system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there so many islands in the Pacific ocean, but not the Atlantic Ocean?
gpt
Many of the islands are volcanic, and the Pacific is much more volcanically active than the Atlantic. The reason for that, in turn, is that the central Atlantic is a *divergent* boundary between tectonic plates (the plates are moving apart, which produces spread-out, 'ooze-y' undersea lava vents). The boundaries around the Pacific are mostly *convergent* plate boundaries (the plates are pressing into the Pacific plate, either one going beneath the other and forming volcanos (Japan) or crumpling up against it a forming a mountain rage (the Andes).

EDIT: A few people have correctly pointed out that in terms of amount of material, the Atlantic is plenty volcanically active. But for the most part the Atlantic doesn't have eruptions of the kind that the Pacific does, and this volcanism occurs slowly enough that the resulting undersea mountains don't reach the surface (with a few exceptions).