Background and Learning Objectives

In this module, you will learn about the devastating basaltic fissure eruption of Lakagígar (Laki Fissures), also known as the Skaftar Fires eruption, which took place between June 1783 and February 1784. The sequence of events and resulting destruction was recorded in great detail by Jón Steingrímsson, a priest in a nearby village who observed the eruption as it occurred. Much of what we know of the eruption and its aftermath comes from the writings of Steingrímsson and his contemporaries, combined with modern observations of the tephra deposits. Although Lakagígar was by no means a small eruption – it produced ~15 km³ of basaltic lava and tephra – what made the eruption particularly deadly was the large mass of volcanic gases and aerosols released into the atmosphere. Some of these aerosols remained in the upper atmosphere, reflecting the sun’s radiation and causing global mean temperatures to drop, while some fell out as acid rain, destroying crops and livestock. Most of the estimated 9,000 deaths in Iceland and as many as 20,000 worldwide that have been attributed to the eruption resulted from famine and disease, which were widespread across western Europe in 1784.

This said week, and the two prior to it, more poison fell from the sky than words can describe: ash, volcanic hairs, rain full of sulfur and salt peter, all of it mixed with sand. The snouts, nostrils, and feet of livestock grazing or walking on the grass turned bright yellow and raw. All water went tepid and light blue in color and gravel slides turned gray. All the earth's plants burned, withered and turned gray, one after the another, as the fire increased and neared the settlements.
- Rev. Jón Steingrímsson, Fires of the Earth, The Laki Eruption (1783-1784)

Upon completing this module, you will be able to:

• Describe the difference between tephra and lava
• Distinguish between Strombolian, Hawaiian, and phreatomagmatic eruption styles
• Explain how mantle convection drives plate tectonics
• Explain how decompression beneath a mid-ocean ridge causes mantle melting
• Identify two ways in which volcanic gases can result in human casualties after an eruption has ended
• Calculate the total mass of volatiles released during an eruption
• Collect data and calculate standard deviation from the mean

Eruption of Ejyafjallajokull, March 27, 2010

By Boaworm (Own work) [CC BY 3.0 [1]], via Wikimedia Commons

What is due for Module 2?

This module will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this module.

Questions?

If you have any questions, please post them to our Questions? discussion forum (not e-mail) in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Lakagígar as it appears today. The fissure eruption started in June 1783 and continued until February 1784. The older Laki table mountain – seen in the background – was bisected by the fissure and lends its name to the event. The crater row visible in the foreground lies to the northeast of Laki mountain. A black tuff cone is partially visible to the right of the crater row, just in front of Laki mountain.

Credit: Maryjo Brounce, 2008

Before proceeding to the description of events at Lakagígar, please watch this video about Icelandic volcanism featuring the great David Attenborough. The footage will give you a sense of what a fire fountain emerging from a fissure looks like up close.

Video: David Attenborough on volcanoes, Filmed at Eldfell Heimaey (7:56)

Note: the description of events given below is taken from contemporary written accounts, summarized and translated from the Icelandic by Thordarson and Self (Bulletin of Volcanology, v. 55, p. 233-263, 1993).

Map of southeastern Iceland showing the extent of the Lakagígar crater row and lava flows. The fissure zone is shown in white, and the numbers correspond to the individual fissure events identified by Thordarson and Self (1993). Each fissure may host several craters. The extent of the lava flows is shown in red, and the location of the older Laki Mountain is shown in green. The dashed yellow line represents the line of sight from Prestbakki looking over Mörtunga, which based on contemporary accounts is used to locate the first fissure. Inset shows the location of the larger image on the southeastern coast of Iceland.

