Category Archives: Earth

Does hydraulically fracturing – or fracking – shale rock have the potential to cause methane contamination of groundwater in England?

By Chris Phoenix Clarke (the report is structured as a scientific paper as part of a final-year university study)

About the report

The report takes a personal viewpoint, targeting curious students and undergraduates, so that they might make up their own minds about fracking in England and how its environmental impact might affect drinking-water supplies.  My aim is to highlight the main protagonists and their respective opinions, along with an evaluation into the claims made and how they withstand scientific scrutiny.  The report will utilise the most relevant and comprehensive scientific evidence available and present it in such a way so that non-specialists can understand, with the hope that hearing both sides of the argument will allow for a proper understanding of the subject.

The report takes a neutral standpoint, with the goal of collating the most up-to-date scientific information on what is undoubtedly one of the most controversial topics of recent times (and yet one that is steeped in uncertainty and lack-of-understanding by the scientific community and the public alike).  Public perception is an important factor when attempting to move forward with any environmentally-sensitive industry, so it is crucial that reliable and accurate information is being offered—and this has never been more applicable than it is with fracking.



The opinion on whether fracking in England can cause contamination of drinking water falls roughly into two camps:  those that believe the dangers are minimal and with proper regulation that fracking should commence (UK government/associated departments); and those that call for a moratorium on fracking until further investigation has been carried out to determine exactly the environmental impacts and its effect on aquifers (Greenpeace, Friends of the Earth (FoE)).

There is, however, a general consensus amongst all parties involved that the most likely cause of contamination is spilled/leaking flowback water from potential well/borehole failure, rather than through the actual process of fracturing shale.  That being said, there is uncertainty—mainly through lack of scientific research—about the possibility of migration of methane towards aquifers as a consequence of shale fracturing.  The available evidence—largely from U.S. case studies—suggests that the closer drinking water wells were to active fracking sites, the higher their concentrations of dissolved methane, with samples less than 1 km away containing 6-20 times more methane than those further away1-2.  A dubious study by Cabot Gas and Oil Corporation of 1701 water wells in Pennsylvania found that concentrations were highly correlated with local topology, with higher readings in valleys than on elevated ground3.  Other results1-2, however, seem to contradict these results.

A study by the U.S. Environmental Protection Agency4 (EPA) provides the only real scientific data on spills/leaks associated with flowback water, showing that spill-rates ranged from 0.4 to 12.2 spills per 100 fracking wells in Colorado/Pennsylvania.  Out of 225 spills examined, 18 saw contaminated water reach drinking water supplies, but of this amount 74% by volume was attributed to surface-container integrity (and not well-failure).  With different, and more robust, regulation in place in England, this risk can be easily mitigated.

The British Geological Survey (BGS) has collected 155 groundwater methane samples to establish a baseline with which future fracking companies can use to monitor any variances during extractions5.  None of the samples exceed the explosive limit for methane.

Possible solutions include: invoking a temporary moratorium on fracking in England until the necessary research and revised regulations are in place; allowing it to go ahead provided baseline methane levels are known and monitored throughout for any fluctuations; and only permit sites to ‘frack’ at sufficient distance from aquifers to reduce risk of contamination.



While fracking in some form or another has been around, on a small scale at least, since the 1940s (Table.1), it hasn’t been until recently that large-scale commercial fracking of shale has become globally widespread, with particular ‘big players’ being the U.S. and China.  Indeed, according to the U.S. Energy Information Administration (EIA)6, the amount of billions of cubic feet of shale gas production in the U.S. has increased from 2.1 in 2008 to 11.4 in 2013, showing that expansion in the industry is rapidly gaining momentum.  This, in turn, makes any environmental concerns associated with groundwater that much more pressing, as any large-scale implementation – in the U.S. or elsewhere – will consequently affect a larger environmental area and a greater number of people.

Table.1 – Shale-gas history summary

The process has, so far, only been undertaken by one company in the UK7 (Cuadrilla), in 2011 near Blackpool, and although the government is “going all out for shale8, no fracking activity has since commenced.

Fracking is the extraction technique of ‘unconventional’ natural gas—predominantly methane—from shale rock.  Fractures are created in the rock by pumping highly-pressurised water, chemicals and sand down well-boreholes (figure.1) and along horizontal pipes to create fissures (the sand, or proppant, used to keep the fractures open)—allowing trapped gas to become liberated (figure.2).


