Mount St Helens : 18 May 1980 Eruption

Good Afternoon!

The 40th anniversary of the catastrophic eruption of Mount St Helens was on 18 May 2020.  On 18 May 1980, 57 people are known to have been killed, most probably by the initial lateral blast, not taking into account anyone unwise enough to have ignored the exclusion zone.  In addition, significant damage was done to the surrounding area.

Geologists Keith and Dorothy Stoffel, on a photography expedition, witnessed the catastrophic failure of the northern flank of the volcano: the north side of the volcano slid down then disintegrated in a massive debris avalanche. This was followed by the horizontal blast and the vertical ash column which reached a height of 20 km within 10 minutes. The threat of being engulfed in the ash cloud forced them to return to base.

Fig 1: Mount St Helens and Spirit Lake before the May 1980 eruption. Image cropped from one by Jim Nieland – US Forest Service, Public Domain

Geological Setting

Mount St Helens lies at the front of the Cascades volcanic arc in Washington. She is the most active volcano in the arc. Her lavas are dacite, andesite / basaltic andesite, basalt / picro basalt, trachybasalt / tephrite basanite and trachyandesite / basaltic trachyandesite(1). The lavas are sourced from depths of 7 km – 14 km beneath the base of the edifice. Eruptions occurring after 1980 were fed from magma at a depth of less than 4 km. 

Volcanism here is driven by the subduction of the Juan de Fuca plate under the North American plate.  The arc starts in northern California and reaches up to British Columbia, including composite volcanoes and volcanic centres, calderas and back arc basalt shield volcanoes.  Lava compositions range from intermediate to high silica lava domes to low silica lave flows in the southern end of the arc.  The presence of low silica lavas at the southern end of the arc is attributed to extensional tectonics permitting the rise of more fluid magmas.

Seismicity in the Cascades

We downloaded the earthquakes between 39.470°N, 130.808°W and 50.870°N, 118.696°W from 1975 to 31 May 2020 to see what the subduction zone below the Cascades looks like.  There were 18,173 quakes over 2.5, of which 48 were between 6.0 and 7.0 and 4 had magnitudes in excess of 7.0. We found two areas where there was some evidence of subduction, one at the northern end of the region (north of latitude 45°N) and the other in the south (south of latitude 42°N), otherwise the area was relatively quiet in this period. 

Fig 2: Seismicity and some volcanoes of the Cascades plotted by the author. © Copyright remains with the author; all rights reserved, 2020.

In order to see the subduction zone clearly we have had to removed the “noise” from the Blanco Fracture Zone.

Fig 3: The subduction zone north of latitude 45N from the above data set, plotted by the author.  © Copyright remains with the author, all rights reserved, 2020.
Fig 4: The subduction zone south of latitude 42N from the above data set, plotted by the author.  © Copyright remains with the author, all rights reserved, 2020.

Eruptive History

Mount St Helens is a relatively young volcano, formed around 40,000 to 50,000 years ago.  The current edifice was built in the last 2,200 years.   Before the eruption Mount St Helens was conical stratovolcano made up of lava, ash, pumice and other deposits with layers of basalt and andesite.  Dacite lava domes extruded from the summit and on the northern flank at Goat Rocks. 

Mount St Helens has had 44 Holocene eruptions according to GVP(1), of which one was VEI 6, and five were VEI 5, including the 1980 eruption.  Two years prior to the1980 eruption USGS volcanologists produced a hazard assessment, predicting an eruption with the next couple of decades. 

The 18 May 1980 Eruption

A 4.2 earthquake on 20 March 1980 and increased seismic activity showed that the volcano was reawakening.  Ash eruptions followed from 27 March 1980. A harmonic tremor, indicating magma ascent, set in.  Between the initial earthquake and to the earthquake preceding the eruption on 18 May 1980 there were 228 earthquakes over 2.5 and thousands more smaller quakes.

By the end of April, a 2 km wide cryptodome (bulge) had deformed the northern flank, swelling upwards at a rate of 1 metre per day to 150 metres above the existing topography by 12 May 1980.  Ground deformation was now presenting a serious hazard.  

Fig 5   The cryptodome on the north flank 27 April 1980, cropped from an image by Peter Lipman in  – CVO Photo Archives Mount St. Helens: A General Slide Set, Public Domain

On the morning of 18 May 1980 magma had reached a level of 2 km below the edifice. The slope failure was triggered by a 5.1 earthquake on the morning of 18 May 1980 (or vice versa).  A debris avalanche of rocks, glacier ice, soil and other debris ensued, reaching Spirit Lake and the Toutle River valley and ending up east of the Camp Baker logging base, 20 km further down the valley. 

