Cataclysmic Fire

Revisiting the topic of biggest ever fires

Back before I started using capital case for my post titles, back when there were just 21 visionary subscribers here, a mere four months after Future Fire first foist itself into the blogosphere, I pondered the topic of the fire to end all fires. Partly stimulated by the statistical extravaganza of burnt temperate forest during the 2019-20 Black Summer fires, and partly by the lure of questions that logically must have an answer, I sketched out some very basic (and almost certainly incorrect) outlines of the search for answers to the questions ‘What was the biggest ever fire?’ and ‘What was the biggest ever fire season?’.

Fast forward 24 months and what citation should appear before my unbelieving eyes on the screen of my soon to be e-less laptop?

Durda, D. D., and D. A. Kring (2004), Ignition threshold for impact-generated fires, J. Geophys. Res., 109, E08004, doi:10.1029/2004JE002279.

That’s right, 20 years ago the Journal of Geophysical Research had published in its 109th volume a paper by the magnificently titled Daniel D Durda, of the Southwest Research Institute in Boulder, Colorado, and his colleague David A Kring, of the Lunar and Planetary Laboratory at the University of Arizona in Tucson.

The first line of the abstract caught my eye

Widespread fires can be generated after large impact events by atmospheric heating caused by the reaccretion of high-energy, vapor-rich plume material.

Glossing over* the opaque but beautifully chunked concept of high-energy vapor-rich plume material reaccretion, I continued, lingering momentarily on the terms threshold irradiance levels and spontaneous and pilot ignition before hitting the intimidating continent-wide spontaneous ignition of wood and the awe-inspiring and terrifying global spontaneous ignition of wood. The abstract ends with these disturbing visuals

Impact craters of at least 85 km diameter are needed to produce continental-scale fires, and craters of
~135 km diameter are needed for global-scale fires.

Aha! So all we need is an asteroid big enough to throw enough stuff way up into the air that… well, I wasn’t entirely clear, but the image was spectacular enough that it settled in my mind like a bunch of plume material.

Composite image showing the comparative sizes of eight asteroids (Source: NASA/JPL-Caltech/JAXA/ESA via Wikimedia Commons)

I soon returned to the paper, which begins with one of the more engrossing introductions to a peer reviewed scientific journal article I’ve read.

Local fires can be generated around an impact site by direct radiation from a fireball [e.g., Shuvalov, 2002] and over larger (perhaps global) regions by atmospheric heating caused by a reaccreting high-energy, vapor-rich plume [Melosh et al., 1990] and possibly extended sheets or rays of lower-energy ejecta. In the case of the 1908 Tunguska blast, which detonated 6 to 10 km above a Siberian taiga, fires were ignited by a thermal radiation pulse over a 200 to 500 km2 area [e.g., Svetsov, 2002]. In the case of the Chicxulub impact event at the K/T boundary, fires may have been generated over a much larger area, generating a globally distributed layer of soot [e.g., Wolbach et al., 1988, 1990] and possibly localized deposits of charcoal and related residues in North America [e.g., Tschudy et al., 1984; cf. Scott et al., 2000].

Fireballs, Tunguska**, thermal radiation pulses***, the boundary between the Cretaceous and Tertiary Periods**** and the world-famous dinosaur-devastating Chicxulub event*****. Man, this intro has it all.

Digging up the Melosh et al 1990 reference, I got to the heart of the reaccretion bizzo:

Recent advances in the understanding of the mechanics of a large impact event [6,11,12] show that in high-velocity impacts the expansion of the plume of vaporized projectile and target material has a larger role than previously anticipated. Theoretical studies show that the plume expands at such high speed that some of the material exceeds the Earth's escape velocity. The vapour plume comprises nearly the whole mass of the projectile along with an equal or greater contribution from the target rocks. In large impacts this plume blows aside the adjacent air and vents most of its material above the atmosphere[13]. At asteroidal impact velocities although some of this material may escape the Earth, most of it falls back into the atmosphere after following ballistic trajectories between the impact site and more distant regions of the Earth (at cometary impact velocities, however, much of this material may leave the Earth altogether [12].

To these untrained ears, that sounds like something freaking massive slamming into the earth so freaking fast that the asteroid and the bit of the earth it hits (kindly called the 'target’) are not only both vapourised, but everything flies back up from the surface so fast that it leaves the planet again, only to rain back down on various other bits of the earth. For an asteroid around the size and speed of the Chicxulub culprit, (~10^16kg, ~20km/s), “most of the vaporized projectile is redeposited ballistically on the Earth at velocities in the range of 5-10km/s.”

Holy shit.

All of this brings us back to Durda and King’s paper, which asks about the specific flavours of cosmic hellfire able to trigger spontaneous combustion of wood at scale. I think we can safely say that impact-generated fires would be very unpleasant. But perhaps we can be charitable to the paleoscientists curious about big fires and fire seasons of the past and ask them to probe the upper limits of fires not caused by rocks from outer space.

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Thanks for reading, have a safe and joyous festive season (if possible), and I look forward to spending some more time with you in 2025.

* Knowing what to gloss over and what to dwell on is a very important, difficult and uncertain skill for academics and non-academics alike.

** I remember Tunguska from an X Files episode, and possibly some other more factual source of information over my years of non-fiction reading.

*** That’s a big thermal radiation pulse. 200 to 500 km2 is roughly the size of the fire currently burning through the Grampians in the west of my adopted home state of Victoria. I’m writing from a safe distance in the Blue Mountains, where the family is spending Christmas. There was a minor scare yesterday when a fire broke out by the local train station, but with little else happening in the area, there were plenty of resources to suppress it quickly. Christmas may be a winter wonderland for some, but here it’s The Most Flammable Time Of The Year.

**** This is now known as the Cretaceous-Paleogene (K-Pg) boundary. The K comes from the German word for chalk, Kreide, because Cretaceous comes from the Latin word for chalk, creta.