XALITZINTLA, Mexico (AP) -- The white-capped volcano that looms over Mexico City emitted a terrifying low-pitched roar Friday and spewed roiling towers of ash and steam as it vented the pressure built up by a massive chamber of magma beneath its slopes. Authorities prepared evacuation routes, ambulances and shelters in the event of a bigger explosion.

Even a large eruption of the 17,886-foot (5,450-meter) cone of Popocatepetl is unlikely to do more than dump ash on one of the world's largest metropolitan areas. But the grit could play havoc with Mexico City's busy airport, and tens of thousands of people in the farming villages on its flanks could be forced to flee.

Popo, as it's commonly known, has put out small eruptions of ash almost daily since a round of eruptive activity began in 1994. A week ago, the eruptions started growing larger and authorities slightly elevated the alert level for people living nearby. [18 photos total]

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1
A plume of ash and steam rise from Popocatepetl volcano seen from San Andres Cholula, Mexico, Wednesday April 18, 2012. Mexico's Popocatepetl volcano is continuing to spout gases and hot rock fragments and it is dusting towns on its flanks with volcanic ash. (AP) #
2
Lights illuminate the Mexican town of Xalitzintla as the looming Popocatepetl volcano spews ash and steam during an early morning eruption, Saturday, April 21, 2012. ( AP / Marco Ugarte) #
3
A plume of ash and steam rises from Popocatepetl volcano as seen from San Nicolas, Mexico, Friday, April 20, 2012. (AP / Dieu Nalio Chery) #
4
Birds fly in the foreground as a plume of ash and steam rises from Popocatepetl volcano as seen from San Andres Cholula, Mexico on Wednesday, April 18, 2012. (AP) #
5
Mexico's Popocatepetl volcano spews super-heated rock fragments into the sky on Wednesday, April 18, 2012. (ZUMA24.com / El Universal) #
6
Guadalupe Perez tends to sheep as a tower of ash and steam rises from the Popocatepetl volcano on the outskirts of Cholula, Mexico, Monday, April 23, 2012. ( AP / Marco Ugarte) #
7
A plane flies overhead as the Popocatepetl volcano releases ash and steam during an eruption in Cholula, Mexico, Sunday, April 22, 2012. (AP / Marco Ugarte) #
8
The Popocatepetl volcano releases ash and steam during an early eruption as seen from Cholula, Mexico, Sunday, April 22, 2012. (AP / Marco Ugarte) #
9
A tower of ash and steam rise from the Popocatepetl volcano overshadowing the Catholic church Nuestra SeÒora de los Remedios or Our Lady of Remedies in Cholula, in the Mexican state of Puebla, early Sunday, April 22, 2012. (AP / Marco Ugarte) #
10
The Popocatepetl volcano southeast of Mexico City vents steam and clouds of ash and glowing red rocks on Friday, April 20, 2012. (ZUMA24.com / El Universal) #
11
A plume of ash rises from the crater of the Popocatepetl volcano seen from the town of San Nicolas de los Ranchos, Mexico, Friday April 20, 2012. (AP / Joel Merino) #
12
Families wait in a line for bottled water and face masks for protection from the ash spewed from the nearby Popocatepetl volcano in Xalitzintla, Mexico, Friday, April 20, 2012. (AP / Dieu Nalio Chery) #
13
A woman leaves a makeshift distribution center with bottled water and face masks for protection from the ash spewed from the nearby Popocatepetl volcano in Xalitzintla, Mexico, Friday, April 20, 2012. (AP / Dieu Nalio Chery) #
14
Residents stand next to an evacuation route sign in the village of Xalitzintla, Mexico, Friday, April 20, 2012. (AP / Dieu Nalio Chery) #
15
A plume of ash and steam rises from the Popocatepetl volcano as seen from Huejotzingo, Mexico, Friday April 20, 2012. ( AP / Dieu Nalio Chery) #
16
Street lights illuminate the Mexican town of Xalitzintla as the looming Popocatepetl volcano spews lava, ash and steam in the town's shadows during an eruption, early Saturday, April 21, 2012. (AP / Marco Ugarte) #
17
Popocatepetl volcano spews lava, ash and steam during an eruption as seen from from Xalitzintla, Mexico, Saturday April 21, 2012. (AP / Marco Ugarte) #
18
Popocatepetl volcano spews lava, ash and steam during an eruption as seen from from Xalitzintla, Mexico, Saturday April 21, 2012. (AP / Marco Ugarte) #

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