Credit: GoogleEarth

Weak seismic tremors were first reported in the areas around Laki Mountain and the Skafta River in mid-May, 1783. The seismic activity grew in intensity over the next several weeks, until people were so unsettled by it that they took to sleeping in tents outside of their homes. The first fissure opened at 9 AM on June 8, 1783, producing fire fountains that were visible from the nearby towns of Skaftártunga, Medalland, and Sída. By reconstructing the line of sight from Prestbakki over Mörtunga, a ranch mentioned by name in Steingrímsson’s account, it can be determined that these first fire fountains were located near Hnúta, at the southwest end of the Laki fissures. Strong earthquakes were felt again on June 9-11, and were followed by more fire fountaining a bit farther to the north (contemporary accounts say the second fires were to the north – most likely they were actually to the northeast, following the trend of the fissure). The eruptions of June 8 and June 10 produced tephra deposits up to 60 cm thick extending to the north and east, and lava flows extending southwest towards the Skaftá River Gorge. Another earthquake swarm on June 13 preceded the rupture of a third fire fountain on June 14. This event produced a significant amount of Pele’s hair, which we learned about in Module 1 [3]. Lava flows from the June 14 fissure followed a somewhat more southerly path before joining with the flows still emerging from the June 8 and June 10 fissures at the Skaftá River Gorge. An earthquake on June 23 was followed two days later by explosive fountaining that peaked on June 27-28, raining ash and tephra on nearby towns. Interaction with the shallow water table resulted in a phreatomagmatic eruption, producing a large tuff cone centered on the fourth fissure. Lava flows emerging from the mouth of the Skaftá River Gorge were more or less continuous from June 12-29, with a surge following the opening of each new fissure. The fifth and most sustained explosive event on the Laki fissures occurred between June 30 and July 25, 1783. The initiation of this stage is not well documented, but it is known that earthquakes occurred on June 30, and tephra fell on local villages July 9-10. It is believed that all of the first five fissures were fountaining during this time. A lava surge—the last to be reported—emerged from the Skaftá River Gorge July 13-14. Tephra fall and seismic activity continued intermittently through July 25. The fissure that opened during this episode bisected the western flank of the older Laki hyaloclastite mountain, from which Lakagígar takes its name.

The dark material visible in the midground is part of the tuff cone formed during the June 27-28 fissure event (Event 4), one of two phreatomagmatic deposits at Lakagígar. A small lake nearby provides evidence of a shallow water table in this area, just west of Laki Mountain. Magma-water interaction at the time of eruption drives the relatively more explosive phreatomagmatic eruption style.

Credit: Maryjo Brounce, 2008

Eruptive activity northeast of Laki mountain commenced on July 29, 1783, marked by explosive phreatomagmatic activity. The phreatomagmatic eruption lasted 2-3 days, producing a second tuff cone. Lava flowed both to the south and to the north, ultimately entering the Hverfisfljót River Gorge to form a second flow channel that would drain the lavas produced by the northeast fissures. The lava flow rate reached a maximum of 4 km/day between August 3-7, continuing at a lower rate until the next surge on September 1 (preceded by an earthquake swarm on August 23). Another earthquake swarm on September 26 was accompanied by intense volcanic activity, and both the Skaftá and Hverfisfljót rivers dried up during this time. Another fissure opened October 24-29, delivering a final surge of lava to the Hverfisfljót River Gorge. Effusive eruption continued at a diminished rate until February 7, 1874.

This image of a fissure eruption at Mauna Loa in 1984 gives us a sense of what the Hawaiian-style stages of the Laki fissure eruption might have looked like. Note the human for scale in the lower left hand corner.

Credit: J.D. Griggs, U.S. Geological Survey.

Segment of the northeast crater row at Lakagígar as it appears today. This linear row of volcanic craters probably formed during a fissure eruption similar to the one pictured above.

Credit: Maryjo Brounce, 2008

Video: Fissure eruption at Eyjafjallajokull, Iceland, in 2010 (0:47)

Basaltic liquids are characterized by low viscosities and relatively low volatile contents, such that basaltic eruptions tend to be more effusive than explosions. However, this doesn’t mean that basaltic lavas always flow quietly from the Earth. Explosive basaltic eruptions and fire fountains can send volcanic tephra tens to hundreds of meters into the air – sometimes more than a kilometer! The table below explains the eruptive styles observed at Lakagígar; it is by no means exhaustive.

