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Volcanoes In Focus


Guest Shetland Coastie

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Guest Shetland Coastie

VOLCANO IN FOCUS - HEKLA - ICELAND

Know as the "land of ice and fire" Iceland lies on the North Atlantic Ridge, making it highly volcanically and hydrothermically active, with numerous well-known volcanos, including the sub-glacial Grimsvotn, Vatnojokull & Loki volcanos.

One of the most active, however, is Hekla, which is a prominent stratovolcano which lies in southern Iceland. The map below shows its location:

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Hekla has erupted frequently in recorded history since 1104 and these eruptions are often large and explosive, often measuring VEI3 - VEI5. Hekla also produces prodigious amounts of lava, with its eruptions tending to follow a pattern of an initial, Plinian-style explosive phase, followed by often large volume lava production and tephra ejection. Indeed, 10% of all Icelands tephra deposits of the last 1000 years have come from Hekla.

Hekla is 1491m high and has a large, 5.5km long fissure called Heklugja, which cuts across it and is the scene of spectacular lava fountains during eruption, as the fissure often unzips for almost its entire length. The tephra produced has an extremely high fluorine content and is therefore dangerous to grazing animals. During non-eruptive spells, Hekla is often covered in snow and has small glaciers.

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The largest eruption from Hekla occurred sometime between 1159 and 950 BC. This was also the largest volcanic eruption in Icelands history. This eruption produced some 7.3 cu km of ejecta (VEI5) and lowered northern hemisphere temperatures for a number of years afterwards, indeed, tree ring evidence in Ireland shows little or no growth for at least a decade after this eruption. Traces of this eruption have also been found in Scottish peat bogs.

There have been some 20-30 large eruptions since 1104. Hekla is a particularly dangerous volcano in that it gives little or no warning that it is about to erupt. Often, seismic events will only start some 30-80 minutes prior to an eruption and these increase rapidly to a crescendo, before dying back again after the initial first hour or two of an eruption. During dormant spells the volcano is largely aseismic, with virtually none of the background seismic activity associated with other volcanos. The magma chamber is estimated to be large with its top some 4km below the surface and its centrepoint some 2.5km below that. A firm correlation has been established between periods of dormancy and the violence of Heklas eruptions. The longer she remains dormant, the more violent and catastrophic her eruptions can be.

In 1846, an eruption deposited volcanic ash as far away as Shetland and Orkney. The eruption which started in March 1947 lasted until April 1948, ending a 100 year period of dormancy which was the longest since the 1104 eruption. This eruption began completely without warning with a loud roar, which was followed in the initial phase by a large, Plinian-style explosive eruption, with a column that reached over 30km high and produced an ash layer some 3-10cms thick. 51 hours later, ash started to fall in Helsinki, Finland, having travelled 2860km in that time.

The most recent eruption was a relatively short-lived affair in 2000, which suggests a more powerful eruption may be due in the near to medium future. The 2000 eruption measured VEI3 and had 4 distinct phases:

1. Initial explosive eruption

2. Lava fountaining

3. Strombolian type activity

4. Lava effusion

This eruption started on 26 Feb 2000 and lasted for some 10 days. The picture below shows the 2000 eruption at night with the northern lights above!

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And a couple more pictures, this time of the 1970 eruption:

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In summary then a volcano that has it all! Large explosive eruptions, lava fountaining, massive lava production and (as discovered in the 2000 eruption) pyroclastic flows. One to watch!

Edited by Shetland Coastie
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Guest Shetland Coastie

Hi all. Just so everyone knows, in consultation with the mods, its been decided to make a seperate thread for this more in-depth look at volcanos.

The original thread can get quite congested with snippets of information, especially if theres an eruption ongoing and things are happening quite quickly so we thought it might be an idea to open this new thread to keep the two seperate.

Im hoping to regularly post more in-depth looks at volcanos around the world in here and it will follow the pattern of the first one about Hekla i.e. the nature of the volcano, its eruptive history, especially any famous eruptions and what the future holds. During fairly quiet periods like now, I will just pick a volcano at random or Im open to requests! When a particular volcano erupts then obviously I will focus on that particular volcano so that you get some background and perspective to the current eruption.

:)

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VOLCANO IN FOCUS - GLACIER PEAK - USA

Glacier Peak is one of 5 major volcanoes in Washington State and one of the most isolated of all the Cascade Volcanic Arc volcanoes. The maps below shows Glacier Peak in relation to other Cascades volcanoes and also its locale.