Depressurisation of the system, resulted in the violent explosive lateral blast. The blast outran the initial avalanche, felling trees in an area of 600km2 and causing the most loss of life.  The blast included heated old lava from the volcano and left a relatively thin ash layer. 

After the blast, pumice was erupted in a vertical eruption column and pyroclastic currents covered the northern side of the volcano.  Pyroclastic deposits reached a depth of 40 metres in the Upper Toutle Valley.  It is estimated that at the time of the eruption the pyroclastic flows had temperatures in the region of 700°C.  Hot ash reaching Spirit Lake caused secondary eruptions as the water flashed to stream.

During the eruption, Mount St Helens lost its pristine cone and around 400 metres height; it was left with a 1.6 km wide crater, open on the northern side.

Fig 6 The day before the eruption and a few days later.  Both images are from Johnson’s Ridge (named after the volcanologist who lost his life in the eruption) and are by Harry Glicken – USGS Cascades Volcano Observatory, Public Domain

Post Eruption

From mid-June 1980, lava emerged into the crater forming a lava dome; subsequent eruptions have been crater-based lava dome.

Could Mount St Helens produce another catastrophic eruption? 

As she has a history of VEI 5s and a VEI 6, the answer is probably, yes.  Her lava types do produce explosive eruptions, including the dacite lava domes. If any of the remaining slopes are unstable, there could be future slope failure should ascending magma cause enough deformation but it’s possible that she may have to rebuild the cone first. 

In the meantime, a risk would be to the unwary straying too near to the crater should she have a smaller e.g. phreatic eruption, which are unpredictable. Phreatic eruptions occur when ground water in the edifice heated by hot rocks or magma flashes to steam.

GVP(1) have added a commemorative feature to its website giving a lot more information, including footage of the eruption.  We recommend that you take a look; it is well worth the visit(2).

The Armchair Volcanologist

2 June 2020

References & Further Reading

  1. Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu/
  2. Mount St Helens 40th Anniversary, GVP: https://volcano.si.edu/projects/sthelens40/
  3. Mount St Helens, Wikipedia: https://en.wikipedia.org/wiki/Mount_St._Helens
  4. 1980 Eruption of Mount St Helens, Wikipedia, https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens

© Copyright remains with the author; all rights reserved, 2020

Seismic Activity in the Tjörnes Fracture Zone

Good Afternoon!

It’s back to Iceland to finish off a post I started before being diverted by the earthquake swarm in Nevada.

Having looked at the recent activity at the Reykjanes Peninsula, let’s now look at the Tjörnes Fracture Zone, where the Mid Atlantic Ridge leaves Iceland to head northwards.  Here, current seismic activity is predominantly tectonic.   Our study is based on the same data set used for the introduction to Iceland and the Reykjanes Peninsula (earthquake data downloaded from the Icelandic Meteorological Office(1) from January 2016 to April 12, 2020, updated to May 3, 2020).

Fig 1: Earthquakes in the Tjörnes Fracture Zone Region from January 2016 to April 12, 2020, plotted by the author.  NVB is the Northern Volcanic Belt.  © Copyright remains with the author, all rights reserved, 2020.

Geological Setting

The Tjörnes Fracture Zone (TFZ) is a complex area of transform and extensional faulting connecting the Kolbeinsey Ridge, the Western Volcanic Zone and the Northern Volcanic Zone.  The Kolbeinsey Ridge, itself, is slow spreading at a rate of 10mm per year.  The main faults in the area are: the EyjaFjarðaráll Rift, the Húsavík-Flatey Fault (the TFZ, itself), the Grímsey Oblique Rift and the Dalvik Fault.  Both hydrothermal and seismic activity cluster on the faults. The Húsavík-Flatey Fault has produced earthquakes with magnitudes in the region of 7.0.

Grímsey is an inhabited island on the Arctic Circle. Its main industries are fishing and tourism(2).

Fig 2: Grímsey Cliffs.  Cropped from an image by MosheA, published under CC BY-SA 2.5

Flatey is a small island in Skjálfandi Bay in northern Iceland. It is inhabited in the summer for the tourist season, being home to puffins, terns whimbrels and plovers, amongst others(3).

