In addition to the five-mile plume, several explosions erupted in the course of the day, the Geophysics Institute said.
The Cotopaxi volcano stands 5,897 meters (19,500 feet) high and is 45 kilometres (30 miles) south of the capital Quito.
Orders have already been given for a million surgical masks to be distributed across the city of 2.3 million, Mayor Mauricio Rodas said.
The Geophysics Institute said Cotopaxi is one of the most dangerous volcanoes in the world because of the large amount of snow at its peak and because nearby areas are densely populated.
The last time it erupted was in 1877.
The government’s emergency management agency raised the alert level in areas near the volcano to yellow, which is in the middle of the scale.
The Environment Ministry declared the volcano off limits to tourists. Fifteen climbers who were preparing to scale the mountain were sent home.
One mountain guide who was in the area during the first explosion recounted tourists’ panic.
I felt “shocked, not knowing what to do as I saw everything moving. Then the strong odour of sulphur was detected across the mountain. The tourists as well were desperate, wanting to leave as soon as possible,” he told Ciudadana radio.
Authorities are monitoring the volcano closely as they decide whether to order evacuations.
However, they have already “decided to mobilise animal species in the area and move them to rescue centres,” said Lorena Tapia, head of the Environment Ministry, without going into further detail.
Airports in Quito and in the town of Cotopaxi remained open despite the plume of ash, the civil aviation directorate said.
In the town of Machachi, 25 kilometres (16 miles) from the volcano, people started wearing surgical masks.
Brian Manzano, a hotel receptionist there, said, “when I got up, I opened the door of the hotel and saw a car covered in ash. I do not know how long it has been falling.”
Cotopaxi is one of eight active volcanoes in Ecuador and is closely monitored by scientists.
Source: The Telegraph