Fig.1 – of hydraulic fracturing (image: Al Granberg/ProPublica)


Fig.2 – Shale gas well design (image: U.S. Department of Energy)

Concerns amongst environmental groups and the public about potential groundwater contamination as a consequence of fracking created much recent controversy, not least from residents in Pennsylvania who reportedly claimed that their proximity to fracking sites actually led to flammable tap water!9  It was speculated that methane liberated by fracking was somehow intruding into drinking water aquifers.

Due to the relative novelty of large-scale fracking operations and the inherent uncertainties associated with the possibility of methane contamination to groundwater, it is no surprise that fracking is a cause for concern as it looks to be implemented on a commercial scale in England.


Are groundwater reserves and aquifers the same thing?
Yes, both describe the same underground reservoirs of water used for drinking water supplies

What is shale?
Shale is deep, sedimentary rock, composed of very fine-grained particles that trap gas from hydrocarbons from escaping

What is the difference between conventional and unconventional gas?
Conventional gas is ‘free’ gas found flowing naturally within the spaces/pores between various rocks; unconventional gas is found trapped within the very low-permeable rock itself and can only be liberated by fracturing



Crudely put, the issue of whether fracking presents a risk to groundwater divides opinion into two camps:  on one side is the UK government who believe that the risks are minimal and that drilling should commence10, and on the other are the environmental groups like Greenpeace and Friends of the Earth (FoE) who call for a moratorium on fracking until further research has been conducted and more stringent regulations in place to mitigate pollution to drinking water aquifers11-12.

The government perceives the risks to groundwater to be more concerned with the wells themselves, rather than the actual process of fracturing the shale.  They recognise a number of hazards dealing with the design, construction and monitoring of the wells that could potentially lead to contamination of aquifers, but believe with robust regulations in place that these risks can be mitigated.  They also state that the risks are no different to well-established and accepted conventional gas drilling methods, and that “a moratorium in the UK is not justified or necessary at present”10.  It has released a number of reports on fracking through its various departments, most notably the Department of Energy & Climate Change (DECC)13, the Energy and Climate Change Committee (ECCC)10 and Public Health England (PHE)14.  It also references a joint report15 by the academic institutions of the Royal Society (RS) and the Royal Academy of Engineering (RAE).

Non-governmental organisations (NGOs) Greenpeace and FoE, whilst agreeing that groundwater contamination from the fractures themselves is unlikely, counter that the chemicals used in the fracking process, coupled with inadequate drilling regulations, still pose a considerable environmental threat  to drinking water aquifers due to well-failure.  They have also released respective reports on the issue11-12, and reference the reports by RS/RAE15 and the British Geological Society (BGS)16.

Where might they be going wrong?  One might speculate that the government’s eagerness to get moving with the industry could be fuelled by the growing energy crisis and the need to bring down costs—possibly rushing its development before the necessary research and regulations are in place.  You might also argue that the anti-fracking lobbyists are campaigning against something that the dangers of which are not yet properly confirmed or understood.

Either way there is much uncertainty surrounding what effect fracking has on groundwater, with only a handful of peer-reviewed studies having been undertaken, and it is these, along with various reports from either side, that are examined in the next section.


What is a moratorium on fracking?
A ban on all fracking activities until further notice.


The evidence

For reasons of clarity the different sources of available evidence will be evaluated individually, with comment as to how they relate to the views held by either side of the argument.

Molofsky et al (2013) conducted a study3 in Pennsylvania into methane sources in groundwater following claims of ignitable drinking water by homeowners in the vicinity.  They concluded that methane is common in the area and is “best correlated with topography and groundwater geochemistry, rather than shale-gas extraction activities”, suggesting that methane from the underlying Marcellus shale had not migrated through extensive pathways created by fracturing, and was not responsible for the methane in the drinking water.  They go on to say that “testing of 1701 water wells… shows that methane is ubiquitous in groundwater, with higher concentrations observed in valleys versus upland areas” (figure 4), implying that valley troughs are the sole cause for increased methane concentrations.