According to GVP(4) a submarine eruption occurred in 1868 on the Manareyjar Ridge, north of Manareyjar Island, at the south eastern end of the system; the lavas were basalt / picro basalt.  A submarine eruption or dyke intrusion in 1999 caused an earthquake swarm 180km north of Grimsey and 100km north of Kolbeinsey Island on the Southern Kolbeinsey Ridge.  Volcanic activity occurred in 1372 and 1755, but its whereabouts is unclear.

Seismic Activity

In the period from January 2016 to May 3, 2020, there were 26,762 earthquakes reported by the Icelandic Meteorological Office (IMO)(2) for the region. 131 earthquakes had a magnitude of 3.0 or more; 67 occurred in month 26 (February 2018) on the Skajálfandadujúp Rift, 52 miles ENE of Grímsey, the largest of which was 5.21M.

Fig 3: Earthquakes in the Tjörnes Fracture Zone Region, February 2018, plotted by the author.  Black stars denote earthquakes of magnitude 3.0 or more; blue triangles are the approximate locations of volcanoes or volcanic islands. © Copyright remains with the author, all rights reserved, 2020.

A depth plot of the February 2018 swarm shows that most earthquakes over 3.0M occur in the lithosphere.

Fig 4: Depth v Longitude plot of earthquakes April 2018. Green dots denote earthquakes less than 2.0M, yellow circles denote earthquakes over 2.0M; red stars denote earthquakes over 3.0M; blue triangles are the approximate locations of volcanoes or volcanic islands.  © Copyright remains with the author, all rights reserved, 2020.

According to IMO, these swarms have occurred before; the most recent being in May & September 1969, December 1980, September 1988 and April 2013.  The data for most of the earlier swarms is not publicly available on IMO’s website, but we can get data for the April 2013 swarm.  In that swarm, there were 84 earthquakes with a magnitude of 3 or more; the largest of which had a magnitude of 5.37.

Fig 5: Earthquakes in the Tjörnes Fracture Zone Region April 2013, plotted by the author.  Black stars denote earthquakes of magnitude 3.0 or more; blue triangles are the approximate locations of volcanoes or volcanic islands. © Copyright remains with the author, all rights reserved, 2020.

A depth plot of this swarm shows that most earthquakes over 3.0M also occurred in the lithosphere.

Fig 6: Depth v Longitude plot of earthquakes in the April 2013 swarm. Yellow circles denote earthquakes over 2.0M; red stars denote earthquakes over 3.0M; the blue triangle is the approximate location of Grímsey. © Copyright remains with the author, all rights reserved, 2020

How does this compare to the activity on the Reykjanes Peninsula? 

In the Tjörnes Fracture Zone, most seismic activity is occurring in the lithosphere. There is no reported volcanic activity associated with the two swarms we looked. 

Apart from the recent large swarm, the Reykjanes Peninsula shows much less activity in the same period; again, most activity was in the lithosphere. The recent swarm, itself, was atypical (still ongoing at the time of writing, but at a reduced rate) and accompanied by ground uplift – hence the increased monitoring put in place there.

I am not Icelandic so apologies for any typos in Icelandic names.

The Armchair Volcanologist

29 May 2020

© Copyright remains with the author; all rights reserved, 2020

References & Further Reading:

  1. Icelandic Meteorological Office: https://en.vedur.is
  2. Grímsey: https://en.wikipedia.org/wiki/Grímsey
  3. Flatey, Skjálfandi: https://en.wikipedia.org/wiki/Flatey,_Skjálfandi
  4. Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu
  5. “Present Kinematics of the Tjörnes Fracture Zone North Iceland, from campaign and continuous GPS measurements”, Sabrina Metzger, Sigurjón Jónsson, Gillis Danielsen, Sigrún Hreinsdóttir, François Jouanne,Domenico Giardini, Thierry Villemin, Geophysical Journal International, Volume 192, Issue 2, 1 February 2013, Pages 441–455, https://doi.org/10.1093/gji/ggs032

Credits:

Raw earthquake data downloaded from the Icelandic Met Office: https://en.vedur.is

Plots are the author’s own work.

Wobbles in the Walker Lane Deformation Belt

Good Afternoon,

An earthquake of magnitude of 6.5, 56 km west of Tonopah, Nevada, started an earthquake swarm on 15 May 2020; at the time of writing there had been 1,032 quakes.  Last year there was a large swarm to the south in the Eastern California Sheer Zone.  This prompted me to take a look at earthquake activity in both California and Nevada.

