Possibly there's some decomposition going on with a bit of attendant heat. Just a couple or three degrees.

Once the sun strikes the uncovered pile, the darker surface helps clear the rest of the surface.

I doubt there is any or much decomposition going on as thinly spread and certainly not heat from that. What looks like a 'pile' here is just snow with a thin covering of mulch. The point is the snow around this - without the mulch on top - melted much faster.

One of the reasons to apply mulches is to aid in the control of soil temperatures so yes mulch can insulate. A big reason why mulch is not applied to frozen ground.

I'm thinking that sun penetrates a few inches into the snow, and the dark color of the mulch began to absorb a tiny bit of heat and accelerate the melting. More melting, more light going through, more heat, and the process accelerates.

Just glad to see it melting! Here in central MO we had 1" of wet snow last night - rare to get April snow. The second measurable snow this month!

I'm not sure I explained this well, but the thing that surprised me was that the snow under the mulch melted much, much less than naked snow right around it. I noticed a few other significant spots that were similar - like sawdust on top of snow - that had also resulted in less melting (and so weird 'lumps' of snow with sawdust on top). So either the mulch/sawdust stopped sun from warming up the snow beneath, or had kept the cold snow beneath cold (stopped circulation of relatively warm air from melting the snow).

To be clear, this wasn't an attempt to do mulching or composting in winter - I was tossing the wood chips in a corner of the yard to do something with in spring. Which thankfully has just started to arrive. I was just surprised how it affected the snowmelt process.

Yep, I'm saying something similar - in my case because mulch had been spread (or just tossed) on top of a thick layer of snow, it had the opposite effect - keeping the heat from the sun/air from melting the snow underneath. Or at least that's what I reckon is going on. Same basic idea though, slowing passage of heat from warm to cold areas.

Kind of confusing isn't it?..a different neighbor that saw him spreading mulch said "I think it's too cold..I want the sun to warm up my soil a little before I get mulch"..so you wonder does mulch keep the soil colder because it slows the sun or retain residual heat from the earth and keep the soil warmer?..

@nicholsworth - I think there's no one answer. If it insulates, it will moderate the swings and heat transfer from cooler to hotter, which is just going to depend on the time of year. It happens with houses on a daily basis - moderating the swing from cooler at night to hot during the day (all relative of course and all things being equal, obviously some houses might be designed to absorb heat during day, etc).

I'm not trying to draw a big lesson from this accidental experiment - will just depend on local conditions and needs (whether excessive sun/heat is a problem, for example). Most likely 'nature's way' is for mulch and leaves to be spread in fall in cold climes, but nature also wouldn't rake up leaves and branches, or worry that much about weeds, or fret about having a nice lawn. So 'adjust to your needs' is all I'm going to say.

armoured..I appreciate your thoughts..we have half an acre in a wooded neighborhood..we compost the leaves we rake and use our compost for mulch around our plants..sometimes we mow the paths with our mulching mower..our lot has a tidy but natural look..some neighbors have a few plants in a sea of mulch whereas I prefer more plants so not much ground (if any) shows..