Fig.4 – Elevation map showing dissolved methane levels in 1701 water wells in Pennsylvania.  (Image: Cabot Oil and Gas Corporation)

Upon inspection, however, the objectivity and reliability of the study must be drawn into question as the paper is copyrighted to Cabot Oil and Gas Corporation and one of the contributing authors is an employee.

Osborn et al had undertaken a likewise study in 20111, but generated very different results.  They found that in active fracking areas, methane concentrations in water-wells increased with proximity to the nearest fracking site (with levels being a potential explosion hazard).  In contrast, wells more than 1km from fracking sites averaged nearly 20-times less—implying the underlying and currently-being-‘fracked’ Marcellus shale must be the source of contaminating methane.

Another similar study2 by Jackson et al (2013) found comparable results, stating that dissolved methane was discovered in 82% of samples, with concentrations averaging 6-times higher for homes less than 1km from fracking sites.  They also concluded that distance from fracking wells were the only significant data, finding that the data associated with distances to valley bottoms were not significant.

Both Osborn and Jackson’s papers were published in the respected Proceedings of the National Academy of Sciences with all contributing authors holding posts at academic institutions of varying scientific disciplines.  The papers seem to conflict with Molofsky, going against his conclusion that topography was the cause and seem to implicate fracking activities.

The Royal Society/Royal Academy of Engineering (RS/RAE) released their fracking review15 in 2012, concluding that the risk is “very low” of fractures extending from the shale rock to overlying aquifers providing that the extraction occurs at sufficient depths.  They explain that the geological nature of the intervening strata prevents this.  What is far more likely is well-failure, rather than the hydraulic fracturing of the shale itself, that can cause leaking of contaminated flowback water into the surrounding medium.  As respected academic institutions, one would rely on the findings to be accurate and free of bias.

Public Health England (PHE) in 2014 produced a very similar report14, and agrees that the process of fracking itself is unlikely to cause methane contamination and that the likely source is leakage through the borehole/well.  They advise that “caution is required when extrapolating experiences in other countries to the UK since the mode of operation, underlying geology and regulatory environment are likely to be different”.  Although a government department, PHE’s findings match very closely that of RS/RAE and appear reliable.

Both the RS/RAE and PHE suggest that effective environmental monitoring of methane in the vicinity of fracking sites is needed before, during and after extraction, and particular focus should be paid to designing and maintaining wells to ensure their integrity in order to mitigate leaks.

The British Geological Survey (BGS) compiled a specific report16 on groundwater contamination in 2012 and seemed more open with their conclusions, stating that contamination could potentially be caused by both flowback water (as a result of well-failure), and the constituents of shale gas itself (as a result of the fracturing process, known as fluid leak-off).  It has also been conducting a baseline methane study5 since 2012 to determine nominal levels of dissolved methane throughout the UK (figure.5), in order for the data to be available to fracking companies so they might monitor any variances during/after extraction.  They conclude all levels are below the explosive limit for methane (table 2).  Also a government department, but nevertheless provide comprehensive and relevant information.


Fig.5 – distribution of methane groundwater data (image: BGS)


Table.2 – Summary of the methane baseline results up to January 2015 (Image: BGS)

The U.S. Environmental Protection Agency (EPA) recognises that leaking flowback water from faulty fracking wells poses a risk of contamination to groundwater and estimates that within the states of Colorado/Pennsylvania, spill rates ranged from 0.4 to 12.2 spills per 100 wells4.  Data from 225 well spills were also collected over a six-year period, and it was found that 18 (8%) of the spills reached surface water/groundwater.  But out of the 225 cases, 74% of the total combined spill-volume was attributed to surface-container integrity and not to the wells themselves4.  It also recognises fluid leak-off as a potential, albeit unlikely, risk.

The Department of Energy and Climate Change’s (DECC) 2014 fracking report13 on water attempts to justify the process by stating “Shale gas deposits are hundreds of metres to kilometres below the surface, much deeper than groundwater.  The geology of the UK means that generally there are layers of rock above the shale rock that are impermeable and act as a barrier to contamination”, referencing the findings15 by the RS/RAE.

The Energy and Climate Change Committee’s (ECCC) 2011 fifth report10 on fracking acknowledges fluid leak-off, referencing the EPA report4 and a 1985 study17 by Glenn.  But ultimately they conclude that this is unlikely, and concur that the only real risk of methane contamination of groundwater is via poor well/borehole integrity, and can be mitigated by robust regulation.

