Originally Posted by
elcyc
I think a reason it doesn't pile up is ...
... and rain/snow.
Not sure about busy highways and freeways ... don't think they're swept?
Still, all that rubber stays in "nano" form for while -- on road surface -- until (natural) chemical breakdown.
The point I was making is that we, as a nation, go through a lot of tire wear per year. If the rubber didn't break down relatively quickly, you'd see drifts of rubber particles along the road way. Some one did a study of this long ago when tires wore out more quickly...sorry I don't have a link...and oxidation of the particles was the conclusion they came to to explain the lack of huge quantities of along our roads.
Additionally, apply Occam's Razor. The Earth is made of "earth". The most likely thing to get splashed off the Earth is "earth". That will be silicates of various forms and various hardnesses. They make up 90% of the Earth's crustal material after all.
It's also been my experience that street sweepers only do a marginal job of removing debris from the roads.
I'm fairly certain I would know if it was silica as that "crunches" when chain is twisted.
Most people would say that their chain "crunches" after riding in the rain.