earthquakes occurred in two swarms.
The first swarm, which was volcanic, occurred in the first two months, and was linked to underwater eruptions at a depth of about 1600 m on the southeast flank of the island.
The second earthquake swarm was mostly tectonic and consisted of a smaller number of higher-magnitude earthquakes (up to magnitude 4.3). These were geographically dispersed and probably associated with underground resetting.
The formation and activity of volcanic islands give us a glimpse of the workings of our world, which operate on scales of time and mass the defy comprehension.
The Atlantic Ocean began to form when the African and South American plates were thrust apart about 120 million years ago. Ocean floor spreading continues at the Mid-Atlantic Ridge where relatively shallow rising magma erupts into basalt lava forming low ridges on the ocean floor, pushing Africa north-eastwards and South America north-westwards at a current rate of about 2cm a year.
Tristan da Cunha is not on the Mid-Atlantic Ridge but is a surface expression of a deep-seated hot spot some 400 km east of the ridge (see map right). Hot spots (The Hawaiian Islands are another example) such as these receive magma from deep in the earth's mantle…
The Tristan Island Group forms the modern south-west end of a range of otherwise extinct submarine volcanoes, termed seamounts or guyots ( former volcanic islands now planed off by sea erosion) known as the Walvis Ridge. This largely submerged mountain range marks the movement of the African Plate north-east over the stationary hot spot. As the ocean floor spreads, the fixed hot spot leaves a trace-line of volcanoes, each of which becomes extinct as it moves away from the stationary mantle source. Eventually, as the African plate moves further NE, Tristan itself will become extinct and a new active hot spot volcano will build to the west.
The present-day Tristan Island Group marks the hotspot activity during the last 18 million years, first forming Nightingale Island (an extinct remnant in the latter stage of erosion), then Inaccessible Island (a younger extinct cone in the middle stage of erosion) and finally Tristan island itself (having the classic conical shape of an active shield volcano)…
Tristan da Cunha is an active strato-volcano formed above a magma hot-spot some 400km east of the Mid-Atlantic Ridge.
The volcano first erupted 3 million years ago from the 3500m deep ocean floor.
Successive eruptions have built a cone 48km wide and 5500m high, with the summit (Queen Mary's Peak at 2,060m above sea level) overlooking a heart-shaped crater lake.
Tristan erupts both effusively (to form sheet-like lava flows) and explosively (when magma has a higher gas content to yield ash and vesicular rocks like the recent pumice). Vesicular rocks are those with many cavities or holes and are commonly called 'floating stones' on Tristan.
Those who call island’s home have no choice but to be cognizant of the condition of the ocean.
Humans have lived on volcanic islands for millennia, but residing on them has never been more precarious:
Climate Change and Migration Issues in the Pacific
Authored by John Campbell and Olivia Warrick
Publication prepared by the United Nations Economic and Social Commission for Asia and the Pacific
Environmental change can contribute to individual’s decision to migrate. Although economic and social reasons maybe the primary reasons for migration, environmental change can also contribute to the decision to migrate. Climate change can cause a reduction in land, livelihood or habitat security for some Pacific communities. For example, low-lying coastal areas and river deltas may become unsuitable for physical settlement, or they remain habitable but income and food security options become marginal; or reduced precipitation or increased disease vectors could cause the deterioration of habitability. The impacts of climate change can be the tipping point which results in an individual or family deciding to migrate. In the longer term, the planned relocation of some communities may be required, particularly in areas where population density and growth rates are high. In the shorter term, the voluntary migration of individuals and households could aid in relieving environmental pressure when coupled with improved in situ adaptation strategies, population management and climate-resilient development. There are five ‘hotspots’ in the Pacific that are likely to become source areas for climate change-related migrants: (a)urban areas; (b) urban atolls; (c) non-urban atolls; (d) coastal, delta and riverine communities; and (e) communities prone to drought. As cited by the International Organization for Migration (IOM), global estimates for the number of migrants moving due to climate change range between 25 million and 1 billion people by 2050, with 200 million people the most commonly cited figure. Inherent uncertainties mean that only rough estimates can be given for the number of people likely to be involved in migration related to climate change. However, the impacts of climate change on migration will be more acute in particular habitats. A review of the existing literature identifies five localities that are potential ‘hotspots’ requiring increased research into climate change impacts, in situ adaptation responses, demographic processes and community security. These include: (a) urban areas; (b) urban atolls; (c) non-urban atolls; (d) coastal, delta and riverine communities; and (e)communities prone to drought. Unmanaged rural to urban migration and population growth strains the capacity of urban areas to cope with the impacts of climate change; as urban populations continue to grow there is likely to be an increased demand from urban populations for international migration.
