Beginning Monday, red-hot rock, ash, and steam have begun spewing from the volcano a half-mile into the sky. In response, Mexico's National Disaster Prevention Centre raised their alert level to stage five (out of seven). Several schools have closed and local officials have readied emergency shelters.
In general, stratovolcanos are large, steep-sided, symmetrical cones composed of "alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs," according to the US Geologic Survey.
Popocatépetl is 17,900-feet tall, glacier-clad and contains a steep-walled, 800-1,500-foot-deep crater. Its name means "smoking mountain" in Nahuatl, the region's indigenous language.
Hazards created by volcanic activity like this may include ash, lahars, and pyroclastic flows. Lahars are mudflows created by water mixing with volcanic materials like ash, while pyroclastic flows are caused by superheated gas combined with rock. Pyroclastic flows are relatively rare at Popocatépetl, however.
During the last major eruption of Popocatépetl in 2000, more than 50,000 people were evacuated.