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The vulcanism of the Cascades Volcanoes is caused by the subduction of the Juan de Fuca plate beneath the North American Plate and the movement of this proceeds at a rate of some 4cm per year. The volcano is some 10,451ft high although only the top 3,000 or so ft is the actual volcano as it sits on top of a ridge, this makes it a fairly small stratovolcano. A couple of views of the volcano below.

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Like its sister St.Helens, Glacier Peaks eruptive pattern is highly explosive and lying only 70 miles NE of Seattle, any major eruption of the volcano would have a major impact. Also, like St.Helens, Glacier Peaks lava is dacitic and highly viscous (sticky) this means that it builds up in the vent, leading to large explosions, indeed USGS believes that a large debris avalanche or lateral explosion of the same order as the 1980 St.Helens blast is a distinct possibility.

Glacier Peak, as its name suggests is covered by some 11 major glaciers and several smaller ones. This of course means that any eruption is likely to produce large scale lahars, mudflows and pyroclastic flows. Indeed huge pyroclastic and mudflow deposits have been identified from previous eruptions, extending as far as Puget Sound to the west. The diagram below shows the path of these flows which have even diverted the course of some rivers. As you can see, the flows pass through several populated areas.

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Glacier Peak last erupted in approximately 1700 (+/- 100 years). Its eruptions are infrequent and unevenly spaced, making prediction of the next eruption extremely difficult. In addition, due to its remoteness it is poorly monitored and studied. A 2005 USGS report identified Glacier Peak as a "very high threat volcano with inadequate monitoring." Indeed at the time the report was made there was only 1 seismogramme on the volcano and that hadn't worked for almost 2 years!

Its eruptions tend to follow a similar pattern to those of St.Helens, with an initial central vent eruption followed by an explosive phase and large scale tephra ejection which may lead to any of the following: lava flows, pyroclastic and mudflows, lahars and lava dome growth. The graphic below gives a comparion of the eruptive history of Cascades volcanoes over the last 4,000 years.

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The volcano is still active and will erupt again. There are active hot springs on the flanks and high on the north and south sides of the volcano there are fresh looking dacite domes surrounded by warm ground areas which are snow free.

So there you have it, a relative unknow quantity compared to its better known counterpart, Mount St Helens but potentially just as dangerous.

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Posted
  • Location: Ponteland
  • Location: Ponteland

I hate to say this SC but until now I had never heard of Glacier Peaks-I guess that makes it pretty near Mount Ranier. On that subject could you do your next " focus an a volcano" on Mount Ranier if at all possible. Thanks Mike.

Edited by Rollo
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Posted
  • Location: Sheffield South Yorkshire 160M Powering the Sheffield Shield
  • Weather Preferences: Any Extreme
  • Location: Sheffield South Yorkshire 160M Powering the Sheffield Shield

For some reason I was thinking of Lassen peak when I read this.

Anyway this bit is interesting.

A little more than 13,000 years ago, a sequence of nine tephra eruptions occurred within a period of less than a few hundred years. Associated with these eruptions were pyroclastic flows. Mixed with snow, ice and water, these formed lahars that raced into three nearby rivers, filling their valleys with deep deposits. Subsequently, the mudflows drained into both the North Fork of the Stillaguamish River (at that time an outlet of the Sauk River) and Skagit Rivers. In Arlington, 60 miles (97 km) downstream, lahars deposited seven feet of sediment. Subsequent erosion of lahar deposits near Darrington led to the current river system with the Stillaguamish River separated from the Sauk/Skagit Rivers. Lahar debris was deposited along both the Skagit and Stillaguamish Rivers all the way to Puget Sound. A small portion of the erupted tephra was deposited locally. However, most of the tephra reached higher levels of the atmosphere, and was transported by the wind hundreds of miles. Deposits from this congregation were as thick as 1 foot (0.30 m) near Chelan and 0.3 inches (7.6 mm) near Missoula, Montana.[6]

So it can erupt frequently if the magma is there.

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Guest Shetland Coastie
I hate to say this SC but until now I had never heard of Glacier Peaks-I guess that makes it pretty near Mount Ranier. On that subject could you do your next " focus an a volcano" on Mount Ranier if at all possible. Thanks Mike.

Yes no problem, I'll do Mount Rainier next :D

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VOLCANO IN FOCUS - MOUNT RAINIER - USA

This latest in my series is by request from Rollo!

