Anastasia Steffen, Valles Caldera Trust & University of New Mexico
Transcript of Anastasia Steffen, Valles Caldera Trust & University of New Mexico
• 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 Unpublished Ph.D. dissertation, Department of Anthropology, University of New Mexico,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