SAN JUAN, Puerto Rico (AP) — Montserrat's rumbling volcano spilled superheated debris toward a populated valley on the Caribbean island's west coast, prompting authorities to order dozens of resident to evacuate.

The debris, including boulders the size of houses, has come within 4 kilometers (2.5 miles) of the populated zone, Montserrat Volcano Observatory director Paul Cole said Friday.

"The pyroclastic flows are getting farther and farther down the valley," Cole said. "They would incinerate anything in their path."

Cole estimated that fewer than 100 people are affected by the nighttime evacuation order. Islanders are allowed to visit their homes during daylight hours, when authorities can provide more warning of danger.

Authorities raised the volcano's "hazard level" to 4 on a scale of 5 on Thursday, triggering the evacuations.

The pyroclastic flows have been accompanied by dozens of small earthquakes. But Cole said the volcano displayed similar activity earlier this year and he has not noticed anything unusual.

The Soufriere Hills volcano became active in 1995 and killed 19 people when it erupted two years later, burying much of the British territory and prompting half its 12,000 inhabitants to leave.