Usually spontaneous combustion implies a fire starting without an ignition source….no flame. This can happen when vegetation material breaks down. The microbial activity breaking it down releases heat (and sometimes combustible gases) that can build up locally enough to exceed the ignition point of the surrounding material.

To start a fire you need fuel, oxygen and heat.
If the vegetation is very dry due to a long period without rain and it gets hot enough (and there’s air) then a fire could start without any other intervention.

How hot is hot in these circumstances?

Remember Farenheit 451? That was the ignition point for paper.

Is that how hot it is just before the natural fire starts in the forests?

Oooo oooo! Pick me!!! I know this one first hand!
Back in the stone age my Dad mowed his lawn with a reel type mower that collected the grass clippings. He would dump out the basket in the corner of our yard.
One day my brother and I were playing in the clipping pile and happened to dig down a little and found a spot that was smoking hot! It was lucky we found it or the pile would have likely lit off. My Dad hosed it down and it was steaming! The grass breaks down and generates heat. It eventually gets so hot it ignites. Check out the classic linseed oil on oily rags videos. Earlier this year a house in our area caught fire because the owner had left linseed oil soaked rags in the work room after a wood working project.

That is amazing.

Do home buyers and property owners get told about these things, just like fire depts etc. educate the public about fire detectors, (installing them and changing the batteries), for example? I only hear about natural fires in forests.

We were taught about oily rags in elementary school science class.
Recent example

“The homeowner says he was using rags with linseed oil on his boat and put the rags in a pile around 6 p.m. By 11 p.m, he and his family woke up to the smoke detectors going off.

“The heat that’s generated from that drying process within the materials, the rags, it’s been sealed in an area that is flammable, those combined makes it dangerous,” said Shawn Gilleland with the Rural Metro Fire Department.”

If you can do it safely, you should try the grass clippings experiment.

In the grass clippings case, under a pile, wouldn’t there be less oxygen there? And wouldn’t it be hotter on the surface? Less oxygen less heat?

This may be the case. But what is critical is that the heat is confined and thus destined to increase. There comes a critical point where ignition must occur, if ANY oxygen is available. The resulting combustion will then draw oxygen in to sustain and increase the growing conflagration. It’s interesting to speculate on what forest fires must have been like in the days where oxygen was so abundant a percentage of our atmosphere that dragonflies could reach the size of present day hawks. A dinosaur today would probably require an oxygen tank strapped to its back in order to survive.

But isn’t it cooler below the surface? Heat rises, basements are cooler, So, if what is on the surface is not smoking/burning, ....?

But remember the initial situation wulfie (for once) aptly describes above. It will work perfectly well in a cool basement if the heat is confined within the accumulated decomposing matter.

So, dry grass clippings are supposed to be particularly flammable. I never would have guessed. I still don’t know where the heat would be coming from esp. enough to start a fire.
https://restorationmasterfinder.com/restoration/flammable-liquids-and-things-within-the-home/
Flour and oranges are flammable. Is it just _orange rind _ by the way, not clementines rind etc.?
And shouldn’t there a lot more fires every day all over the place?

It happens that not only are several things combustible but actually explosive if conditions are right. Every farmer knows about the dangers of the dust from grains not properly ventilated in storage silos.

Ok. It’s really interesting to me.

Thanks all.

@chefl You wrote “In the grass clippings case, under a pile, wouldn’t there be less oxygen there? And wouldn’t it be hotter on the surface? Less oxygen less heat?”
You’d think so but a quick experiment would show you that is not true. Make a pile and let it sit for a few days. Then (gently) put your arm inside. It will be hot. Depending upon the grass clipping moisture it can be very hot! My Dad would put the clippings in a metal garbage can. That made the situation even worse. But it sure was fun to play in.

Here is an article about spontaneous combustion in hay bales on the farm. Harvesting wetter hay is more likely to spontaneously combust.

If you are interested in explosions you can check out the many videos of corn and grain silos blown apart from a static electricity spark in the dust. Everything needs to be dry.
That is another experiment you can try. Blow some dust over a candle. Wow! Do it outside!

@LuckyGuy I don’t need to do an experiment, I know you’re not talking garbage. I was just trying to understand the how of it. So, even if I did the experiment it wouldn’t help, so I’m leaving it alone.