Question: The volcano, named after the legendary King featured in Historia Regum Britanniae, last erupted when?

Knowledge 1: King Arthur
The historical basis for the King Arthur legend has long been debated by scholars. One school of thought, citing entries in the Historia Brittonum (History of the Britons) and Annales Cambriae (Welsh Annals), sees Arthur as a genuine historical figure, a Romano - British leader who fought against the invading Anglo - Saxons some time in the late 5th to early 6th century. The Historia Brittonum, a 9th - century Latin historical compilation attributed in some late manuscripts to a Welsh cleric called Nennius, contains the first datable mention of King Arthur, listing twelve battles that Arthur fought. These culminate in the Battle of Badon, where he is said to have single - handedly killed 960 men. Recent studies, however, question the reliability of the Historia Brittonum. [SEP] Knowledge 2: Volcano
Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake activity or significant new gas emissions. Most scientists consider a volcano active if it has erupted in the last 10,000 years (Holocene times) -- the Smithsonian Global Volcanism Program uses this definition of active. Most volcanoes are situated on the Pacific Ring of Fire. An estimated 500 million people live near active volcanoes.
Knowledge 1: King Arthur
The historical basis for the King Arthur legend has long been debated by scholars. One school of thought, citing entries in the Historia Brittonum (History of the Britons) and Annales Cambriae (Welsh Annals), sees Arthur as a genuine historical figure, a Romano - British leader who fought against the invading Anglo - Saxons some time in the late 5th to early 6th century. The Historia Brittonum, a 9th - century Latin historical compilation attributed in some late manuscripts to a Welsh cleric called Nennius, contains the first datable mention of King Arthur, listing twelve battles that Arthur fought. These culminate in the Battle of Badon, where he is said to have single - handedly killed 960 men. Recent studies, however, question the reliability of the Historia Brittonum. [SEP] Knowledge 2: Bank of England £5 note
Note First issued Last issued Ceased to be legal tender Colour Size Design Additional information White (1793) 1793 1944 1 March 1946 Monochrome (printed on one side only) 200 × 113 mm (may vary) White (1945) 1945 1957 13 March 1961 Monochrome (printed on one side only) 211 × 133 mm Incorporated metal thread for first time: permanent feature until the series G polymer note Series B 21 February 1957 1963 27 June 1967 Blue 158 × 90 mm Front: Helmeted Britannia Back: Lion Saint George and the Dragon, with Britannia on the front Series C 21 February 1963 1971 31 August 1973 Blue 140 × 85 mm Front: Queen Elizabeth II Back: Seated Britannia First £5 note to carry portrait of monarch Series D 11 November 1971 29 November 1991 Predominantly blue 145 × 78 mm Front: Queen Elizabeth II Back: Duke of Wellington Series E 7 June 1990 July 2002 21 November 2003 Multicoloured (predominantly turquoise - blue) 135 × 70 mm Front: Queen Elizabeth II Back: George Stephenson Notes issued from March 1993 featured the denomination symbol ``£5 ''in bolder colours Series E (variant) 21 May 2002 2016 5 May 2017 Multicoloured (predominantly green) 135 × 70 mm Front: Queen Elizabeth II Back: Elizabeth Fry Series G (polymer) 13 September 2016 Multicoloured (greeny - blue) 125 × 65 mm Front: Queen Elizabeth II Back: Winston Churchill First Bank of England note in polymer
Knowledge 1: Middle Ages
Chivalry and the ethos of courtly love developed in royal and noble courts. This culture was expressed in the vernacular languages rather than Latin, and comprised poems, stories, legends, and popular songs spread by troubadours, or wandering minstrels. Often the stories were written down in the chansons de geste, or "songs of great deeds", such as The Song of Roland or The Song of Hildebrand. Secular and religious histories were also produced. Geoffrey of Monmouth (d. c. 1155) composed his Historia Regum Britanniae, a collection of stories and legends about Arthur. Other works were more clearly history, such as Otto von Freising's (d. 1158) Gesta Friderici Imperatoris detailing the deeds of Emperor Frederick Barbarossa, or William of Malmesbury's (d. c. 1143) Gesta Regum on the kings of England. [SEP] Knowledge 2: Arthur's Seat
Like the rock on which Edinburgh Castle is built, it was formed by an extinct volcano system of Carboniferous age (lava samples have been dated at 341 to 335 million years old), which was eroded by a glacier moving from west to east during the Quaternary (approximately the last two million years), exposing rocky crags to the west and leaving a tail of material swept to the east. This is how the Salisbury Crags formed and became basalt cliffs between Arthur's Seat and the city centre. From some angles, Arthur's Seat resembles a lion couchant. Two of the several extinct vents make up the 'Lion's Head' and the 'Lion's Haunch'.

Answer: lava samples have been dated at 341 to 335 million years old