Geoscatter plot of the Tonopah swarm, May 2020
Fig 1: Latitude v Longitude plot of Nevada swarm 15 May to 21 May 2020 by the author. © Copyright remains with the author; all rights reserved, 2020

The May 2020 swarm occurred in the Walker Lane Deformation Belt.  According to USGS(1), this area has produced two dozen earthquakes with magnitude over 5.0M, mostly to the west and south.  In December 1968, there was a 6.8M quake 50 km to the north; and, in January 1934, a 6.5M 40km to the north west.

Geological Setting

The North American Plate and Pacific Plates slide past each other at the San Andreas Fault(2) on the west coast of North America.  There are small plates, the Gorda Plate(3) and Juan de Fuca Plates to the north of California, believed to be the remnants of the much larger Farallon Plate which subducted under the North American Plate.

The San Andreas Fault, a transform fault, accommodates 75+% of the relative motion between the North American and Pacific Plates.  The Walker Lane Deformation Belt(4), itself, takes up between 15% to 25% of the boundary motion.

The Mendocino Fracture Zone(5) links the junction between the San Andreas Fault and Cascadia Subduction Zone to the Gorda Ridge on the western boundary of the Gorda Plate.

The Walker Lane Deformation Belt is a roughly 800km long trough, roughly aligned with the California / Nevada state border: the northern end lies at the junction of the Honey Lake Fault Zone, Warm Springs Valley Fault Zone, the Pyramid Lake Fault Zone and the southern boundaries of the Modoc Plateau and Columbia Plateau; and, the southern end lies at the intersection of Death Valley and the Garlock Fault.  

The Eastern California shear zone is a portion of the Walker Lane Deformation Belt that links Owens Valley to the San Andreas Fault.  It is an area that has produced several quakes of 7+M:  1872 Lone Pine quake in Owens Valley; 1992 Landers Earthquake; 1999 Hector Mine earthquake; and, most recently, the 2019 Ridgecrest swarm.

Seismic Activity 1975 to May 21, 2020

This swarm is not the first in the area.  If we look at seismic activity in California and Nevada, we should see the activity from the Walker Lane Deformation Belt to the San Andreas Fault.

We downloaded earthquake data between 43.860°N, 128.786°W and 33.68°N, 114.548°W with magnitude over 2.5 from USGS’ site(1).  In the plot below, we can see earthquakes clearly delineating the plate boundaries of the Gorda Plate, Pacific Plate and the North American Plate.

Fig 2: Latitude v Longitude plot of earthquakes occurring between 1975 and May 21, 2020.  Yellow stars denote earthquakes over magnitude 6.0. Plotted by the author. © Copyright remains with the author; all rights reserved, 2020

A geodensity plot of the above data set, weighted by earthquake magnitude, shows the areas of most activity:  Mammoth Lakes, an area to the west of Sacramento, and the Mendocino Fracture Zone, east of the southern boundary of the Gorda Plate.

Fig 3: Geodensity plot of earthquakes occurring between 1975 and May 21, 2020 by the author. © Copyright remains with the author; all rights reserved, 2020

We looked through the data set to find the larger earthquake swarms; we found 17 swarms (groups of earthquakes exceeding 30 per day).  With the exception of two swarms in April 1992, this was successful.  In April 1992, there appears to be two swarms occurring at the same time so we split them based on geographic location.  

The swarms correlate with the areas of activity displayed in the geodensity plot, with the exception of the area to the west of Sacramento.  The activity to the west of Sacramento has not experienced any large swarms or quakes over 6.0.

Fig 4: Earthquake swarms occurring in California and Nevada between 1975 and 21 May 2020 plotted by the author.  The swarms are colour coded from magenta as the earliest, through red, orange, yellow, green, blue and black as the latest.  Earthquakes with magnitude over 6.0 are denoted as yellow stars. © Copyright remains with the author; all rights reserved, 2020

Mammoth Lakes(6) is located near the Long Valley Caldera and Mammoth Mountain, an area of rhyolitic, rhyodacite and dacite lava domes and hot springs.  Mammoth Mountain last erupted around 700 years ago, with a small phreatic eruption, but still produces large volumes of CO2. 