Data presented by the EPA4 seem to support the call for a moratorium on UK fracking by Greenpeace and FoE, with the former stating in their 2013 report11as the Gulf of Mexico oil spill has shown, regulating away accidents, spills and failures has proven difficult especially because fracking involves thousands of wells”.


What is flowback fluid?
During and after extraction, a proportion of the water produced by the fracking process returns up the well and may contain a number of dissolved constituents—including liberated methane

How does fluid leak-off work?
It is theorised that fracturing shale can lead to the opening of pathways/faults that might allow the migration of methane/gasses into nearby aquifers



From the evidence evaluated it appears there are a number of clear, albeit competing, solutions to the problem

  • Invoke a temporary moratorium of fracking activities in England pending further research into the potential threats to groundwater and appropriate and revised regulations put in place to mitigate well failure
  • Continue to allow the development of the industry but make mandatory that baseline methane readings are known for every potential site and constant monitoring of these levels undertaken during and after extraction (to see if contamination is occurring as a consequence of extraction)
  • Only allow shale rock deposits to be ‘fracked’ if their proximity to aquifers is sufficiently far enough away to present little or no threat of contamination (further research would be required to establish safe distances)

So, does hydraulically fracturing shale rock have the potential to cause methane contamination of groundwater?  The answer is yes, this does appear to be the case, although there is much uncertainty as to how or to what extent.  The evidence –insofar as current limited studies have allowed – suggests that the actual process of creating fractures deep underground is an unlikely cause of either the U.S. methane results, or potential future methane contamination of English aquifers.  It has been identified, however, that the possibility cannot be ruled out due to fluid leak-off, and it is certainly here that the focus of any further research should be conducted to better understand the probability and potential magnitude of this occurring.

The main cause for possible contamination by methane is flowback fluid transiting through wells of poor design, construction and monitoring, leaking/spilling out into the surrounding medium, eventually finding its way into groundwater reserves.  There is evidence to support that this has already happened in the U.S. (albeit the little data available suggesting surface containers are mostly to blame), and although extrapolating issues in the U.S. to those in England is never an exact science, the severity of the risks demand that well integrity and adequate regulations be addressed and solutions found for each, before any large-scale implementation of fracking in England takes place.

By Chris Phoenix Clarke


Further reading

Environment Agency (EA) –  The risk assessment for fracking in the UK.  The EA is a government department responsible for water quality and resources.

British Geological Survey (BGS) –  Information about UK shale deposits and aquifers.  The BGS is a government department and the foremost scientific authority on UK geological issues.

Department of Energy and Climate Change (DECC) –  Information about fracking regulation and monitoring.  The DECC is the government department tasked with awarding permits and regulating fracking in the UK.

David E. Newton – Fracking:  A Reference Handbook.  A personal, albeit scientific, viewpoint presented as the comprehensive book on fracking.


1Osborn, G. S., Vengosh, A., Warner, R. N., Jackson, B. R. (2011) ‘Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing’, Proceedings of the National Academy of Sciences, vol. 108, no. 20, 8172-8176 [online]. Available at (accessed 29/09/15)

2Jackson, B. R., Vengosh, A., Darrah, H. T., Warner, R. N., Down, A., Poreda, J. R., Osborn, G. S., Zhao, K., Karr, D. J. (2013) ‘Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction’, Proceedings of the National Academy of Sciences, vol. 110, no. 28, 11250-11255 [online]. Available at (accessed 29/09/05)

3Molofsky, J. L., Connor, A. J., Wylie, S. A., Wagner, T., Farhat, K. S. (2013) ‘Evaluation of methane sources in groundwater in Northeastern Pennsylvania’, Groundwater, vol. 51, no. 3, pp 333-349 [online]. Available at (accessed 29/09/15)

4U.S. Environmental Protection Agency (2015) ‘Assessment of the Potential Impacts of Hydraulic Fracturing for Oil and Gas on Drinking Water Resources’, pp 17 [online]. Available at (accessed 29/09/15)

5British Geological Survey (2015) National methane baseline survey: results summary [online]. Available at (accessed 29/09/15)

6United States Energy Information Administration (2014) Shale gas production data [online]. Available at (accessed 28/05/15)

