• Systematic descriptions of the range of heating effects on a suite of

archaeological materials (obsidian, ceramic, architectural stone) across forest

types and burn severity classes, including re-burns.

• Controlled laboratory experiments to quantify heat flux associated with

archaeological fire effects.

• Site condition assessments and comparison of pre-fire and post-fire

conditions, especially in re-burn areas, and post-fire erosion hazard.

• Modeling to link fire behavior and impacts to cultural resources.

We are seeking to develop the following tools:

• CR Damage Classification and Assessment Protocol

• Vulnerability Assessment Atlas (GIS based)

• Operational tools to guide CR managers during fire suppression and BAER.

However, other observed markings included branching, Wallner mist-hackle

configuration, mist lines, and double tails (parabolas) with mist lines. These

occur only rarely in knapping and are indicative of relatively high-velocity

fracture. Tsirk concluded that fracture facets with these markings likely did not

occur in a knapping context.

High levels of stress and available energy contributed to nodule breakage. The

effects of fire can lead to high (nominal) tensile stresses inside a nodule or near

its outer portions; fractures may have occurred at heat up or cool down.

Tsirk’s analysis strongly suggests that the presence of

these features, especially mist, could be used as a readily

apparent, easily learned, visual indicator of fire fracture.

How do forest fires affect obsidian?

Literature Cited 1Steffen, A. 2005 The Dome Fire Obsidian Study: Investigating the Interaction of Heat, Hydration, & Glass

Geochemistry. Unpublished Ph.D. dissertation, Department of Anthropology, University of New Mexico,

Albuquerque.

2Macdonald, R., R. L. Smith, and J. E. Thomas 1992 Chemistry of the Subalkalic Silicic Obsidians. Professional

Paper 1523. U. S. Geological Survey, Washington D. C.

3Ambrose, W. R. 1976 Intrinsic Hydration Rate Dating of Obsidian. In Advances in Obsidian Glass Studies:

Archaeological and Geochemical Perspectives, edited by R. E. Taylor, pp. 81-105. Noyes Press, New Jersey.

4Stevenson, C. M., M. Gottesman, and M. Macko 2000 Redefining the Working Assumptions of Obsidian

Hydration Dating. Journal of California and Great Basin Anthropology 22(2):223-236.

Shattered: Direct Effects of the Las Conchas Fire at Jemez Obsidian Quarries

Anastasia Steffen, Valles Caldera Trust & University of New Mexico

Jemez volcanic glasses The Jemez Mountains are a major source for high-quality obsidians in the Southwest. There are several

geochemically-distinct Jemez obsidian deposits, including the two that burned in Las Conchas: Valles Rhyolite

at Cerro del Medio and Cerro Toledo Rhyolite (aka Obsidian Ridge, Rabbit Mountain) in the Dome area.

Not only does geochemical variation allow the geologic source of artifacts to be determined, it also can cause

different glasses to respond differently to high temperatures. However, obsidian “sourcing” relies on variation

in trace elements—essentially, the random chemical noise in a magma chamber. In contrast, vesiculation is

affected by water content, which is related to how the volcanic eruption occurred2.

•Slower lava-extruding eruptions have plenty of opportunity to de-gas and tend to have low water contents.

These have the more familiar low (<0.5%) water content and are more common.

•Rapid, explosive, pyroclastic eruptions can have higher and more variable water content. This may range as

high as 6%, but 2% or less is more usual in artifact-sized nodules.

Water content also is relevant for how obsidians hydrate. All else being equal, high-water glasses hydrate more

rapidly than low-water glasses3,4,5. If water content is variable across a source, hydration rates may vary

likewise. Measuring water content across obsidian source deposits provides important information for 1)

assessing potential response to fire, and 2) hydration tendencies.

Jemez fire history The 2012 Las Conchas Fire overlapped the fire areas of several late

20th century forest fires. In the last 15 years, nearly a third of the Jemez

Mountains have burned in forest fires. This repeat burning provides an

extraordinary opportunity to investigate the effects of multiple fire

exposure to archaeological sites and other cultural resources. Many of

these fires represented a “worst-case” at the time of each fire, with

subsequent fires providing an “even-worse-case”. Las Conchas Fire

outdid this pattern of increasing intensity.

Las Conchas burned over 150,000 acres, making it by far the largest

wildfire in NM history. The previous record-breaker was the 43,500

acres Cerro Grande fire in 2000. By comparison, the Las Conchas fire

burned 45,000 acres in the first 12 hours. Las Conchas exceeded all

prior experience.

Las Conchas Lessons Learned The high burn severity and spatial expanse of this forest fire provided an

unusual opportunity to observe effects across multiple obsidian source areas.

Results confirm that not all obsidians respond to fire in the same way and thus,

all research on obsidian fire effects must control for geochemical variation.

Further research is needed to determine the cause(s) of fire fracture.

The stunning damage caused by the Las Conchas Fire at obsidian quarries

drives home the importance of fire use and prescribed burning to decrease the

incidence of fires with this magnitude and intensity. Lesser fires do have lesser

impacts to obsidian.