Looking more closely at the area to the west of Sacramento (38.74°N, 122.9°W to 38.86°N,122.7°W), we found that it has had 2,746 earthquakes spread consistently throughout the period from 1975 to May 21, 2020, of which the maximum magnitude was 5.01.  the activity is located near Clear Lake(7), California, and the Clear Lake Volcanic Field.  The Clear Lake Volcanic Field has not erupted for thousands of years but is not wholly inactive, with volcanic type earthquakes, hot springs and seepage of volcanic gas. 

Fig 5:  Location of the activity to the west of Sacramento by the author.  Green dots represent earthquake locations. © Copyright remains with the author; all rights reserved, 2020

Although 44 years of earthquakes feels like a long period to study, this is a very short period of time geologically-speaking so any apparent trends may not be representative and, while very interesting and worthy of investigation, may not be enough for future predictions. 

So how would geologists make predictions or update risk assessments?  In addition to the type of work above, they would look at ground deformation and movement (the ground may stretch before fracturing), and, for volcanoes, volcanic gas emissions, and gravitational and temperature changes.

The Armchair Volcanologist

21 May 2020

© Copyright remains with the author; all rights reserved, 2020

References & Further Reading

Earthquake data was downloaded from USGS; the rest from Wikipedia.

  1. USGS https://earthquake.usgs.gov/
  2. San Andreas Fault, Wikipedia: https://en.wikipedia.org/wiki/San_Andreas_Fault
  3. Gorda Plate, Wikipedia: https://en.wikipedia.org/wiki/Gorda_Plate
  4. Walker Lane, Wikipedia: https://en.wikipedia.org/wiki/Walker_Lane
  5. Mendocino Fracture Zone, Wikipedia: https://en.wikipedia.org/wiki/Mendocino_Fracture_Zone
  6. Mammoth Lakes, Wikipedia: https://en.wikipedia.org/wiki/Mammoth_Lakes,_California
  7. Clear Lake, Wikipedia: https://en.wikipedia.org/wiki/Clear_Lake_(California)

Plots are the author’s own work.

Recent Seismic Activity on the Reykjanes Peninsula

Good Afternoon!

There has been a large earthquake swarm on the Reykjanes Peninsula over recent months, still ongoing at the time of writing, albeit with reduced intensity.

So let’s take a look at what’s been going on.

Fig 1: Earthquake epicentres January 2016 to April 12, 2020 plotted by the author. © All rights reserved, 2020.

Geological Setting

The Reykjanes Peninsula lies at the south west tip of Iceland on the Mid Atlantic Ridge, the boundary between the North American and Eurasian Plates. The North American Plate is moving westwards in relation to the Eurasian Plate; transform and extension faulting accommodate the relative Plate motions.  The Reykjanes Volcanic Belt lies on the Reykjanes Peninsula, comprising five north east trending volcanic systems: Reykjanes, Svartsengi, Krýsuvík, Brennisteinsfjöll and Hengill.  The volcanic systems are fissure swarms. 

Earthquakes

The line of earthquake epicentres in Fig 1 shows the path of the Mid Atlantic Ridge.  These earthquakes were extracted from the data set used to generate the plots in the earlier post introducing Iceland.  The raw earthquake data is publicly available data downloaded from the Icelandic Meteorological Office for the period January 2016 to 12 April 2020. The plot above is still data-heavy (too much data to see what is happening) so I have extracted the earthquakes by month to see where and when most activity occurred. 

In the plots there is an impression of seismic activity trending along the Peninsula from the east to the west.  This impression is born out most strongly in the activity from month 48 onwards.  Although it should be noted that the level of activity for these months is unusually high.

Fig 2: Earthquake epicentres Month 48 (December 2019) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems.  © Copyright remains with the author; all rights reserved, 2020.
Fig 3: Earthquake epicentres Month 49 (January 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems.  © Copyright remains with the author; all rights reserved, 2020.
Fig 4: Earthquake epicentres Month 50 (February 2020) plotted by the author.  Earthquakes with magnitude ≥ 3.0 are shown as black stars.  Blue triangles are the approximate location of the volcanic systems.  © Copyright remains with the author; all rights reserved, 2020.
Fig 5: Earthquake epicentres Month 51 (March 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars.  Blue triangles are the approximate location of the volcanic systems.  © Copyright remains with the author; all rights reserved, 2020.
Fig 6: Earthquake epicentres Month 52 (April 1 to April 12, 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars.  Blue triangles are the approximate location of the volcanic systems.  © Copyright remains with the author; all rights reserved, 2020.

