Volcanic ash is tiny particles of rock.
If you fly an airplane through a cloud it will sand off the paint on the plane. The rock will melt in the hottest part of a jet engine and resolidify in bigger chunks when it cools down.
Your car’s air filter is also not ratted or even capable of trapping such fine silica, it will get past it like smoke through a screen door. Once inside the engine some inevitably get on to that nice wet sticky surface where the oil film is inside the cylinders. Then it will grind ,rings ,bearings and valve stems into ruin. Your oil filter is not rated to capture these sized particles either. They are small enough to get past a OEM grade filter but still large enough to kill bearings and rings nasty little problem with single digit micron sized particles.
Almost no one has submicron bypass oil filters on their vehicles and even then they only filter 25% capacity per hour it’s bypass trickle ffiltering not bulk flow.
Also tiny particles of volcanic glass. Volcanic glass is one of the sharpest edged materials there are—even sharper than diamond scalpels. When there are millions of them covering a surface they can make millions of very visible micro scratches. Not the best stuff to breathe either.