ENSENADA, Chile — Authorities urged 2,000 people living near the Calbuco volcano to evacuate Friday after potentially devastating mudflows of volcanic debris were detected in a nearby river, the result of two huge eruptions this week that sent ash across large swaths of southern South America.

Chilean officials said the evacuations were precautionary but necessary because flows of volcanic mud, known as lahars, are capable of leveling anything in their path once in motion.

The area had been evacuated after the volcano first erupted Wednesday afternoon, but by Friday many people had begun to return home even as Calbuco continued to billow lesser ejections of smoke and ash. Authorities said the evacuees from the towns of Chamiza, Lago Chapo and Correntoso would stay at shelters in the nearby city of Puerto Montt.

The volcano, which had been dormant four decades, sent a plume of ash about 11 miles high during Wednesday’s blast. A second, spectacular outburst came early Thursday, with lightning crackling through a dark sky turned reddish orange by the explosion.

The head of the National Mining and Geology Service said Friday that the volcano’s eruptive process could last weeks and even months and warned that a third eruption was possible.

“What I can say for certain is that this process is not going to end now,” the service’s director, Rodrigo Alvarez, said. “It’s highly likely that we will have other eruptions, maybe not with the same amount of energy, but with activity that can be worrisome.”

The two mighty blasts left Ensenada a ghost town, abandoned by most of its 1,500 residents. Sitting at the foot of the volcano, the town was covered in thick soot and some roofs collapsed under the weight of the ash.

Just about 10 miles from Calbuco’s peak, Ensenada is within the official evacuation zone, and most residents complied. But about 30 refused to leave because of worries about their homes and animals.