“The earth has known fire for over 400 million years,” Stephen J. Pyne, professor in the Biology and Society Program at Arizona State, tells us in a NOVA Online feature.
“Life made it possible. Marine life pumped the atmosphere full of oxygen; terrestrial life lathered the crust with fuels. When oxygen and fuel meet a spark under the right circumstances, a fire kindles. (Lightning is an ancient and ample ignitor.) The fundamental chemistry of combustion lies at the core of the living world. When it happens within a cell, it’s called respiration. When it happens outside organisms, it’s called fire. It’s that basic.”
Lightning, sparks from rockfalls, spontaneous combustion, and volcanic eruption ignite most natural wildfires. Though discarded cigarettes and arson usually occur to us as being the main human causes, sparks from equipment and arcing electricity from a stressed or downed power line, as in 2011’s Las Conchas wildfire, are also strong manmade igniters.
Even a spark from a blown-out tire can start a fire. So can the slash-and-burn techniques which humans have used to clear land for agriculture since prehistory, military operations, mining, and logging, which leaves behind abandoned roads. Overgrown by flammable grasses and other vegetation, these logging corridors foster wildfire and enable it to spread quickly.
According to the New Mexico Forestry Division, 106 fires on state and private land (federal lands not included) in the past year were started when trees fell on power lines. Only equipment use, lightning, and burning debris caused more fires in the state than power arcs.
- Surface fires, which consume mostly grasses, detritus (fallen leaves, needles, bark, sticks, branches, and live tree stems), downed trees, small living shrubs, and other combustibles at ground level, are dangerous but can usually be controlled. When burning enters subterranean roots and buried organic matter such as peat, it can smolder for days to months. Lower down, mine fires or coal seam fires can burn for decades or even centuries.
- When a fire rises above ground level, its chances of going out of control also soar. An uncontrolled ground fire becomes a ladder fire in wooded areas. Fire in low-level vegetation, climbing ferns, vines, mosses, and the branching, leafy understory of trees make up a ladder of combustibles. As the flames burn continuously upward, fed by the ladder fuels, the danger increases. It reaches a critical level when the fire begins to ignite the canopy, atop the trees.
- Once a fire crowns out, it can spread from tree top to tree top. Extreme heat, intensifying winds, convection currents and updrafts, and tornado-force fire whirls then take the fire to extremes. Crown fires are the most deadly and most damaging type of wildfire. Crews find them nearly impossible to control. Sometimes they rage on until the weather changes.
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