scotland is geologically alien to europe, comprising a sliver of the ancient continent of laurentia (which later formed the bulk of north america). during the cambrian period the crustal region which became scotland formed part of the continental shelf of laurentia, then still south of the equator. laurentia was separated from the continent of baltica (which later became scandinavia and the baltic region) by the diminishing iapetus ocean. the two ancient continents moved toward one another through the cambrian and ordovician periods, with tectonic folding during the silurian pushing the first scottish land above water. the final collision occurred during the devonian period, with the scottish segment of the laurentian plate smashing into avalonia (which contained what is now most of england and wales), a motile subcontinent which had previously joined with baltica. this impact threw up a massive chain of mountains (at least as tall as the present-day alps) and saw the formation of the granitic west highland and grampian mountain chains and (through the carboniferous) a period of volcanic activity in central and eastern scotland. during the permian and triassic periods, with the iapetus ocean entirely closed, scotland lay near the centre of the pangaean supercontinent. at the start of the tertiary, a constructive plate boundary (at which tectonic plates move apart) became active between laurentia and eurasia, pushing the two apart (and parting scotland from laurentia). this recession opened the atlantic ocean for the first time, and the consequent subduction zone at the western plate margin led to a renewed period of volcanism, this time on scotland's west coast, producing fresh mountains on skye, jura, mull, rùm, and arran.