The Aljojuca volcano in central Mexico doesn’t have the obvious look of a volcano: It’s a giant crater with 1,200-foot cliffs and a half-mile-long deep-blue lagoon at the bottom.
Families from the adjacent town of Aljojuca — almost four hours east of Mexico City — have been coming to the lagoon for generations. Adriana Martinez, mother to 6-year-old Ximena and 3-year-old Samanta, recalled the story she was told when she herself was a child:
One day, the legend says, when Aljojuca was in the middle of a long drought, neighbors noticed a cow returned with its hooves wet from grazing, so they followed the cow and the little girl who owned it back to its pasture and discovered a small puddle.
And on that day, the legend goes, the little girl and her cow walked into the puddle, turned into a mermaid and made the puddle swell into a lagoon.
It was when Martinez grew up that she learned the more likely origin of the lagoon: it formed at the bottom of a crater that a volcanic explosion left behind.
"I like both stories," Martinez said with a shrug and a laugh.
Eight students from Northern Arizona University and the National Autonomous University of Mexico are studying this volcano and another one nearby in what is called the Serdan Oriental volcanic field. They want to make a map of what’s beneath the surface to better understand what’s above.
Earlier this month, they pounded a sledge hammer against a metal plate along hundreds of points on the volcanoes’ surfaces. They also used sophisticated magnets to measure the gravitational pull and the electrical resistance at different points.
The students used the sledge hammer to generate tiny earthquakes and send vibrations through the Earth, which hit rocks beneath and bounce back to special sensors the students placed on the surface, said Ryan Porter, an assistant professor of geophysics at Northern Arizona University. The students recorded the readings.
"Ultimately, what we’re trying to find is the plumbing of these systems and take the lessons that we learn here to apply to other systems like northern Arizona, where we have similar type of volcanism," Porter said.
Indeed, the volcanic field here shares similarities with the San Francisco volcanic field just north of Flagstaff, Porter said: The volcanoes are 20,000 years old or younger. Many of them are monogenetic volcanoes, which even though they’re the most common type of volcano in the world, relatively little is known about them. They had only one eruption cycle, which happened when magma came in contact with underground water, created steam, pressure and then an explosion.
"In the San Francisco volcanic field, a lot of them erupted out of a side, so they're asymmetrical," said Shannon Rees, a master’s student whose research focuses on the San Francisco field. "They’ll be taller on one side and the vent will be sideways."
Which helps explain why volcanoes come in so many different shapes. What most people imagine as typical volcanoes are only one kind: stratovolcanoes. Those include Mount St. Helens in Washington or Mount Ngauruhoe in New Zealand, which was used in the Lord of the Rings film trilogy.
Here in central Mexico, the students from Arizona and Mexico collected seismic data that they’ll be studying this year. It won't tell them anything about a legend or a mermaid, but they believe it’ll tell them when the volcanoes exploded, for how long and where the magma came from. With that, they hope they can better explain why the surface of these types of volcanoes looks the way it does here, in Arizona and elsewhere.