Mount Rainier, like the aforementioned Glacier Peak, is one of the Cascades Arc volcanos that lie along the Pacific Coast of the USA. It lies just some 54 miles South-East of the city of Seattle, within the boundaries of the Mount Rainier National Park and this is why Rainier is considered to be highly dangerous, but more on this later.

Rainier is a Stratovolcano which is some 14,411ft high and is capped by 26 major glaciers. Indeed it has some 35 square miles of permanent snowfields and glaciers. It has two volcanic craters at its summit , each of which is some 1000ft in diameter and the slightly larger east crater overlaps the west one. The crater rims are kept free of snow and ice by geothermal heat, athough down in the craters themselves there is a small crater lake and the worlds largest glacier cave network.

The volcano has a broad summit plateau with three named summits. The highest called Colombia Crest, the next highest is Point Success and then slightly lower down, Liberty Cap. On most days the volcano can be clearly seen from Seattle where most residents tend to refer it simply as "The Mountain." On exceptionally clear days it can be seen as far away as Portland, Oregon and Victoria, British Colombia. The picture below shows well the view from Seattle and the map (right) shows the location of the National Park, showing clearly its proximity to populated areas.

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The present cone of Rainier is estimated to be some 500,000 years old, however, extensive lava flows dating back as far as 840,000 years ago suggest a smaller 'proto-Rainier' was in existence before that. It is also believed that Rainier actually stood much higher at some 16,000ft but a major debris avalanche some 5,000 years ago hurled the top 1,600 ft or so down the nearby river valleys. This caused what is known as the Osceola Mudflow, which reached as far as modern day Tacoma and south-eastern Seattle. This is important to know because Rainier has shown itself to be rather prone to these major, devastating debris avalanches, 3 having occured within the last 6,000 years and this is by far the biggest threat to life in the vicinity of the mountain, although, of course, being a volcano its not the only one! Rainier has also historically produced lava flows, tephra falls and pyroclastic surges.

The last 'recorded' eruption was sometime between 1824 and 1850, although eye witness accounts suggest 1894 as the last eruption. Like its Cascades brethren, Rainiers eruptions tend to be infrequent but mightily powerful when they do happen. The biggest threat to life, as previously stated, is from lahars and mudlfows. Due to the large number of glaciers, there is, of course, a huge amount of ice on Rainier and furthermore, the structure of the mountain itself is being constantly weakened by hydrothermal activity. A repeat of the Osceola mudflow cannot, therefore, be ruled out, with all the devastation that that would ential for this populated area. It has been estimated that such a mudflow on a similar scale would completely destroy the towns of Enumclaw, Kent, Auburn and Renton and possibly reach the south eastern suburbs of Seattle itself via the Duwamish Estuary. This in turn would likely trigger tsunami in Puget Sound and Lake Washington. According to USGS figures, some 150,000 people currently live on top of old lahar deposits from the volcano. Another picture of Mount Rainier below and also a map of the likely path of any lahar/debris flows from the mountain.

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As a result of this, in 1998 USGS began the task of installing a lahar warning system for towns in the likely path of a huge debris avalanche in the Puyallup River Valley. This has now been completed and is now run and monitored by the local authority. The city of Tacoma (pop almost 200,000) lies at the mouth of the Puyallup River and is only some 37miles west of Rainier.

There is currently no immediate threat of an eruption from Mount Rainier (indeed all the Cascades volcanos are currently at alert state GREEN) but the mountain is still considered as active and will erupt again at some point in the future. There are ongoing signs of the volcanos inner heat with occasional steam explosions near the summit and many steam vents, fumaroles and hot rocks within the summit area.

Edited by Shetland Coastie
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Posted
  • Location: portsmouth uk
  • Weather Preferences: extremes
  • Location: portsmouth uk

yes indeed there all brillant post thankyou.

there is one in alaska thats becoming more active and has a pending eruption,

id love to know more about this one as it was on orange alert.

and any other pending eruptions would be great cheers. :) B)

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VOLCANO IN FOCUS - WHITE ISLAND - NEW ZEALAND

New Zealand is geologically very active, particularly the North Island which has several active volcanos which have produced some of the worlds largest eruptions (especially Taupo some 26,500 years ago which is a dormant but potentially active 'supervolcano').

