Greenland ice cores
Krakatoa, the Indonesian volcano that erupted so violently in 1883, produced many recorded effects that are frequently used as yardsticks when discussing the possible consequences of similar events in the past, particularly the second millennium BC destruction of Thera, a leading contender in the Atlantis stakes. The Krakatoan eruption had a detrimental effect on global climates for some years.
However, this was not the first time that the eruption of Krakatoa had calamitous global consequences. David Keys (image below), an archaeological journalist, details the effects of an eruption of Krakatoa in 534/5 AD, in his book, Catastrophe. This book was the subject of a documentary on the UK’s Channel Four(d). A few years before his book was published Keys wrote an article entitled: Comet may have caused catastrophe on Earth(e), in which he dismissed a volcanic eruption as the cause of the 6th century crop failures, plagues, wars, social unrest and widespread deaths, yet his subsequent book advocates[p.269] a massive eruption of Krakatoa as the culprit. Around the same time, Mike Baillie was about to publish his Exodus to Arthur  in which he argued
strongly that the mid-6th century range of catastrophes were caused by a cometary impact. Five years later, Baillie co-authored, with Patrick McCafferty, another book linking comets with Irish mythology, The Celtic Gods, in which they point out that Keys’ proposed huge eruption has not been reflected in any of the various Greenland ice cores in the form of a volcanic-acid spike[0112.164]!
This debate regarding the cause of the global catastrophes in the mid-6th century would appear to be far from over. A 2015 report(f) suggests that a series of North American volcanic eruptions in 536 AD had such a detrimental effect on the climate of Europe that it contributed to the final demise of the Roman Empire. Furthermore, there is now evidence(g) that the eruption of the El Chicon volcano in Southern Mexico around 540 AD led to the disruption of the Maya civilisation. Matthew Toohey from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, has suggested the possibility of a double event!
Early in the 19thcentury the eruption of Tambora, also in Indonesia, was even more powerful(a). However, the most violent eruption of the last two million years also took place in Indonesia 74,000 years ago, when Mt. Toba erupted with devastating consequences for the Indian sub-continent and further afield(b). The Toba caldera is now Lake Toba. A recent study suggested(c) that the Mt.Toba event led to the near extinction of humans.
The Theran eruption was equivalent to the 19th century Krakatoa event when measured according to a volcanic explosivity index (VEI), based on quantitative criteria, as discussed in Walter Friedrich’s book on Thera.*Within a decade, the explosivity figure for Thera was reassessed by Professor Floyd McCoy of the University of Hawaii, who has written and broadcast extensively on the matter of the Late Bonze Age eruption of Thera. This included a paper delivered to the 2005 Atlantis Conference. In it, he noted that “New finds of tephra – ash and pumice – both on land and on the seafloor indicate a far larger eruption than previously assumed, suggesting a volume of at least 100 km3 of tephra (bulk volume) ejected, perhaps more. Such a volume ranks the eruption on the Volcanic Explosivity Index (VEI) at 7.0, equivalent or larger than the 1815 eruption of Tambora (‘the year without a summer’), ten times larger than the eruption of Krakatau in 1883, and approximately 100 times that of Mt. St. Helens in 1980.” [629.311]*
Today, when we watch the 20th century eruption of Mt. St. Helens or the Montserrat volcanoes on our televisions, it gives no real notion of the incredible power of these events or the absolute terror that was experienced by those living close by.
*(d) https://www.davidkeys.co.uk/davids-documentaries/ (Link broken Nov. 2018)*
Immanuel Velikovsky (1895-1979) was by profession a doctor of medicine, specialising in psychiatry. However, his fame is based on being arguably the most controversial catastrophist of the 20th century. He daringly proposed that the Earth had a number of close encounters with other planetary bodies that resulted in catastrophic consequences, including interference with the rotation of our planet. He speculated that Atlantis was probably destroyed during one of these cataclysmic events.
Some have seen the influence of Ignatius Donnelly’s Ragnarok, written seventy years earlier, in Velikovsky’s cosmic collision theories. In fact a number of commentators have noted how Velikovsky seemed reluctant to credit earlier writers, such as W. C. Beaumont and Johann Radlof (1775-1846)(b), with their contributions to the development of the theory of planetary catastrophism. Rens Van Der Sluijs has written an interesting two-part paper(d)(e) listing the catastrophists who preceded Velikovsky demonstrating a certain lack of originality on his part! Others take a more critical view of his ideas(g). In 1950, he responded to this criticism with a defensive piece(n), but I consider it inadequate as he continued to ignore the work of Radlof and Beaumont.
Van Der Sluijs has written a two-part(k)(l) article on Velikovsky’s radical views regarding Venus as a comet-like body and how Aztec sources support some of his contentions.
Carl Sagan (1934-1996), was a well-known American astronomer, author and lecturer. He is considered a leading debunker of Velikovsky’s theories. He devoted much of his Broca’s Brain  to this end. Charles Ginenthal (1934-2017) produced an extensive rebuttal of Sagan’s criticisms in Carl Sagan & Immanuel Velikovsky .>However, criticism of Velikovsky continue with varying degrees of ferocity, such as that of Leroy Ellenberger, a former supporter of Velikovsky, who contends that the data from the Greenland ice cores fails to support Velikovsky(s).<
Velikovsky was initially inclined to link the disappearance of Atlantis with the eruption of Thera, but later came to support a location between the Azores and the Mid-Atlantic Ridge(i). He was an early questioner of Plato’s figure of 9,000 years for the age of Atlantis, suggesting that it was exaggerated by a factor of ten[0037.152]. ”Whatever the source of the error, the most probable date of the sinking of Atlantis would be in the middle of the second millennium, 900 years before Solon, when the earth twice suffered great catastrophes as a result of ‘the shifting of the heavenly bodies.’ These words of Plato received the least attention, though they deserved the greatest.”
Velikovsky offered intriguing evidence that on at least one occasion the early Egyptians experienced the sun rising in the west and setting in the east(q)! His other major contribution was in his questioning of the accepted Bronze Age chronologies of the eastern Mediterranean. Later writers, such as David Rohl and Peter James have built on his chronology work, while Gary Gilligan has added support for Velikovsky’s planetary theories as well.
One website(a) provides us with a considerable amount of Velikovsky’s unpublished work, while another offers an encyclopedia of his work(c). A more general German site(f), in English, is also worth a visit.
The three of Velikovsky’s most popular books as well as some of his lesser known papers are available as pdf files(j)(m).
Jan Sammer was an assistant to Velikovsky (1976-1978) and an archivist and editor for the Velikovsky Estate (1980-1983). He advises us that he was involved in the completion of Velikovsky’s unpublished book, In the Beginning, which can be read online(h).
In 2012, Laird Scranton, published The Velikovsky Heresies, in which he reviews Velikovsky’s controversial theories in the light of scientific discoveries since his death. Not unexpectedly, Scranton does find evidence that supports some of Velikovsky’s contentions.
Some readers may wish to see a video by Wallace Thornhill, of Electric Universe fame, in which he discusses Velikovsky’s Astrophysics (o). There are a number of free papers and books, including some Velivovskian material, available online(p).