A chunk of obsidian from a prehistoric quarry, heated in the 2011 Las Conchas fire, came out as a frothy, bubbled mass with no trace of the artifact it had been. In the Jemez Mountains of northern New Mexico, fire has always been part of the landscape. But the fires of the last four decades are burning bigger and hotter than before, and they are destroying parts of the archeological record that had survived for centuries or millennia.
Figure 1. A torch-ignited surface fire spreads underneath a Ponderosa pine stand during the Pino prescribed fire in the Jemez Mountains of New Mexico, 2014. Ignition patterns were designed to maintain low intensity, short duration fire within site boundaries. U.S. Geological Survey/Rachel A. Loehman.
Obsidian from a prehistoric quarry, melted and bubbled ("vesiculated") in the 2011 Las Conchas fire. Credit: NPS/A. Steffen.
Why the fires changed
Before the 19th and 20th centuries, frequent low-severity surface fires burned through mid-elevation Ponderosa pine forests, with rarer, more severe fires higher up, often during droughts. Fire suppression and heavy livestock grazing then nearly ended surface fire, and fuels built up. Warming now amplifies drought: at Jemez Springs, records from 1914 to 2021 show mean annual temperature rising about 1.8°F per century, driven by spring and summer. Warmer air dries trees and lengthens the fire season, and days of high fire danger are projected to increase.
Four big fires
| Fire | Acres | What it taught |
|---|---|---|
| La Mesa, 1977 | ~15,500 | the first time archeologists were deployed during active fire suppression to protect sites |
| Dome, 1996 | ~16,500 | how heat penetrates buried deposits, and how fire alters obsidian quarry sites and obsidian hydration dating |
| Cerro Grande, 2000 | ~16,500 | erosion control at large pueblo sites, and the common effects on ancestral Pueblo masonry and homestead-era sites |
| Las Conchas, 2011 | ~156,000 | about 45% burned at high or moderate severity; about 2,500 documented sites were directly affected — prompting the ArcBurn project |
Direct and indirect damage
- Direct (heat): masonry cracks, spalls and discolors and walls destabilize; pottery sherds change color or decoration until their style, age or use is unreadable; stone tools crack; obsidian shatters or bubbles, and fire-fractured pieces can be mistaken for artifacts. Heat can also reset obsidian's hydration "clock," which archeologists use to date it.
- Indirect (after the fire): erosion and flooding strip away the soil that holds buried sites, jumble artifacts out of context, and cut gullies through deposits undisturbed for millennia; falling fire-killed trees tear up features.
An ancestral Puebloan masonry site after the Las Conchas fire: sooting, spalling and cracking of stone, and a destabilized wall. Credit: NPS.
ArcBurn
In the laboratory at the U.S. Forest Service's Missoula Fire Sciences Laboratory, ArcBurn exposed ceramics, obsidian, chert and masonry to simulated crown, surface and ground fires, and judged which effects were substantial — enough to lose information — rather than temporary, like soot. Surface fires did the most kinds of damage to pottery; obsidian fractured in surface fires but was little affected by smoldering ground fires; masonry cracked mainly under long, hot exposure (900°C for 90 seconds), the kind a heavy pile of fuel can also produce. Field modeling linked the severity of fire effects to topography and fuel moisture.
Protecting sites
The guidelines now used at Bandelier National Monument and Valles Caldera National Preserve include:
- thin forests to remove the fuel that built up during fire exclusion;
- avoid creating concentrated fuel "jackpots," scattering heavy logs over sites, or leaving dense beds of shredded fuel on them;
- at masonry sites, move fuels outside site boundaries to burn in piles, or scatter them thinly;
- at obsidian scatters, never pile fuel within the site, to protect dating potential;
- tailor treatments to the most common or most sensitive artifact types present.
Future fires will likely reburn areas already burned severely, and how artifacts fare after repeated exposure is still unknown.
In memory
The article honors Dr. Ana Steffen, archeologist at Valles Caldera National Preserve from its establishment in 2000, who helped define and shape the field of fire archeology, guided ArcBurn and was a leading expert on obsidian. She died on February 16, 2024.
Sources
Based on "Drought, Fire, and Archeology in the Jemez Mountains, New Mexico" by Anastasia Steffen, Jamie A. Civitello, Rachel A. Loehman and Robert R. Parmenter (National Park Service and U.S. Geological Survey), a work of the United States government in the public domain. The ArcBurn results are detailed in Loehman and others (2016). Pictures an earlier version of this page left out were restored on 2026-09-26.
In these publicationsBandelier National MonumentValles Caldera National PreserveIntermountain Park Science: Drought in the Southwest
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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