Hawaiian: Hawaiian-style eruptions are characterized by continuous fountains of lava that can go on for hours or days. Such fire fountains typically form scoria or spatter cones (Pu’u O’o [4] is a classic example).

Credit: J.D. Griggs, U.S. Geological Survey

Strombolian: The defining characteristic of a strombolian eruption is that it is not continuous – short bursts of volcanic activity throw tephra tens to hundreds of meters into the air. The tephra falls back to the Earth along a parabolic trajectory that is sensitive to wind direction.

Credit: B. Chouet, U.S. Geological Survey

Phreatomagmatic: In a phreatomagmatic eruption, magma interacts with groundwater at shallow depths beneath the surface. The violent interaction between the hot magma and cold water causes steam explosions that fragment the lava and overlying solid rock layers.

Credit: D. Swanson, U.S. Geological Survey

Two types of tephra are found today at Lakagígar, providing physical evidence of the styles of volcanism during the 1783-84 eruption. The two tephra types are classified by Thordarson and Self (1993) as strombolian and phreatomagmatic. The strombolian tephra is most common and is characterized by a glassy skin covering the surface of each clast (lapillus), indicating that it was still partially molten as it was falling. These lapilli in fact could be the products of either strombolian or hawaiian style volcanism. The phreatomagmatic tephra has no glassy skin and is entirely vesiculated, indicating that it was completely fragmented upon eruption.

Strombolian tephra with Pele’s hair from the third fissure eruption at Lakagígar on June 14, 1873.

Credit: Maryjo Brounce, 2008

Video: Eruption Animation (0:19)

Iceland is unique in that it is the only place in the world where a mid-ocean ridge protrudes above sea level. Iceland straddles the Mid-Atlantic Ridge, with the North American Plate to the west and the Eurasian Plate to the east, and ~2 cm/yr relative motion in opposing directions. Much, although probably not all, of the magmatism at Iceland is a result of mantle decompression beneath this divergent plate boundary.

Location of Iceland on the Mid-Atlantic Ridge. Although there are several islands that lie close to the Mid-Atlantic Ridge, Iceland is the only part of the ~16,000 km long ridge that is exposed above sea level. Decompression of the mantle beneath the plate boundary, possibly with the help of a hot mantle plume, results in nearly constant volcanic activity on Iceland.

Credit: U.S. Geological Survey

Mantle Convection and Plate Tectonics

In Module 1, we learned that tectonic plates move across Earth’s surface relative to a more or less fixed reference frame of mantle plumes. So what are these plates, and why do they move? The Earth’s lithosphere – which consists of the crust and the rigid upper portion of the mantle – is broken up into 15 major plates, plus several micro-plates. The tectonic plates move with respect to one another – some moving apart at divergent boundaries, some coming together at convergent boundaries, and some sliding past each other at transform boundaries. The lithospheric plates ride on top of the flowing, plastic mantle asthenosphere.

Earth’s Tectonic Plates, Wikimedia Commons. Some plates, such as North America and Eurasia, move away from each other at divergent boundaries. Others, such as Nazca and South America, move toward each other at convergent boundaries. Less commonly, two plates may slide past each other at a transform boundary – the most famous example being the San Andreas fault zone, which lies between the North American and Pacific plates.

Credit: Wikimedia Commons

The geothermal gradient inside the Earth is such that temperature increases with depth. The higher temperatures at the core-mantle boundary (~2,900 km depth) relative to the lithosphere-asthenosphere boundary (~100-200 km depth) drive convection in the plastic, flowing asthenosphere. This works very similarly to water convecting in a pot that’s being heated on the stove – hot fluid is less dense and therefore it rises, while the dense cooler fluid sinks. Just remember that the “fluid” in the mantle is actually a flowing solid; it is more viscous and flows much more slowly than water in a pot – at a rate of millimeters per year. Most geoscientists agree that there is a close relationship between mantle convection and plate tectonics, although it remains unclear to what extent the convecting mantle “drags” the lithospheric plates along its surface, or if instead the sinking of lithospheric plates at subduction zones serves to initiate convection cells in the mantle. In the most general sense, we can imagine that tectonic plates move away from each other at places where the mantle is rising, and together at places where the mantle is sinking.

