A lava fountain surged out of Mount Etna in Sicily on Thursday, sending a glowing burst of orange out of a crater and down its flank.

Dark ash from the eruption fell on towns at the foot of the volcano, and residents were seen in video footage sweeping a thick layer of dust off streets and sidewalks.

Earlier this week, activity also intensified at Stromboli, a volcano nearby in the Aeolian Islands, with huge clouds of ash and debris falling into the sea.

Salvatore Cocina, head of Sicily’s civil protection department, said that the authorities were evaluating the possibility that large amounts of lava falling into the sea from the Stromboli volcano could cause a tsunami.

He said that boats for day trippers had been halted and that the Italian Navy was ready to carry out evacuations if necessary.

Stromboli and Mount Etna are active and regularly erupt, but they are experiencing a particularly high level of activity, said Claudio Peri, an inspector with Sicily’s firefighters.

Marco Neri, an Italian volcanologist, said that the crater from which the lava fountain burst out of Etna had been dormant for about four years but began erupting about two weeks ago, culminating in the explosion on Thursday.

He added that the cloud of ash had reached nearly 15,000 feet, with winds then sending the plume over surrounding towns.

The mayor of the Sicilian port city of Catania, Enrico Trantino, on Friday banned the riding of motorcycles there for two days because of the risks related to the presence of ash, and he set a speed limit of about 20 miles an hour for cars.

The runway at the Catania airport became unusable on Friday because of the volcanic ash, the airport posted on social media, adding that arrivals and departures were suspended until at least the afternoon.