7Hellier, D. (2015) ‘Fracking: who’s who in the race to strike it rich in the UK’, Guardian, 22 August [online]. Available at (accessed 29/09/15)

8Watt, N. (2014) ‘Fracking in the UK: ‘We’re going all out for shale’ admits Cameron, Guardian, 13 January [online]. Available at (accessed 29/09/15)

9BBC (2011) ‘Bill Ely sets fire to his water supply in Pennsylvania’ 28 November [online]. Available at (accessed 29/09/15)

10Energy and Climate Change Committee (2011) Fifth Report: Shale Gas [online]. Available at (accessed 29/09/15)

11Greenpeace (2013) Fracking: What’s the Evidence? [online]. Available at (accessed 29/09/15)

12Friends of the Earth (2014) All that glitters… Is the regulation of unconventional gas and oil exploration in England really ‘gold standard’? [online]. Available at (accessed 29/09/15)

13Department of Energy & Climate Change (2014) Fracking UK shale: water [online]. Available at (accessed 29/09/15)

14Public Health England (2014) Review of the public health impacts of exposures to chemical and radioactive pollutants as a result of the shale gas extraction process [online]. Available at (accessed 29/09/15)

15Royal Society, Royal Academy of Engineering (2012) Shale gas extraction in the UK: a review of hydraulic fracturing [online]. Available at (accessed 29/09/15)

16British Geological Survey (2012) Potential groundwater impact from exploitation of shale gas in the UK [online]. Available at (accessed 29/09/15)

17Glenn, P. S. (1985) ‘Control and modelling of fluid leakoff during hydraulic fracturing’, Journal of Petroleum Tech, Vol. 37, no. 6, pp 2257-2272


Image references

Figure.1 – (2015) Hydraulic fracturing [online]. Available at (accessed 29/09/15)

Table.1 – Department of Energy & Climate Change n.d. Shale gas background note, pp 3 [online]. Available at (accessed 29/09/15)

Figure.2 – Geological Survey (2012) Potential groundwater impact from exploitation of shale gas in the UK [online]. Available at (accessed 29/09/15)

Figure.3 – Geological Survey (2012) Potential groundwater impact from exploitation of shale gas in the UK [online]. Available at (accessed 29/09/15)

Figure.4 – , J. L., Connor, A. J., Wylie, S. A., Wagner, T., Farhat, K. S. (2013) ‘Evaluation of methane sources in groundwater in Northeastern Pennsylvania’, Groundwater, vol. 51, no. 3, pp 339 [online]. Available at (accessed 29/09/15)

Figure.5 – British Geological Society (2015) National methane baseline survey of UK groundwater [online]. Available at (accessed 29/09/15)

Table.2 –  Geological Society (2015) National methane baseline survey: results summary [online]. Available at (accessed 29/09/15)



Volcano apocalypse – the 5 most notorious eruptions of all time

by Chris Phoenix Clarke

Volcanoes are pretty badass, let’s be honest.  In this particular blog I’ll be reviewing 5 of the most destructive natural events in Earth’s history, caused by 3 different volcanic mechanisms: stratovolcanoes, supervolcanoes and flood basalt eruptions.

The 'Volcano Man' G Brad Lewis photographed his friend standing in front of exploding lava at the edge of the Kilauea volcano in Hawaii.
The ‘Volcano Man’ G Brad Lewis photographed his friend standing in front of exploding lava at the edge of the Kilauea volcano in Hawaii.  Image credit: G Brad Lewis/Barcroft Media.

Everyone’s typical picture of a volcano is a tall, conic mountain with a crater at the top bellowing out ash and spewing out lava (a stratovolcano – see picture of Mount Fuji below).  Unfortunately, this is no more accurate than the stereotypical view that the English all drink tea and talk like the Queen, and Australians converse solely about barbecues and drink Fosters.  

There are many different types of volcano, ranging from the picture-postcard snow-crested peak of Mount Fuji in Japan (below) to the linear fissure vents of Iceland and Hawaii.  Some explode violently sending gas and dust high into the atmosphere, whereas some seep thick lava down gently-sloping flanks (known as shield volcanoes).  Others have enormous calderas — the area of a volcano that collapses in on itself and into the empty magma chamber following an eruption, leaving an incredibly vast, open, cauldron-like hole.  Volcanoes of this type and size are usually coined supervolcanoes, but are classified officially on their VEI rating — the Volcanic Explosivity Index — a logarithmic scale with each rise of 1 corresponding to a ten-fold increase in the amount of ejected material, with the highest rating being an 8; the lowest being a zero).

