San Jose, Apr 14 (IANS): Costa Rica closed a national park after a volcano erupted suddenly, spewing ash and cracking the crater's dome.

The Costa Rica National Parks System (SINAC) said the closure on Thursday was aimed at minimising the exposure to ash and gases that can irritate the eyes, nose and throat, and skin of people and animals, Xinhua news agency reported.

The temporary closure will also give scientists an opportunity to "carry out necessary studies and assessments" of the unusual phreatic eruption, or blast of steam mixed with volcanic material.

"We went around the active crater and what most caught our attention was that the dome is cracked," Raul Mora, a volcanologist with the University of Costa Rica, told the daily La Nacion.

"Part of what the volcano spewed was part of the dome's structure, and much of that material fell on top of the visitor centre and the mirador," he said.

OVSICORI, Costa Rica's volcano and earthquake monitoring agency, estimated the eruption lasted some 40 minutes.

Poas Volcano is located just 40 km north of the capital San Jose, in Alajuela province.

Although it is active, the period of eruptive activity ended in 1954.

The country's Turrialba volcano, located in the east, is usually the one making headlines for sending plumes of ash and smoke 4,000 metres into the sky.