There are several possible reasons for the east to west trending of the earthquake swarms:

  • the plates do not move smoothly past each other, so friction generates faults and earthquakes;
  • rifting is occurring to accommodate the upward motion of land further to the east, generated by the mantle plume in the vicinity of Vatnajökull; and /or,
  • magma intrusion in local volcanoes.

Let’s take a look at depth plots of earthquakes under the Reykjanes / Svartsengi area.  This shows that the earthquakes over 3 M are largely in the lithosphere.

Fig 7:  Analysis of earthquakes in the Svartsengi area by magnitude and depth by the author.  © Copyright remains with the author; all rights reserved, 2020

Plotting depth against longitude, effectively looking northwards through the swarms, also shows that most larger quakes are in the lithosphere but some over 2 M track towards the surface.  On its own, this is not enough to draw any conclusions over the likelihood of an eruption; field observations, including gas emissions and ground deformation are required to determine how close to the surface magma may be.

Fig 8:  Depth plot by the author of the earthquakes in the Reykjanes / Svartsengi area.  Green circles are earthquakes less than 2.0 M, yellow circles are earthquakes between 2.00 and 3.00 M, red stars are earthquakes over 3.00 M.  © Copyright remains with the author; all rights reserved, 2020.

These swarms were accompanied by local uplift, as shown by local GPS stations.

Fig 9:  Uplift in the vicinity of Mt Thorbjörn as shown in recent GPS plots published by IMO: https://en.vedur.is/. THOB moved south eastwards and ELDC moved westward; both showed uplift.

IMO(3) has reported that the most likely explanation for the recent swarms and uplift is a magma intrusion near Mt Thorbjörn at depth; Mt Thorbjorn is located near the Blue Lagoon, Svartsengi. As to whether or not magma will reach the surface for an eruption and where it emerges, we will have to wait and see.  In the meantime, let’s look at the recent activity in historical times.

Historic Volcanic Activity

As noted earlier, the volcanic systems are fissure swarms.  Activity is driven by rifting which enables magma to reach the surface.  The most recent onshore volcanic activity took place between 940 AD and 1340 AD; later activity has been offshore.  Onshore lavas from these eruptions tend to be tholeiitic basalts (1). The systems are still active as demonstrated by current geothermal activity.

Hengill

Fig 10:  Image cropped from one by Hansueli Krapf, published under cc licence: CC BY-SA 3.0

The Hengill volcanic system is a series of fissure vents, crater rows and small shield volcanoes, with a highest point of 803m.  It lies at the triple junction of the Reykjanes Peninsula volcanic zone, the Western volcanic zone and the South Iceland seismic zone.  The lava types are basalt / picro-basalt, andesite / basaltic andesite and rhyolite. Hengill’s lavas are more complex that those to the west, reflecting its position at the triple junction. GVP(2) lists 13 Holocene eruptions ranging between VEI 0 and VEI 2; the last known eruption was a  VEI 2 in 150 AD.

Brennisteinsfjöll

The Brennisteinsfjöll volcanic system is a series of crater rows and small shield volcanoes, with a highest point of 610m.  Its lava types are basalt / picro-basalt.  GVP(2) lists 9 Holocene eruptions ranging between VEI 0 and VEI 2. One eruption, previously attributed to Hengill, occurred during a meeting of the Icelandic parliament at Thingvellier in 1000 AD.  The most recent eruption was a VEI 2 in 1341.

Krýsuvík

Fig 11:  Image cropped from one by Reykholt, published under creative commons licence: CC BY-SA 3.0

The Krýsuvík volcanic system is a series of crater rows and small shield volcanoes, with a highest point of 360m.  Like Brennisteinsfjöll, its lava types are basalt / picro-basalt.  GVP(2) lists 11 Holocene eruptions ranging from VEI 0 to VEI 2; the most recent of which was in 1340.  The Krýsuvík Fires spanned a period between 1151 and 1188, producing 36 km2 of lava.

Reykjanes & Svartsengi

Fig 12:  Image of Svartsengi Power Station from one by Jóhann Heiðar Árnason, published under creative commons licence: CC BY-SA 3.0

The Reykjanes volcanic system is a series of crater rows and small shield volcanoes, which extends offshore and includes several small islands.  Reykjanes highest point is 140m. The Reykjaneshryggur volcanic system is a submarine system which is considered part of Reykjanes.  GVP includes Svartsengi as a crater row of Reykjanes. Reykjanes lava types are basalt / picro-basalt.   GVP(2) lists 22 Holocene eruptions ranging between VEI 0 and VEI 4, of which the most recent was a VEI 0 in 1970; the VEI 4 was in 1226.  The 1226 eruption was part of the Reykjanes Fires which started in 1210 and lasted until 1240.