Mount Fuji is a stratovolcano that is also Japan's highest mountain, situated 100 km south of Tokyo. It last erupted in 1708.
Mount Fuji is a stratovolcano that is also Japan’s highest mountain, situated 100 km south of Tokyo. It last erupted in 1708.  Image credit: AFP/Getty Images.

Some volcanoes lie patiently dormant while others have died and gone to volcano heaven (or hell, shall we say) and are extinct.  Some volcanoes can even erupt for a million years and take centre stage in the largest mass extinction event our planet has ever known: the Permian-Triassic extinction event.  Said to have wiped out 90% of all living things, this catastrophe of 250 million years ago very nearly ended life on Earth before humans had even begun to evolve.

What follows are 5-of-the-best eruptions, so to speak, but one must remember that although fascinating, volcanoes are also furnaces of death and destruction and should be regarded equally with fear and caution as with curiosity and awe.

5) Mount Tambora, 10th April 1815.  Location: Sumbawa, Indonesia.  VEI rating: 7

Officially the largest eruption in recorded history, Mount Tambora is claimed to be the only VEI-7 in almost 2000 years.  It killed in excess of 70,000 people and caused 1816 to become known as the ‘year without a summer’ due to the effect it had on North American and European weather.  The lowering of global temperatures was such that crops failed and animals died, causing widespread starvation – the worst famine of the 19th century.

This volcanic winter, as it is known, is the reduction of global temperatures as a consequence of ash particles and sulphuric acid droplets physically blocking sunlight from reaching the surface of the Earth.

The eruption was heard over 2000 km away and the ejected material (ejecta) from the volcano measured over 160 km3 .

Mount Tambora is still active to this day, but almost reaching the 200th anniversary of the catastrophe it hasn’t shown any signs of exploding again with the same magnitude.

Aerial view of Mount Tambora. Image credit: Jialiang Gao (
Aerial view of Mount Tambora. Image credit: Jialiang Gao (

4) Yellowstone caldera, 2.1 – 0.64 million years ago.  Location:  [currently] Wyoming, United States.  VEI rating: 8

Like the pits of Hell portrayed in Dante’s Inferno, here lurks a reservoir of fire and brimstone in the heartland of the United States that has been responsible for many of the largest explosive volcanic eruptions in all of history.  I am, of course, talking about the now world-renowned Yellowstone hotspot.

90% of the planet’s volcanic activity is found at the boundaries between tectonic plates; the other 10% at hotspots.

Hotspots are areas of volcanic activity at seemingly random locations across the surface of the planet that are hypothesised to be caused by anomalously super-hot parts of the underlying mantle or particularly-thin sections of the Earth’s crust (or, indeed, both).

While the hotspots do themselves appear to drift very slowly (the mantle behaves like a highly-viscous liquid over geological time), the constant, and comparatively faster, movement of the tectonic plates which make up the crust move across the hotspots — tracing out a trail of volcanic structures above and away from them.  This is evident in island chains such as Hawaii and on land from the Yellowstone hotspot trail.  As the Pacific Plate has headed in a north-westerly direction over the last few tens-of-millions of years, the magma from the hotspot has intruded through the seafloor to build up volcanic islands that rise above sea level.

Once an island is formed it is very gradually dragged away from the hotspot by the tectonic plate until it is no longer positioned above the active area.  The process continues on and another island is made in its place — repeating again and again until an island chain starts to take shape.  The main island of Hawai’i is the newest (300,000 years old) and the oldest is the island of Kaua’i (4 million years old), but the chain actually stretches away from what is termed as the state of Hawaii all the way up to the Aleutian Trench near Russia — the eroded islands there being many tens-of-millions of years old.