White Island is New Zealands only marine volcano, lying some 48km east of North Island in the Bay of Plenty. It is part of a group of 4 islands called the Olive Islands and the nearest mainland towns are Whakatane and Tauranga. The island measures some 2 x 2.4km and the summit is just 321m above sea level, however, this doesn't tell the whole story as this is just the tip of a submarine volcano some 1600m high. It was named White Island by Captain Cook in 1769 as the volcano always seemed to be blanketed in white clouds (although Cook didn't realise it was in fact a volcano). Its Maori name is Te Puia o Whaakaari which means ' Dramatic Volcano.' The maps below show the islands location and its topography:

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From about 1880 onwards, numerous attempts were made to mine sulphur on the island but this brought dramatically to a halt in 1914. Part of the western crater wall collapsed causing a large lahar which destoyed the mining camp and all 10 of the miners disappeared without trace. Indeed the only survivor was the camp cat, Peter The Great! Mining was attempted again in 1923 but was finally abandoned in 1930 as the small amounts of sulhpur were not economically viable. The picture below shows an abandoned mining camp which has now been heavily corroded by the volcanic gases.

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White Islands eruptions are frequently explosive, producing great steam clouds, ash and tephra, as well as lava flows, lahars and pyroclastic flows. The island is continually at alert state 1 (on a scale of 0-5 indicating ongoing volcanic unrest) and there is ongoing activity including fumaroles, steaming and boiling mud. The island is heavily monitored and attracts scientists from all around the world. At one time the island had an extensive Pohutakawa forest (a tree of the Myrtle family) but this was destroyed in several explosive eruptions between 1981-83. The last major eruption of the island was in 2001. Although White Island is at least 16,000 years old (and probably older) since 1976 it has been more active than it has ever been, suggesting that it is going through an upswing in its eruptive activity. No eruptions thus far have been large enough to threaten the Bay of Plenty coast, however, recent scientific investigation has shown that White Island does have the potential for large scale eruptions. A couple of shots of White Island below, one from overhead and another showing one of the perpetual steam plumes:

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The reason for the suspicion of the potential for large eruptions is because the activity at White Island is being driven by a large magma chamber deep below the volcano. This magma is sufficient to continually produce the enormous volumes of steam and volcanic gases seen from the volcano over a sustained period of time. These gases percolate rapidly up the volcanic conduits where they meet shallow groundwater just below the surface, which produces the almost constant steam. These conduits can often become blocked, however, by solidified magma. This leads to a build up of gases which in turn results in explosive eruptions and these can happen at any time, with little or no warning. The volcano is also prone to large steam explosions which result in lahars and pyroclastic flows. Added to this, the crater of the volcano is fairly unstable and a large debris avalanche and crater collapse, like that which killed the miners in 1914, is likely to occur again at some point in the future. The volcano produces steam CO2, SO2 (at the rate of 400 tonnes per day) and smaller amounts of chlorine and fluorine. The picture below shows scientists collecting gas samples at White Island.

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White island is privately owned and has been made a Private Scenic Reserve which means people can only visit the island with permission, usually through authorised tour operators.

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Guest Shetland Coastie
Bob......is it possible possible to have a lahar in conjunction with a pyroclastic flow?....or would the prescence of water prevent this?

The answer is yes inasmuch as pyroclastic flows are often the source of a lahar. Lahars are often pyroclastic material mixed with water. This mixture of ash and water can flow very rapidly but once brought to a halt can also set rapidly like concrete.

A case in point being the 1980 eruption of Mount St.Helens which produced a pyroclastic flow which then melted the snow and ice on the upper slopes forming a massive lahar which was heated to 90C!

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The answer is yes inasmuch as pyroclastic flows are often the source of a lahar. Lahars are often pyroclastic material mixed with water. This mixture of ash and water can flow very rapidly but once brought to a halt can also set rapidly like concrete.

A case in point being the 1980 eruption of Mount St.Helens which produced a pyroclastic flow which then melted the snow and ice on the upper slopes forming a massive lahar which was heated to 90C!

Thanks for the reply Bob,fascinating stuff :lol:

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VOLCANO IN FOCUS - GALERAS - COLOMBIA

As Galeras has erupted the other day, I thought I would take a closer look at this dangerous volcano.

Galeras is a complex stratovolcano which lies in the North Volcanic Zone of the Andes mountain chain, in southern Colombia. Its summit is some 14,029ft high and is the most active volcano in Colombia. In the native languages it is called Urcunina. The map below shows Galeras in relation to other Colombian volcanos.

