Nature does it by charging electrons until they’re hot enough to flare in bolts of lightning. Such events were more likely to occur in prehistoric Africa than in Europe. Today, the greatest number of cloud to ground strokes occur in the highlands of eastern Congo, the black spot on the map below that lies on the equator.
NASA’s charting shows Europe experiences relatively few lightning strokes. Our part of the country gets a moderate number, but only one of our recent fires was caused by lightening. The Jaroso ran through the headwaters of the Santa Cruz river near Trailrider Wall in 2013.
The other major fires were caused by exposing flammable material to intense heat. The Dome spread from a poorly extinguished camp fire in 1996, the Cerro Grande from a controlled burn in 2000, and Las Conchas from a downed power line in 2011. All occurred during times of high winds, and none were declared out until monsoon rains had drenched them.
In the past, the easiest way to create such a spark was by striking pyrite with something harder like chert or flint. The impact sheered the iron and ignited the sulphur. Pyrite is iron sulfide (FeS2), flint and chert mainly silicon dioxide (SiO2).
Our recent experiences have taught us the condition of grass and the spacing of trees makes a difference in the severity of a conflagration. High intensity fires spread when they burn in tree canopies or crowns where heat transfers easily from dried leaf to instantly dried leaf.
On the road to Jémez Springs the trees were so closely spaced in 2000 the grasses at the bases of some could ignite the tops of others. This was probably a rare situation caused by lumbering in the early twentieth century. Trees are not normally so dense nor are they all the same age and height.
Low intensity fires spread along the ground through plants and organic debris. It’s difficult for them to jump from trunk to trunk, because there’s too much oxygen between the two poles of carbon for temperatures to sustain them. Instead, they creep up the trunks, charring as they rise.
At moderate temperatures, ground fires may occasionally flare high enough to torch lower leaves. Usually, the trees are too widely spaced for one canopy to ignite another.
The nature of the grass and underbrush also matter. Few lightening strokes produce fires during the monsoon season when it’s raining. Most occur in the dry spring. One Forest Service scientist, Guido Kaminski, found rotted wood ignited but went out quickly. When cheat grass was heated, "the flame of the pilot jumped across the entire fuel bed, consuming it before the test operator could contain it."
Another Forest Service employee, William Pitts, found a mix of pine needles and recently cut tall, wet-season fescue burned at a lower temperature than the other tested fuels, but the quickest flame occurred with dry-season fescue if there was a wind. He found cheat grass also required wind to burn, and a had tendency to smolder. Needles from loblolly pines were the only kindling that would fire without wind.
After the Cerro Grande fire, people who surveyed the grounds saw areas of ash, iron oxides, and clays where once there’d been trees. The reddened soils were ones where the water-containing iron compounds (iron hydroxides) had become so hot they lost their water and some form of hematite emerged. To produce those results, soil temperatures were at least 480 degrees (250C). The severely burned surface may be sterile for some time.
More common were areas where ground litter burned and only charred logs remain. Here soil temperatures were above the boiling point of water and could have been lethal down 2". Even if only ash were present, there were nutrients left to support seeds that blew in from adjoining areas.
Surrounding them were the scorched trees that hadn’t ignited. Over the nearly 6,000 acres reviewed by Raymond Kokaly’s team, nearly half were ash, char or bare ground. More than 15% were conifer dehydrated by heat. Almost 4% were grasses dried into straw. The rest, 28.9%, were still green.
Randy Balice’s group found, outside the lands managed by LANL, 48.6% of the acres burned at high intensity and 42.6% at moderate temperatures. The remaining 8.8% suffered low intensity heat. Fire intensity and fire severity refer to two different phenomena, the one to chemistry and air temperatures, the other to biological consequences and soil temperatures.
In Africa hominins no doubt also walked through fire scarred lands and saw the effects of heat. They may have observed sticky resins released from partly charred trees and transformations in rocks. They also may have noticed differences in the tastes of plant foods roasted by passing heat and tender rejuvenating growths. They certainly would have recognized any hematite.
When we drive through the damaged lands years after the Cerro Grande, we see curiosities. The hominins were trained to observe rocks and plants.
Notes: Fahrenheit conversions of Centigrade temperatures are rounded for ease of understanding.
Balice, Randy G., Kathryn D. Bennett, and Marjorie A. Wright. Burn Severities, Fire Intensities, and Impacts to Major Vegetation Types from the Cerro Grande Fire, 2004; good maps and pictures.
Kaminski, Guido C. Ignition Time vs Temperature for Selected Forest Fuels, November 1974. He was studying fires caused by sparks from chain saws.
Kokaly, Raymond F., Barnaby W. Rockwell, Sandra L. Haire, and Trude V.V. King. "Characterization of Post-fire Surface Cover, Soils, and Burn Severity at the Cerro Grande Fire, New Mexico, Using Hyperspectral and Multispectral Remote Sensing," Remote Sensing of Environment 106:305-325:2007.
Pitts, William M. Ignition of Cellulosic Fuels by Heated and Radiative Surfaces, March 2007. He was studying fires started by hot mufflers and catalytic converters. Loblolly pine is Pinus taeda. His fescue was Festuca arundinacea.
Graphics: All photographs were taken 4 July 2013 along Route 4 from the entrance to Bandelier National Monument to the base of Cerro Grande. The altitudes are from the camera’s GPS interface.
1. United States National Aeronautics and Space Administration, National Space Science and Technology Center Lightening Team, and the Global Hydrology Resource Center. Data from space-based sensors reveal the uneven distribution of worldwide lightning strikes, 10 December 2001. Data obtained from April 1995 to February 2003. Units: flashes/km2/yr.
2. Ponderosa pine on steep hill, burned so severely, little has come back in 13 years. Altitude: 8049'.
3. Area of charred wood. It looks like a dozer knocked down trees like the one laying in back with short needles. The grasses have not revived in the drought, but shrubs have regrown and the trees in the distance weren’t killed. Altitude: 7054'.
4. Close-up of gambel oak in #3. The trunks were scorched enough to kill the buds buried in the sapwood, but the roots have been able to sucker.
5. Gully was not destroyed and juniper have come back. Other sloping ground in the area is still infertile, but the ponderosa pine on the far bank survived. Trees in the distance, closer to Cerro Grande, are still black specks. Altitude: 7211'.
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