On land, the Yellowstone hotspot has erupted nearly 20 times in the

The trail of calderas span several U.S. states as the North American tectonic plate moves across the hotspot. Image credit: Smithsonian National Museum of Natural History
The trail of calderas span several U.S. states as the North American tectonic plate moves across the hotspot. Image credit: Smithsonian National Museum of Natural History.

last 16.5 million years, with the trail of calderas originating at the Nevada-Oregon border, going right across Idaho and finishing at its current location at the most north-westerly tip of Wyoming — in Yellowstone National Park (if only Yogi Bear was made aware of the real nature of his surroundings!).  Fast-forward a little longer and a new caldera will undoubtedly form in Montana.

Inside the national park the Yellowstone Plateau consists of 3 calderas that date back to 2.1, 1.3 and 0.64 million years ago — the first and last of these being supereruptions, and the other a very sizeable VEI-7 (getting on twice the size of the Mount Tambora eruption of 1815).  The ejecta from the two supereruptions was approximately 2,500 km3 and over 1000 km3 , respectively, making the former the second largest VEI-8 eruption of all time.

3) Krakatoa, 26th August 1883.  Location:  Sunda Strait, Indonesia.  VEI rating: 6

A photograph of an Indonesian newspaper dated one month after the eruption, showing a drawing of Krakatoa as it was before the eruption of 1883.
A photograph of an Indonesian newspaper dated one month after the cataclysm of 1883, showing a drawing of Krakatoa as it was before the eruption.

The second, but unfortunately not last, entry on our list from the Indonesian archipelago, Krakatoa is largely regarded as the most notorious eruption in recorded history.  Having claimed the title as the loudest noise ever heard by human ears and creating the biggest tsunami wave ever seen with human eyes, it also has the rather more infamous statistic of killing approximately 120,000 people (figures of 36,000 made 130 years ago are alleged to have been grossly under-estimated) — a total that is probably the most number of human fatalities ever caused by a volcanic eruption.

Having only been a VEI-6 eruption, you might be wondering why Krakatoa was so devastating.  The truth is that no one really knows.  The favoured hypotheses suggest that some form of subsidence or landslide, either above ground or submarine, allowed the mixing of sea water with the volcano’s magma chamber causing a highly energetic phreatic explosion.  Either way, two thirds of the island disappeared overnight as a result of the cataclysm.

Such was the ferocity of the main explosion that sailors in the Sunda Strait had their ear drums shattered, and the noise could be heard as far away as Australia (over 3000 km away) — a distance that is comparable to travelling between London and Moscow!

The pressure wave caused barometers the world over to go crazy and reverberated around the world 7 times; the explosion having had the energy of 2 hundred million tonnes of TNT (or 200 megatonnes) making it 12,500 times more powerful than the bomb dropped on Hiroshima and 4 times as powerful as the largest nuclear device ever detonated (the Russian Tsar Bombe in 1950).

The great 9.0 Japanese earthquake of 2011 generated tsunami waves of 10 metres. Image credit: Mainichi Shimbun/Reuters
The great 9.0 Japanese earthquake of 2011 generated tsunami waves of 10 metres. Image credit: Mainichi Shimbun/Reuters.

But the two most deadly features of the Krakatoa explosion were the tsunamis and pyroclastic flows that followed.  A pyroclastic flow is a superheated, fast-moving cloud of noxious gas and dust that incinerates and suffocates as it propagates away from a volcano, sometimes for many tens, or even hundreds, of miles.  Indeed, the one generated by Krakatoa traversed 40 km of ocean on a cushion of heated air and inundated flabbergasted natives on the island of Sumatra, killing over 1000 people.

The tsunami surges reached their peak at 40 metres — some 3 to 4 times higher than the Boxing Day tsunami of 2001 and Japanese tsunami of 2011.  Tidal gauges registered increased levels as far away as the English Channel (although some scientists claim this was due to the globally circumnavigating pressure wave).  It was the sheer immensity of the flooding that caused over 90% of the deaths from the catastrophe.

Never before has humankind played witness to oceanic destruction on such a colossal scale.  The sight must have been truly terrifying.  It is therefore understandable that Krakatoa makes it into the top 3 eruptions of all time.

2) Lake Toba,  70,000 b.c.  Location:  Sumatra, Indonesia.  VEI rating: 8

The eruption at Lake Toba (also situated in Indonesia) is widely recognised as the largest volcanic event on Earth in the last 25 million years and the largest VEI-8 of all time.  Considered supervolcanic due to its VEI rating, the explosion ejected approximately 3000 km3 of material into the atmosphere, blanketing most of southern Asia in 15 cm of ash and causing a volcanic winter that lasted for nearly a decade.  Moreover, it is postulated that global temperatures didn’t recover fully for a further 1000 years (there is geological evidence in ice cores of dramatic and catastrophic climate change during this period).