Mantle convection cell, Wikimedia Commons. In this generalized model, relatively cool mantle from near the lithosphere-asthenosphere boundary sinks at convergent plate boundaries (i.e., subduction zones), while relatively hot mantle from near the core-mantle boundary rises at divergent plate boundaries (e.g., mid-ocean ridges).

Credit: Wikimedia Commons

New oceanic crust is more or less continuously being formed at mid-ocean ridges, which are a type of divergent plate boundary. It was mentioned above that the North American and Eurasian plates are moving away from each other at ~2 cm/yr at Iceland. As it turns out, this is a pretty representative average spreading rate for the Mid-Atlantic Ridge, which means that if you were to fly from John F. Kennedy International Airport in New York to London’s Heathrow Airport today, the trip would be 1 meter longer than if you had taken the same flight 50 years ago!

Map of the world’s oceans showing the ages of the sea floor. Red indicates the youngest crust (<10 million years old), while purple indicates the oldest crust (160-180 million years old; older crust exists only in a small part of the eastern Mediterranean Sea). Note that the youngest crust is found at the mid-ocean ridges, where it is being formed. The relatively narrow band of young crust at the Mid-Atlantic Ridge reflects slower spreading in the Atlantic compared to the Pacific.

Credit: National Geophysical Data Center, National Oceanic and Atmospheric Administration (NOAA)

In Module 1, we learned how the mantle sometimes melts as a result of a thermal anomaly (“hot spot”) that elevates the local geotherm above the mantle solidus. A close look at the diagonal orientation of the mantle solidus on a pressure-temperature diagram suggests that the mantle should also be able to melt under the influence of a pressure anomaly. That is, a rapid decrease in pressure can cause the mantle to melt, even without an increase in temperature. We call this kind of melting adiabatic – or, more commonly, decompression melting. Decompression melting commonly occurs at divergent plate boundaries, where two tectonic plates are moving away from each other. Mid-ocean ridges are the classic example, but adiabatic melting also occurs during continental lithospheric extension and in some mantle plumes.

Adiabatic melting of the mantle, also known as decompression melting. During decompression melting, upwelling of the mantle results in a rapid decrease in pressure. Temperature may remain constant, or even decrease slightly. The local geotherm intersects the mantle solidus as the pressure decreases, resulting in partial melting. Depending on the angle at which the geotherm intersects the solidus, the degree of melting can be quite high, with as much as 40% of the mantle melting to form basaltic magma.

Credit: Winter

Iceland is unique in that many researchers believe that a mantle plume is rising up through the Mid-Atlantic Ridge here. Thus melting at Iceland probably involves both high-temperature melting due to a thermal anomaly and decompression melting related to the divergent boundary. This may explain why eruption rates are so high (a significant eruption occurs once every 2-3 years), and why it is the only place on Earth where a mid-ocean ridge is exposed above sea level. The relative travel times of seismic waves beneath Iceland (seismic waves travel more slowly through hot or partially molten materials) have been used to map a narrow anomaly extending to at least 400 km depth, which many believe to be evidence of an Icelandic mantle plume. However, it is important to keep in mind that 400 km depth is still only the uppermost part of the asthenosphere, nowhere near the 2,900 km depth of the core-mantle boundary (which is generally believed to be the origin of hot mantle plumes). Some scientists believe that the plume does extend into the deep mantle, but cannot be imaged at depth because it is so narrow that it cannot be resolved using seismic imaging techniques.