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Galeras has been active for at least 1 million years and there have been two major caldera-forming eruptions. The first occurred some 560,000 years ago, ejecting some 15 cu km of material. The second was a smaller episode between 40,000 & 150,000 years ago, which ejected some 2 cu km. The volcano has frequently been weakened by hydrothermal activty which has led to caldera wall colapses and major debris avalanches. Eruptions subsequent to the second major eruption mentioned above have constructed the present smaller cone within the horseshoe shaped caldera. The pictures below show the crater of Galeras:

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Typical eruptive behaviour for Galeras is a central vent eruption, followed by an explosive phase leading to pyroclastic surges and lahars. There have been 6 major explosive eruptions in the last 5000 years and over 20 lesser explosive eruptions since 1500. There is extensive tephra deposit and pyroclastic flow evidence all around the volcano which speak to its violent history. The chemical make-up of Galeras' magma has been very consistent over time. Mainly andesite and highly crystalline which may indicate a long-lived and very large magma chamber underneath the volcano.

Activity continues right up to the present day with the recent short-lived explosive eruption on 14th February. Indeed Galeras has had several, short, explosive eruptions in the last 3 years which have often led to the evacuation of the local population from areas around the volcano. It has been discovered that prior to an explosion, Galeras emits a low-frequency, long-period seismic signal, know as a 'tornillo event.' These signals have been observed in 4/5ths of Galeras' explosions and it is believed this is connected with the pressurisation of the magma prior to an eruption.

Galeras lies only 6 miles away from the Provincial Capital of Pasto, a city with a population approaching 500,000. This proximity and its explosive and violent history have led to Galeras being classified as a 'Decade Volcano.' It was this classification which led to perhaps its most famous recent eruption. A 'Decade Volcano' conference was being held in Pasto in 1993 when a team of scientists, led by Stanley Williams, decided to make an impromptu field trip to the volcano summit. Several tourists had also decided to do likewise. Despite there being some seismic evidence of a possible eruption, Williams proceeded with the field trip. The volcano erupted suddenly, killing 6 of the scientists (including one from the Open University in the UK) and 3 of the tourists. Williams himself survived, although he was severely wounded by lava bombs. He later wrote a book about the experience entitled "Surviving Galeras."

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Pictures above from l-r: Galeras with Pasto in the foreground; Galeras at night; cover of Stanley Williams book "Surviving Galeras"

A very active and highly dangerous volcano which, due to its proximity to populated areas, deserves to be taken very seriously.

Edited by Shetland Coastie
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Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL
VOLCANO IN FOCUS - WHITE ISLAND - NEW ZEALAND

Attached a photo I took of it from the shore. Hardly the best you'll ever see though!

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Guest Shetland Coastie

VOLCANO IN FOCUS - MOUNT EREBUS - ANTARCTICA

Mount Erebus is the most southerly of planet earths active volcanos and lies on Ross Island. It is some 3794m in height. Ross Island also contains three other extinct volcanic centres in Mount Terror, Mount Bird and Hut Point Peninsula (which is now home to the McMurdo Station).

The map below shows its location on Ross Island:

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The volcano was first discovered by Antarctic explorer Sir James Clark Ross on 27th January 1841 who named it and nearby Mount Terror after his ships. The volcano was erupting at the time. It was first climbed to the rim of the volcano by members or Sir Ernest Shackletons expedition team in 1908.

It is the most active volcano in Antarctica and is one of the few volcanos in the world to have a long-lived and persistent lava-lake. The volcano has been regularly active for some 1.3 million years. It is classified as a polygenic stratovolcano, meaning that the bottom half is a shield with the top half a cone (making it similar to Mount Etna in Sicily)

The picture below is a satellite image of the lava lake (courtesy NASA-JPL):

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Eruptive activity tends to be of the Strombolian type either from the lava lake or from several subsidiary vents around the main crater. Indeed the volcano has Strombolian eruptions from the lava lake several times a day. There are also occasional ash eruptions and the occasional lava flow, although this tends to be confined to the crater. This was not always the case though as there is ample evidence on the volcanos slopes of prodigious lava flows from some time in the past.

Due to its long-term, ongoing activity, the volcano has been intensively studied, mainly by scientists from the nearby McMurdo Station (US) and Scott Base (NZ).

