The deserts of New Mexico hosted one of Americas most advanced pre-colonization civilizations, including the oldest surviving buildings in what is now the United States. This puzzled later arrivals since the area is a stark contrast from the lush river valleys and fertile farmlands that gave rise to great cities elsewhere. Now a partial explanation has emerged in the form of lava tubes, which apparently allowed people to survive the region’s harsh droughts.
To Spanish explorers, western New Mexico was so uninviting they called it El Malpais, the bad lands. Yet the people who lived there, known as the Ancestral Puebloans, were numerous enough to build multi-story “great houses”. To do so they must have had access to water supplies that lasted through multi-year droughts.
Exploring a lava tube in El Malpais National Monument, Professor Bogdan Onac of the University of Southern Florida found what he thinks is the explanation. Even today the tube is filled with ice – reflecting its altitude of 2,268 meters (7,500 feet) above sea level and the way the cave shape draws in cold air and forces out warmth.
Bogdan discovered charcoal deep in the tubes, which he has found dates to between 1,970 and 1,070 years old, with one further deposit around 600 years ago, long after the nearby great houses had been abandoned. He realized people brought wood they could burn into the caves and used the heat to melt ice, and must have either left the cave while the process was underway to avoid dying of smoke inhalation or carefully managed the fire size. Although Onac acknowledges there could have been religious significance to these fires, the melted water would sometimes have been all that stood between the Ancestral Puebloans and death from thirst.
In Scientific Reports, Onac reveals that the dates of the charcoal deposits match those of major droughts derived from tree rings and lake sediments elsewhere in the American West. "This discovery sheds light on one of the many human-environment interactions in the Southwest at a time when climate change forced people to find water resources in unexpected places," Onac said in a statement.
The tube where the discovery was made, known as Cave 29, is just one of 453 known in the ancient volcanic province.
As exciting as the discovery is for archaeologists, it came about for disturbing reasons – the ice is now melting faster than it ever did when people deliberately encouraged the process. “The melting cave ice under current climate conditions is both uncovering and threatening a fragile source of paleoenvironmental and archaeological evidence," Onac noted. The remnant ice is a small portion of what once filled a large portion of the cave.
Onac believes that in ordinary years the ice would have melted of its own accord, leaving pools near the entrance that the Ancestral Puebloans could drink from. During drier periods the ice would have retreated, as whatever melted was not replaced by water filtering through fissures in the rock. This would have forced those dependent on the water to travel into the cave, start fires, and carry what they melted out. Along with the charcoal, Onac found a shard of pottery. Charcoal has been found in many of the other lava tubes, but not in the abundance of Cave 29.