Vulcanologists have posted "yellow alert" warnings on highways, indicating that the mountain, one of 31 active volcanoes in the country, could explode "imminently". Hours after thousands of pupils were evacuated from schools on Tuesday, the rumblings grew dramatically louder, while wind carried sooty grey particles from the crater and dumped them on the city of 1.2 million.
"We have to sweep it up," shrugged Quito's mayor, Roque Sevilla. Ever since the volcano started spewing smoke and particles on 7 October, city dwellers consider the ash more a nuisance than a hazard. It cannot be hosed away, because once it gets wet it blocks drains as effectively as cement.
Last month communities near the Tungurahua (Throat of Fire) volcano, 78 miles from Quito, were evacuated by government order. Recently police used tear-gas to deter villagers who pressed to return to their homes to retrieve belongings. Lava from the volcano could quickly turn the nearby town of Banos into another Pompeii.
Quito is not threatened by lava flows but a big upheaval of ash and rubble from Guagua Pichincha and subsequent mudslides and earthquakes put it at considerable risk. Hugo Yepez, of the Geophysical Institute, said it "has a higher level of energy" and predicted "a major explosion" was likely.