Pictured below: Left - a fumarolic ice tower on Erebus & Right: lenticular clouds over Erebus

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On 28 November 1979, Air New Zealand Flight 901 crashed into Mount Erebus in whiteout conditions, killing all 257 persons on board. The flight was one of a number of Erebus sightseeing flights which had been started by Air New Zealand a couple of years earlier. The flights were stopped after this tragic accident. During the Antarctic summer snow melt, debris from this flight can clearly be seen from the air.

Edited by Shetland Coastie
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Guest Shetland Coastie

VOLCANO IN FOCUS - OL DOINYO LENGAI - TANZANIA

Ol Doinyo Lengai, which in Masai means "Mountain of the God", is a 2886m high stratovolcano which rises abrubtly from the surrounding plains. It lies in northern Tanzania, just south of Lake Natron, and is in the Eastern Rift section of the Great Rift Valley.

The map below shows its location:

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Ol Doinyo Lengai is utterly unique in that it is the only active volcano in the world to erupt natrocarbonatite lava. Most volcanic lava has a fairly high silica content but Lengais has very little silica and instead is rich in the rare sodium and potassium carbonates, nyerereite and gregoryite. These materials are unstable at the earths surface and the lava is subject to rapid weathering, turning light grey/white within hours of being erupted which can leave the crater and the slopes of the volcano looking more like the surface of the moon!

This lava erupts at much lower temperatures than is usual, often between 500-600C, making it, according to the USGS, the worlds coolest lava. This low temperature gives the lava a black, tar-like appearance with little or no red glow normally associated with lava flows. The lava also tends to be very runny with a viscosity approaching that of water and its chemical makeup is fairly similar to dishwashing liquid!

The picture below shows the tar-like appearance of the lava:

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The historical record of eruptions from Ol Doinyo Lengai goes back to 1883, with major eruptions in 1917, 1926, 1940 & 1966. An eruption which started in 2007 is currently ongoing with occasional bursts of eruptive activity. The 1940 eruption saw volcanic ash deposited as far away as the town of Loliondo, some 100km away. The eruption which began in 2007 was preceded by weeks of earthquake activity which caused near panic in Tanzania & Kenya, with the strongest quake measuring Mag 6. These quakes were felt as far away as the Kenyan capital, Nairobi.

Eruptions can be both effusive, oozing lots of very runny carbonatite lava or explosive, where the lava has a much higher silica content which reduces its viscosity and can lead to spectacular lava fountaining. This explosive activity is usually made up of ash production and the ejection of volcanic bombs from the crater. Regular eruptions since 1983 have now produced so much lava that this is now starting to overflow the main crater and run down the volcanos flanks.

The pictures below show the crater of Ol Doinyo Lengai:

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A remarkable and unique volcano.

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Guest Shetland Coastie

VOLCANO IN FOCUS - POPOCATEPETL - MEXICO

Popocatepetl is a 5426m (17,802ft) high Stratovolcano which lies some 70km south-east of the capital, Mexico City. It is the second highest volcano in North America and the 3rd highest active volcano in the Northern Hemisphere.

The name means "smoking mountain" in Aztec and is often just called "El Popo" by the locals. The name is apt, as the volcano frequently emits puffs of smoke, often accompanied by minor explosions. The summit crater is steep sided and measures some 400m x 600m. The volcano is part of the Trans-Mexican Volcanic Belt. It is joined to the nearby Iztaccihuatl volcano by a ridge called the Paso de Cortes. The map below (courtesy USGS) shows the location of the volcano, relative to Mexico City and other Mexican volcanos. The picture on the right shows Popo (left), the Paso de Cortes and Iztaccihuatl (right):

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Threvious previous cones have existed on the same site and all of these eventually collapsed due to gravity, leading to major debris avalanches. The deposits from these cover a large area to the south of the volcano. There have been at least three Plinian eruptions in historical times, the most recent in 800AD and more than 20 major eruptions since the arrival of the Spanish in 1519. The current cyle of activity, which continues to date, began with a major eruption in 1947. In 1994 Popo spewed gas and ash which carried some 25km downwind of the volcano, leading to the evacuation of some nearby towns and villages. In December 2000 an eruption again led to an evacuation, this time tens of thousands of people from near the volcano.

Some pictures of Popo below:

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The volcano can be clearly seen year-round from the city of Puebla, only 40 km away and from Mexico City, depending on atmospheric conditions. This, of course, means that the volcano presents a significant threat to both these cities if another major eruption were to occur.