The Economics of Volcanoes
*Johanna Choumert, Anaïs Lamour and Pascale Phélinas
September 12, 2019
Volcanic hazard pose a potential threat to almost 500 million people worldwide representing9% of the world’s population (Small and Naumann, 2001). Moreover, according to Ewert and Harpel (2004), the population exposed to volcanic hazards is expected to keep increasing: sustained population growth, migration to areas in close proximity to volcanoes (both urban and rural), but also the possibility of larger eruptions are the main drivers of this trend. Unfortunately, the rise in potential exposure will occur mainly in low and middle-income economies where the population has limited resources but where most of active volcanoes are located. The so-called “Ring of Fire” circles the Pacific Ocean along the Pacific coast of America and Southeast Asia; the Great Rift Valley is a 6,000-mile crack stretching from Lebanon to Mozambique that is still in progress. As a result, many large cities in low and middle-income countries are at threat of a volcanic eruption, such as Mexico City, Manila, Guatemala City, San Salvador, Managua and Quito…
The analysis of the welfare losses inflicted on exposed households by a natural disaster risk uses concepts developed by two arrays of microeconomic studies, the first one focusing on ex-post impacts, when a shock occurs, and the second one considering ex-ante strategies used by households to manage a risk, in anticipation of a future shock. In addition, due to the characteristics of volcanic risk and their localization in low-income regions, both ex-post and ex-ante effects in this case can be seen in the light of the linkages between this risk, the poverty status of exposed individuals and local market inefficiencies. On the one hand, household assets strongly define whether the household will be able to avoid wellbeing losses when a disaster strikes (high vulnerability) and to recover from them (low resilience). For a vulnerable household, the occurrence of a natural disaster can thus increase the probability that it becomes unable to meet basic needs and caught in a poverty trap, provoking dramatic adverse effect on its welfare. Poverty appears as both a driver and a consequence of disaster-induced losses. On the other hand, the threat itself of a disaster cannot be efficiently transferred away (missing or incomplete insurance market) from exposed households and small entrepreneurs, affecting their welfare and investment given risk aversion and in spite of sophisticated but costly ex-ante self-insurance arrangements. The responses of individuals to risk thus perpetuate poverty
The potential economic impact of volcano alerts
by Society for Risk Analysis/ phys.org
MARCH 17, 2021
A new study published in Risk Analysis suggests that, when an alert remains elevated at any level above "normal" due to a period of volcanic unrest, it can cause a decline in the region's housing prices and other economic indicators. Because of this, the authors argue that federal policymakers may need to account for the effects of prolonged volcanic unrest—not just destructive eruptions—in the provision of disaster relief funding…
With natural hazards, the mere presence of information about hazard potential in the form of a public alert level notification may have an adverse effect on local economies.
This sheds light on a systemic issue in disaster resilience, the authors argue. The federal government currently provides disaster relief for direct impacts of volcanic eruptions and other natural disasters, but limited or no assistance for the indirect effects experienced from long periods of volcanic unrest. Durations of volcanic unrest are often protracted in comparison to precursory periods for other hazardous events (such as earthquakes, hurricanes, and floods). As Peers points out, this makes the issue of disaster relief for indirect effects particularly important in high-risk volcanic regions.
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