For decades to come, Barcelona and Inter Milan must expect the preambles to every meeting to turn a little volcanic. In the strictly literal sense, that is.
From the perspective of Catalonia, it is only because of a volcano that Inter have ever been able to call themselves European champions this century. As for Inter, they certainly look back on the previous episode in this rivalry as a seismic event.
Barca host Inter on Wednesday evening, as joint leaders of Uefa Champions League Group B, for the first time since their landmark meeting in the last four of the competition in 2010.
A reminder, first, of the strange background to that semi-final: in the middle of April that year, a volcano, Eyjafallajokull, erupted in Iceland. Only a limited number of people have ever been able to pronounce its name correctly, but the effects of it belching out its clouds of ash made it very famous indeed. Air travel across much of Europe was suspended, because of the danger the debris could damage the engines of planes.
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It so happened that Barcelona, defending European champions at the time, were due at San Siro for the first leg at just that time. Grounded, they had no alternative but to cover the 450 miles by bus.
Barca lost the game 3-1, and although their then manager, Pep Guardiola, would not explicitly blame the 13-hour road trip for his players' shortcomings, the result was an upset, Inter very much the underdogs.
Come the Camp Nou leg, which the Italian club flew to, through cleared skies, things turned really volcanic, all sparks and explosions, and it is safe to report the relationship between the two managers involved, Guardiola and Inter’s Jose Mourinho, was never quite the same again.
Barca won 1-0, a goal shy of what they needed. Inter had Thiago Motta sent off with well over an hour of the contest left, and his anger at what he saw as Sergio Busquets’ simulation after Motta had challenged him with little more than a firm push was vivid.
The red card would cost Motta a place in the final, which Inter scrambled their way towards, defending deep, risking nothing, in an exhibition of concentration and caginess.
Inter's celebrations, with a fiery-eyed Mourinho conspicuous, on making it to a first European Cup final for 38 years grated with the home team. The Barca goalkeeper Victor Valdes confronted Mourinho; Barcelona’s groundstaff turned on the water-sprinklers, soaking Inter’s players as they cavorted on the pitch. In the tunnel to the dressing-rooms there were scuffles.
There will be a few involved on Wednesday night who were part of that drama: Gerard Pique, who scored the 84th minute goal and Busquets, although Lionel Messi, so frustrated by Inter eight years ago, will watch from distance. His hand injury, sustained at the weekend, rules him out of the Barca line-up.
Inter, who depended on experienced men to win their treble of 2010, have none from that team in their present ranks, although there is one Inter player who was at Camp Nou, within close range of all the fireworks.
Mauro Icardi, then 17, was sitting in the stands, along with several of his colleagues from a talented Barcelona youth team. It has been reported that some of his companions were puzzled to see Icardi react to Barca’s elimination less gloomily than they did. He was to move to Italy, to Sampdoria, the following January.
Icardi would not be the first teenager to thrive at Barcelona’s fabled La Masia academy - and he scored prolifically for the age-group teams after he had been scouted in his native Argentina - and then come up against a cul-de-sac in his progress towards the seniors.
Perhaps his timing was wrong. He looked like a deluxe target man as a young player at La Masia, and that has not been a fashionable sort of role in Barca’s pass-and-move football for much of the last 15 years.
What Icardi has turned into, since Inter signed him from Sampdoria in 2013, is much more than a penalty box predator. At 25, he is the figurehead of Inter’s drive to recover, after some lean years, the status they enjoyed back in 2010. He is their creator, galvaniser, captain, and both a great goalscorer and a scorer of great goals.
He is in form, too. Icardi has scored in both Champions League outings so far, both of them 2-1 wins coming from behind, against Tottenham Hotspur and PSV Eindhoven.
In Serie A, where Inter are on a five-match winning streak, he has four goals from his last three games, the latest the last-gasp decider in the Milan derby on Sunday. A first senior goal at the stadium he imagined, as 17-year-old, might be his permanent stage would feel just as precious.