Popo has been somewhat more active recently, with increased seismicity, frequent explosions and gas/emissions, over and above the normal 'smoking' of the volcano. One to keep an eye on perhaps in the coming weeks and months.

Edited by Shetland Coastie
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VOLCANO IN FOCUS - SANTORINI - GREECE

Santorini is the southernmost of the Cyclades island group, lying some 200km south east of the Greek mainland in the southern Aegean Sea. Also known as Thera, Santorini is best known for a cataclysmic eruption during Minoan times, approximately some 3,600 years ago.

The map below shows the location of Santorini:

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The modern day Santorini is what remains of the explosion at the climax of the Minoan eruption, which left the current, almost oblong caldera, surrounded by the main island of Thera, the smaller inhabited island of Therassia, two smaller uninhabited islands – Aspronisi and Palea Kameni and the emergent cone of the new volcano on the central island of Nea Kameni.

The Landsat image below left shows the caldera and the islands, going clockwise from the right - main island of Thera, Aspronisi, Therassia, left centre Palea Kameni and centre Nea Kameni. The picture below right shows modern day Santorini with the houses clinging precariously to the caldera edge!:

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Santorini lies in approximately the centre of the South Aegean Volcanic Arc, which runs for some 500km and is some 20-40km wide, extending from the Greek mainland in the west to Turkey in the east. This region first became volcanically active some 3-4 million years ago and vulcanism on Santorini began about 2 million years ago. This vulcanism is a result of the subduction of the African Plate beneath the Eurasian plate, at a rate of approximately 5cm per year in a north-easterly direction.

In the last 200,000 years, Santorini has erupted literally hundreds of times and has followed a familiar pattern of explosive shield-building eruptions culminating every 20,000 years or so in a massive, caldera-forming eruption, of which there have been 4 to date, the most recent being the infamous Minoan eruption 3,600 years ago.

The illustration below top shows the position of Santorini on the South Aegean Volcanic Arc and below bottom shows the caldera itself with its assocatied faults and vents:

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The islands of Thera, Therassia and Aspronisi are what remains of this massive eruption. The islands of Palea and Nea Kameni have been formed since then by explosive lava eruptions. There are several vents on Nea Kameni, which is still active. It last erupted in 1950 and still emits gases. There are numerous, deep faults beneath the area and magma exploits these to rise to the surface. Also of interest is the nearby sub-sea volcano of Colombus, which is currently constantly active and has been the subject of some intense scientific study. It has been suggested that Colombus may the the emerging summit of a possibly large volcano. Earthquakes of Mag 4 are frequently being recorded beneath it and it has been estimated that currently there is a 5km wide magma chamber just beneath the volcano.

The picture below is an image taken by the International Space Station in which you can clearly see lava flow deposits on Nea Kameni:

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Of course, the most famous eruption was that of Minoan times in approximately 1645BC i.e. the late Bronze Age. This eruption has variously been suggested to have caused the end of the Minoan civilisation, been the origin of the Atlantis legend and the origin of the Biblical plagues as described in Exodus. Whatever the result, it is clear that this eruption was so cataclysmic, that it made a lasting impression on the human race.

Latest scientific research from 2006 shows that the eruption was larger than initially thought. It is now thought that the eruption ejected some 61 cu km of material (it was previously thought to be 30-40 cu km) and has been rated at VEI6. Following the eruption the magma chamber beneath collapsed, enlarging the existing caldera and creating a tsunami which is believed to reached Crete, over 110km to the south.The eruption was centered on a small island just north of where Nea Kameni now lies. In the last 5,000 years, only the eruptions of Mount Taupo in New Zealand and Tambora in Indonesia have ejected more material into the atmosphere.

It is now thought that the islanders may have had some warning of the forthcoming explosion as evidence suggests there was some light ashfall from some phreatic activity some 3-4 months before the main eruption. The main eruption itself produced an eruption column some 36 km high which caused pumice fall-out between 10-600cm thick. This was followed by a second phase which produced multiple pyroclastic surges as a result of huge, phreatomagmatic explosions (caused by the hot magma coming into contact with the cold water). The third phase comprised of voluminous ash flows and the beginnings of the caldera collapse followed finally by phase 4 the major magma chamber collapse and explosion.

As a result of its violent history, likelihood of another significant eruption and proximity to populated areas. Santorini is classified as a Decade Volcano.

Edited by Shetland Coastie
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