Flights to and from the Spanish island were cancelled in the week as a result of the volcanic eruption, which began last Sunday, but this is the first time the airfield has been forced to close altogether.
Experts said the volcano is “entering a new explosive phase” after tons of of lava poured down the slopes and flew high into the air over the weekend.
The Canary Islands Volcanology Institute, Involcan, added the newly opened “emission vent” was to the west of the initial vent.
La Palma airport operator Aena said on Twitter the site was “inoperative due to ash accumulation”. Other airports in the Canary Islands were still operating Saturday but some airlines were suspending flights, it said.
“Cleaning tasks have started, but the situation may change at any time,” Aena told worried travellers and residents.
Workers wearing PPE cleared up the ash as travellers attempted to change their tickets to get on the next available flights.
However, the closure meant those attempting to flee the island were delayed, forcing some to attempt to book ferries to a nearby Canary Island where they could instead fly from.
“We were due to leave today but the flight has been cancelled. There are no flights to Tenerife tomorrow, the next one is on Monday so we have to stay two more nights,” Laura, 46, from Madrid, told Reuters.
She had been due to fly back to the Spanish capital via Tenerife for work.
“Volcanic surveillance measurements carried out since the beginning of the eruption recorded the highest-energy activity so far during Friday afternoon,” emergency services said.
On Friday, authorities evacuated the towns of Tajuya, Tacande de Abajo and the part of Tacande de Arriba that had not already been vacated.
Those evacuated were told earlier they would not be able to return to their homes to retrieve their belongings because of the “evolution of the volcanic emergency”.
Since it began erupting a week ago, the Cumbre Vieja volcano has spewed out thousands of tons of lava, destroyed hundreds of houses and forced the evacuation of more than 6,000 people.
No fatalities have been recorded yet.
La Palma, with a population of more than 83,000, is one of an archipelago making up the Canary Islands.
Additional reporting by agencies