VALLES CALDERA NATIONAL PRESERVE

Vesiculation

Puffy obsidian as a forest fire effect was first documented

following the 1977 La Mesa Fire, and it was then that the

term “vesiculated” was coined by Trembour6. Research1

following the 1996 Dome Fire demonstrated the link

between high water content and vesiculation, arguing that

this extraordinary fire effect was due to a tendency in the

material rather than unusual burning conditions. Past

experiments1 indicate that vesiculation in the Dome area

obsidian can occur at temperatures as low as 815°C

(1500°F).

Vesiculated obsidian is abundant across the Dome area

quarries. After the Las Conchas Fire re-burned the

same Dome quarries, the amount of vesiculated

obsidian is now 2x -5x greater than in 1996. This fire

represents the real “worst-case” burning conditions.

Las Conchas Fire Outcomes:

Fire effects differ dramatically between the two obsidian source areas

Society for American Archaeology IAOS Symposium, Obsidian Studies Across the Americas:

Alaska to Argentina and Beyond 77th Annual Meeting, Memphis

April 20, 2012

Cerro

del Medio

quarries

Dome

Area

quarries

Fire fracture causes complete destruction of obsidian

objects through breakage. Not only are objects lost, the

resulting fragments falsely mimic actual artifacts.

With training, it is possible to reliably identify

fire fracture

Fire fracture is ubiquitous across the burned quarries

on Cerro del Medio (CDM). Before the Las Conchas

Fire we had observed an abundance of fire fracture

in surface assemblages, so this fire effect was

predicted for CDM quarries. However, the extent

and magnitude of fracture following the Las

Conchas Fire has been startling.

All images of fire fractured obsidian shown here are from the Las Conchas Fire on Cerro del Medio. Fire Fracture

Unexploded nodules

The two most extreme forms of obsidian heat damage, vesiculation and

fire fracture, were common in the Las Conchas Fire.

Fracture was more common on Cerro del Medio quarries, while

vesiculation was more common in the Dome area.

The degree of direct fire effects to obsidian artifacts and nodules

depends on temperature ï and duration of heat exposure. For

vesiculation, water content plays a key role.

Previous research1 has shown that the Cerro Toledo Rhyolite obsidian

source in the Dome area is unusually high in water content (i.e., 0.3% to

1.3%) while the Valles Rhyolite obsidian source at Cerro del Medio has

more normal low water content (i.e., 0.2% to 0.4%).

Water content certainly directs the tendency for Dome area obsidians to

vesiculate, but probably has nothing to do with the tendency for the

Cerro del Medio obsidians to fracture.

No prior research has examined the causes of fire fracture.

All images of vesiculated obsidian shown here

are from the Las Conchas Fire in the Dome area.

T

Exploded nodule

Fire fracture is so pronounced at CDM quarries that it now is clear that past forest fires must have decreased

the abundance of large obsidian nodules on the surface. The counter balance is that post-fire erosion would

have been a significant agent in exposing new large nodules.

With subsequent erosion, fire fracture pieces are

rearranged and dispersed, making id less obvious.

Fractography

Partial and full vesiculation

Gull wings and

Wallner wakes

Double tails with mist-hackle lines

and regions of mist

Double tails and

mist lines

Double tail (left), hackle (center),

mist (right)

Fractographer Are Tsirk examined four specimens of fire

fracture obsidian collected following the 1996 Dome Fire.

He observed and photographed several fracture surface

markings on the fire fracture surfaces and provided a report

of his investigation7. Markings include gullwings, Wallner

wakes and other Wallner lines, twist hackles, wake hackles /

tails, hackle scars, and mist. All of these markings can occur

during knapping.

Mist and double tails

Note co-occurrence of vesiculated

nodules and fire fracture.

5Stevenson, C. M., E. Knaus, J. J. Mazer, and J. K. Bates 1993 Homogeneity of water content in obsidian

from the Coso Volcanic Field: Implication for obsidian hydration dating. Geoarchaeology 8(5):371-384.

6Trembour, F. W. 1979 Appendix F. A Hydration Study of Obsidian Artifacts, Burnt vs. Unburnt by the La

Mesa Fire, in The 1977 La Mesa Fire Study: An Investigation of Fire and Fire Suppression Impact on

Cultural Resources in Bandelier National Monument, ed.by D. Traylor, L. Hubbell, N. Wood & B. Fiedler.

7Tsirk, A. 2003 Obsidian Fractures from a Forest Fire Zone in the Santa Fe

National Forest. Report No., 020307, submitted to A. Steffen, SFNF and UNM.

Rectangular photos used here are by A. Tsirk.

Las Conchas Fire as a fire effects laboratory Collaborators from several agencies are teaming up to assess the effects of this

extraordinary fire on archaeological resources across the extent of the Las

Conchas fire area. In all, more than 2,500 documented sites were burned on a

mosaic of federal and tribal lands. CR managers from the Valles Caldera

National Preserve, Santa Fe National Forest, Bandelier National Monument,

Rocky Mountain Research Center Fire Science Laboratory, and Forest Guild are

developing a program to conduct field and lab research, including:

However, the same burn intensities on Cerro del Medio resulted in very little

vesiculation. On CDM, vesiculated pieces are sparsely scattered and found almost only

in long-duration burning contexts (i.e., under fully burned large logs, or around stump

burnouts).

Before After

Blue arrows show fracture direction