The area is one that has had a period of intense volcanic activity, so the recent earthquake swarms have generated a lot of interest.

Regular updates on seismic and volcanic activity in Iceland are published by the Icelandic Meteorological Office(3). 

The Armchair Volcanologist

14 May 2020

References & Further Reading:

  1. David W Peate, Joel A. Baker, Sveinn P. Jakobsson, Tod E. Waight, Adam J. R. Kent, Nathalie V. Grassineau, Anna Cecile Skovgaard , 2009. “Historic Magmatism on the Reykjanes Peninsula”, Contrib Mineral Petrol (2009) 157:359-382
  2. Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu
  3. Icelandic Meteorological Office: https://en.vedur.is

Credits:

Raw earthquake data downloaded from the Icelandic Met Office: https://en.vedur.is

Plots are the author’s own work.

© Copyright remains with the author; all rights reserved, 2020.

A Brief Introduction to Iceland

This is a test post as a starter for my blog. I hope you find this and later posts interesting.

Iceland, home to Eyjafjalljökull and Grimsvötn, whose eruptions in 2010 and 2011, resp., disrupted European airspace, is recommended as a great place to study volcanology, with many types of volcanic activity and relatively easily accessible. It is also a great holiday destination, not that I have been privileged enough to visit.

Iceland, located between 67.2°N 23.0°W and 63.0°N 13.0°W, has an area of 103,000 km2 and a population of 364,000.  Lying on both the North American Plate and the Eurasian Plate, it is the only large surface expression of the MAR where its volcanic activity can be easily studied by field volcanologists.  Iceland, itself, was formed from magma and accretion.

The Mid Atlantic Ridge in the northern hemisphere is the boundary the North American Plate and the Eurasian Plates.  As the plates separate, the ridge widens and allows rising magma to come to the surface.  The magma rises under its own buoyancy; hotter magma is less dense than the colder surrounding rock.

In addition to the MAR, the Icelandic Hotspot, a mantle plume, contributes to both plate separation and volcanic activity. The Iceland Plateau, itself, is a large basaltic igneous province. The plume head is thought to be located in the region of the Vatnajökull icecap. 

The MAR crosses Iceland in a series of transform and extensional faults, starting at the Reykjanes Peninsula in the south west to the Tjörnes Fracture Zone in the north.  Plotting the earthquakes reported by the Icelandic Meteorological Office (IMO) from January 2016 to 12 April 2020 shows the path of the MAR. 

Fig 1.  Earthquakes from 2016 to 2020 (12/04/2020).  Circles roughly denote four of the many volcanic zones. © Copyright remains with the author; all rights reserved, 2020.

If we take a look at a three-dimensional plot of the same earthquake looking from the south, we can see that there are deeper earthquakes under the volcanic regions Myrdalsjökull and Vatnajökull. 

Fig 2. Three-dimensional plot of earthquakes from 2016 to 2020 (12/04/2020).  © Copyright remains with the author; all rights reserved, 2020

A geodensity plot of the same earthquakes, weighted by magnitude, shows most activity (yellow areas) at the western end of the Reykjanes Peninsula and on the Tjörnes Fracture Zone to the north.  Other areas of interest are Myrdalsjökull (Katla), Vatnajökull (Barðabunga) and Herðubreið.

Fig 3. Geodensity plot for the same earthquake data set as above.  © Copyright remains with the author; all rights reserved, 2020

Iceland is often touted as having every form of volcanism going but it does not have an obvious active subduction zone; eruptions tend to be associated with fissures.  Not to worry, Iceland has many active volcanoes, including: Askja, Bárðabunga, Eyjafjalljökull, Katla, Grimsvötn, Hekla and Surtsey – plenty to offer us in terms of volcanic and seismic activity.

Our plot is data-heavy (in the region of 104,000 earthquakes) so we cannot see much more without further analysis. We will look at each volcanic region in turn in later posts.

Thank you for reading this and I look forward to sharing the next post with you.

The Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2020

Sources:

All plots are the author’s own work.

Raw earthquake data: Icelandic Meteorological Office, IMO:  https://en.vedur.is/

Software used for 3D and geoplots: MatLab

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