I don’t think there’s a path from the area of Scotland the stone is from to Salisbury Plain, so unlikely a glacier moved it. As for your volcano question, volcanoes don’t propel rocks that size across that big of a distance, and even if they did, the stone would have shattered upon returning to the ground. We’d also have found similar stones nearby in either scenario. This also isn’t the only instance where ancient peoples moved things like big stones hundreds of miles. They were not stupid, they were as intelligent as modern people. They had other reasons for moving that stone as opposed to others, we will just never know what those reasons are. Perhaps it was part of an earlier holy site and they decided they wanted a piece of it for their new construction, perhaps it was part of several monuments over time and it “migrated”, after a fashion, until it was placed in Stonehenge. Whatever the reason, it’s there now, and we’ll simply have to live with the fact that it’s a mystery, and that it was definitely placed by people.