Landsat image of Lake Toba. The caldera lake is 100 km long and 30 km wide. Image credit: NASA.
Landsat image of Lake Toba. The caldera lake is 100 km long and 30 km wide. Image credit: NASA.

But possibly the most extreme part of Lake Toba’s CV is the suggested ‘human bottleneck’ that ensued.  Homo sapiens were only just beginning to get itchy feet by considering to venture out of Africa at the time, and along with the other hominids (including the Neanderthals) it made for a pretty meagre world population.  The Earth was experiencing a glacial period (between 110,000 – 15,000 years ago) so technologies such as agriculture were far from being realised and to put it simply: humans hadn’t been around long enough to populate to anything resembling considerable numbers.

After the Toba catastrophe it is estimated that less than 10,000 humans remained on the planet.  Global temperatures — already low due to the glacial period — had dropped by 3-5 °C courtesy of the volcanic winter, and in doing so created more planetary ice cover, which in turn raised the Earth’s albedo (a measure of how reflective to sunlight a surface is) — further compounding the problem.  

It is any wonder we managed to survive at all and quite scary to think we very nearly faced extinction as a species.  For this reason Lake Toba makes it to number 2 on our list.

Lake Toba as it looks today.
Lake Toba as it looks today.

1) The Siberian Traps, 250 million years ago.  Location:  [what is now] Siberia, Russia.  VEI rating: n/a

The Permian-Triassic mass extinction (or ‘The Great Dying’ as it is commonly referred to) is the single largest extinction event since the emergence of multicellular life on Earth.

Wiping out 90% of all living things, The Great Dying has often been attributed to a gargantuan asteroid impact (far larger than the one responsible for the death of the dinosaurs) or a volcanic eruption of almost incomprehensible enormity and duration.

The 'Great Dying' -- or Permian-Triassic extinction event -- is the most deadly mass extinction of life in Earth's history.
The ‘Great Dying’ — or Permian-Triassic extinction event — is the most deadly mass extinction in Earth’s history.

It just so happens that an enormous volcanic event did occur around the same time, in what is now known as Siberia, Russia (the continents had very different geography hundreds of millions of years ago) — creating the Siberian Traps.  Covering an area of 2 million square kilometres (the size of Western Europe) the Traps are evidence of an eruption so massive that it spewed out up to 4 million km3 of lava!  The Earth literally split open in a number of different places and the magma inside bled out for — wait for it — one million years.  That’s one million years in case you didn’t hear me the first time.

The word ‘traps’ is derived from the Swedish trappa (meaning stairs) and is an example of a flood basalt.  This type of event is the massive eruption of lava over a wide area of land or ocean that creates huge plateaus and mountain ranges — often layered in composition and forming ridged edges that resemble stairs or steps (see below).

The Siberian Traps. Layers of flood basalt cover an area over 2 million square kilometres.
The Siberian Traps. Layers of flood basalt cover an area over 2 million square kilometres.

It’s unclear why this event took place; the best hypothesis supposes that a meteorite impact triggered the enormous splits to open, and is also backed by [literally] solid evidence.  A number of candidate craters have been discovered — the best situated in east Antarctica.  The crater is 500 km wide and is still intact, suggesting it was formed in the last few hundred million years.  It would have also been roughly antipodal to the placement of the Traps, meaning it is at the opposite location on the other side of the Earth.  The theory is controversial at best but some scientists hold that an impact on one side of a planet can affect its antipodal location — as if the force propagates along a straight line through the centre of the Earth connecting the two.  Either way, an impact of this size would doubtless trigger tectonic unrest the world over, causing massive volcanic activity.

Whether initiated by colossal asteroid or gaping fissures in the crust, or a combination of both, The Great Dying very nearly extinguished all life on Earth.  The flood basalt of the Siberian Traps is inextricably linked with the worst extinction event in history and is without doubt the most massive example of volcanic activity on the planet.  It is for this reason that it tops the list of the most notorious eruptions ever; a real volcano apocalypse — evident in the cold, icy plateaus of northern Russia.