Topline:
Six days after firefighters put out the Lachman Fire, as strong, dry winds whipped across the Santa Monica Mountains, its remnants erupted from deep within the ground, quickly spreading through nearby communities. It became the deadly and destructive Palisades Fire.
The science: Underground, smoldering combustion occurs as the surface of the fuel itself burns; however, due to lower oxygen levels, flames aren’t produced. Rotten roots are particularly vulnerable to smoldering because they have lots of fissures, which expose surface area where a small amount of oxygen can creep in and feed the fire, said Scott Stephens, a fire scientist at UC Berkeley.
Why now: Federal prosecutors said Wednesday, in charging documents filed against a former Palisades resident, that the Palisades Fire was a "holdover fire."
Read on ... for more about the science of reignition.
At around midnight on Jan. 1, 2025, Jonathan Rinderknecht started the Lachman Fire in the hills of the Pacific Palisades, according to a federal indictment released Wednesday.
The fire was kept small that night — contained to just 8 acres by helicopters and hand crews from the L.A. city and county fire departments. Six days later, as strong and dry winds whipped across the Santa Monica Mountains, its remnants erupted from within the ground, quickly spreading through nearby communities and taking on the name the Palisades Fire.
On Wednesday, officials revealed that Palisades, one of California’s most devastating blazes, was a “holdover fire.”
“Unbeknownst to anyone, the fire continued to smolder and burn underground, within the root structure of dense vegetation,” reads the criminal complaint filed against Rinderknecht in the Central District of California. “On Jan. 7, heavy winds caused the underground fire to surface and spread above ground.”
Fires, mop up and reignition
Typically, after a wildfire is extinguished, firefighters walk the area and “mop up.” They look for evidence that anything still is burning and extinguish it.
"It's a lot of man hours to do mop up,” said Derek Bart, a retired L.A. County Fire captain who described the process of spraying ash pits with water and digging up smoldering roots as exhausting and mind numbing.
And sometimes, tiny embers can be missed.
“There may be things that are not visible to your eye,” Bart said. “You can't smell them. It's not putting out any smoke.”
Bart said he’s seen that with oak trees, where embers will creep deep into crevices, slowly burn the interior of the tree over time before catching on fire again and spreading embers to nearby grassy areas.
LAist reached out to the Los Angeles Fire Department about the mop up after the Lachman Fire but did not receive a response before publication.
The science of smoldering
Big, visible flames are produced when fuel heats up to the point that it chemically degrades, and the vapor coming off mixes with oxygen and combusts.
Underground, smoldering combustion occurs as the surface of the fuel itself burns; however, due to lower oxygen levels, flames aren’t produced.
Rotten roots are particularly vulnerable to smoldering because they have lots of fissures, which expose surface area, and small amounts of oxygen can creep in and sustain the burning, said Scott Stephens, a fire scientist at UC Berkeley.
“It really is more common than I think people realize,” Stephens said.
It just doesn’t usually reignite another fire.
However, those bits of smoldering material can be as small as a quarter and continue to burn for months, sometimes through winter. Strong, dry winds can get things going again.
Something along those lines is believed to have happened during the Oakland Hills Fire in 1991, when a rekindle from an earlier, smaller fire kicked back up. Twenty-five people died and 400 homes were lost in that fire.
How do you know when it’s safe?
After fire crews mop up, be on the lookout for flames or smoke coming from the area for weeks or months after the event occurs.
And be ready to call the fire department, who can come back out and tackle the blaze.