Seismic wave anomaly beneath Iceland. Seismic waves travel more slowly through hot or partially molten rock. The observed anomaly is considered by some to be evidence of a deep mantle plume beneath Iceland.

Credit: Wolfe et al., 1996

The Skaftar Fires eruption was one of the five deadliest eruptions of the past 250 years. More than 9,000 people are estimated to have died within Iceland, nearly all due to starvation and disease in the months that followed the eruption itself. This accounts for ~20% of the population of Iceland at the time of the eruption, but the devastation was not limited to the island. Approximately 10,000-20,000 additional deaths across Europe in 1784 are attributed to respiratory ailments, a severe winter, and failed crops, which very likely were exacerbated by - if not entirely a result of - the Lakagígar eruption.

The Icelandic word móðuharðindin, meaning “mist hardships”, is used to describe the difficult times following the Lakagígar eruption. Acid rain destroyed crops and about three quarters of the island’s livestock. Animals that grazed on plants or drank from rivers contaminated with volcanic fluorine developed skeletal fluorosis, making it painful to eat or move. Sulfur is a minor component of the Earth’s mantle that tends to be concentrated in basaltic magmas. When basalt erupts as lava above the surface, the large pressure decrease allows the sulfur to be released in the form of sulfur dioxide (SO₂) – the process is similar to the way carbon dioxide is released from a bottle of soda when you reduce the pressure by removing the lid. Scientists estimate that ~122 megatons of SO₂ were released during the Lakagígar eruption. Upon combining with water in the atmosphere, this would have been converted to ~200 megatons of atmospheric aerosols, which were predominantly H₂SO₄. A little less than 20% of these aerosols are released from the lava flows and remain close to the surface as a local haze, while the remaining ~80% are carried up into the lower stratosphere by the eruption column (fire fountains and phreatomagmatic events) and transported over long distances by atmospheric circulation.

Illustration showing the mass fluxes of sulfur from the 1783-84 Lakagígar eruption. About 20% of the magmatic sulfur is released directly from the surface flows, remaining concentrated locally. This local haze is what devastated the population of Iceland at the time. The remaining ~80% is carried 10-15 km up into the lower stratosphere with the explosive eruption column and subsequently transported across the sea by the polar jet stream. The effects of this distal haze were felt across Europe.

Credit: Thordarson and Self, 2003 (Journal of Geophysical Research)

Of the total aerosols that were produced during the Lakagígar eruption, almost 90% would have fallen out as acid rain (mostly over Europe), with a little over 10% remaining in the upper atmosphere and circulating for several years. Aerosols in the upper atmosphere increase the Earth’s albedo (percent of the Sun’s energy that is reflected into space before reaching the surface), resulting in global cooling, usually on the order of ~1˚C, lasting for months or years. This effect is referred to as volcanic winter, and may last right through the summer growing season and into the next year. Coming on the heels of an unusually hot summer, the winter of 1784 was one of the coldest and longest winters on record in much of Europe and North America. Crop yield was extremely poor, and thousands of people died of starvation.

Annual mean temperature in Europe and the northeastern United States during the time period 1768-1798. The arrow indicates the onset of the Lakagígar eruption. Annual temperatures are shown as deviation from the 31-year mean (DT = T_annual-T_mean), and the dashed lines show the 2s standard deviation for this time period. Note the distinct drop in annual mean temperature from 1784-1786, during which time temperatures were ~1.5˚ less than normal.

Credit: Thordarson and Self, 2003 (Journal of Geophysical Research)

In addition to being a father of the American Revolution, Ambassador to France, First Postmaster General of the United States, Governor of Pennsylvania, and inventor of the lightning rod, Franklin stove, and bifocal glasses, Benjamin Franklin was also an amateur meteorologist and oceanographer. Franklin observed that the severely cold winter of 1784 was preceded by an unusual haze, and he was the first to suggest that atmospheric contamination from volcanic eruptions might reduce the intensity of the sun’s energy reaching the Earth, resulting in winter weather so cold as to pose a hazard to society. The theory excerpted below was originally written as a letter and subsequently presented at a reading in 1784. In the letter, Franklin suggests that the “dry fog” observed in Europe and North America may have originated from Hecla (sic). Hekla, an Icelandic volcano, erupted in 1766 and not again until 1845. The eruption Franklin is referring to in his letter is almost certainly that of Lakagígar.

