It's Turrialba's most powerful eruption in two decades, Reuters reported.
Four explosions were observed Thursday, with ash reaching the capital San Jose around 30 miles away, closing the airport there.
Emergency services officials closed access roads near the volcano, and were evacuating a radius of just over a mile around it.
"We have recommended the evacuation of people and small animals as a precaution," a spokesperson from the Volcanological and Seismological Observatory of Costa Rica said. "The last explosion was very strong."
Turrialba erupted in October, spewing magma for the first time since approximately 1863, according to Reuters.