METEOROLOGICAL IMAGINATIONS and CONJECTURES

by BENJAMIN FRANKLIN.

During several of the summer months of the year 1783, when the effect of the sun's rays to heat the earth in these northern regions should have been greater, there existed a constant fog over all Europe, and great part of North America. This fog was of a permanent nature; it was dry, and the rays of the sun seemed to have little effect towards dissipating it, as they easily do a moist fog, arising from water. They were indeed rendered so faint in passing through it, that when collected in the focus of a burning glass they would scarce kindle brown paper. Of course, their summer effect in heating the earth was exceedingly diminished. Hence the surface was early frozen; Hence the first snows remained on it unmelted, and received continual additions. Hence the air was more chilled, and the winds more severely cold.

Hence perhaps the winter of 1783-4, was more severe, than any that had happened for many years.

The cause of this universal fog is not yet ascertained. Whether it was adventitious to this earth, and merely a smoke, proceeding from the consumption by fire of some of those great burning balls or globes which we happen to meet with in our rapid course round the fun, and which are sometimes seen to kindle and be destroyed in passing our atmosphere, and whose smoke might be attracted and retained by our earth; or whether it was the vast quantity of smoke, long continuing to issue during the summer from Hecla in Iceland, and that other volcano which arose out of the sea near that island, which smoke might be spread by various winds, over the northern part of the world, is yet uncertain . It seems however worth the enquiry, whether other hard winters, recorded in history, were preceded by similar permanent and widely extended summer fogs. Because, if found to be so, men might from such fogs conjecture the probability of succeeding hard winter, and of the damage to be expected by the breaking up of frozen rivers in the spring; and take such measures as are possible and practicable, to secure themselves and effects from the mischiefs that attended the last.

Published in Memoirs of the Literary and Philosophical Society of Manchester, 1789, (pp. 373-377). T. Cadwell in the Strand: London.

Benjamin Franklin served as the first United States Ambassador to France from 1776-1785, from which post he wrote a letter concerning his observations of unusual weather in 1784 and possible connection to volcanic activity in Iceland.

Credit: Wikimedia Commons

In Part 1 of this lab activity, you will conduct the famous "Mentos and Diet Coke eruption", under more-or-less controlled experimental conditions. In Part 2, you will make some degassing calculations of your own. First, you will calculate the amount of CO₂ released from a 2-liter bottle of Diet Coke. Then you will apply the same principles to calculate the amount of SO₂ released during the 1783-84 eruption of Lakagígar.

Mentos and Diet Coke Eruption

Credit: Michael Murphy, Wikipedia [5]

Summary

In Module 2, we learned about fissure eruptions by studying the 1783-84 eruption of Lakagígar in Iceland. Hopefully you noticed that although the type of lava erupted here - basalt - is the same type of lava erupted in Hawaii, the style of volcanism during this eruption was very different from what we typically see at Kilauea today. This basaltic fissure eruption produced fountains of lava that shot high into the air, and more importantly, emitted massive amouts of volatlie gases and aerosols into the atmosphere, with grave consequences for thousands of people living in the Northern Hemisphere. We also learned about mantle convection, mid-ocean ridges, and decompression melting. For our hands-on experiment this week, we created Diet Coke and Mentos "volcanoes" and used a little math to calculate gas emissions.

Final Tasks

Congratulations! You have completed Module 2. Please return to the Assignments table on Page 1 to be sure you have completed all tasks.

Questions?

If you have any questions, please post them to our Questions? discussion forum (not e-mail) in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.