Per my understanding, the ocean is largely very deep.
I can understand why there might be small, not-mountainous islands near the coasts of major landmasses, as it would only take a marginal elevation anomaly to create one there, but: How do places like Polynesia exist?
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Places likes Polynesia are formed by volcanic activity. Magma, which is molten rock, rises from the mantle to the earth’s surface via a “hotspot” underwater and hardens. Sometimes, there’s enough of that rock piled up that it reaches above sea level, and we get an island. Hawaii is formed this way.
These volcanic islands are essentially giant mountains where only the tip is above water. If we drained the ocean, we would see that Hawaii’s tallest mountain is actually 30,000 feet tall from its base at the sea floor. That’s higher than the elevation of Mount Everest!
Edit: to go slightly beyond ELI5 – you may notice that some sets of islands appear to line up with each other in a row (see a map of Hawaiian islands). This is because the crust (made of up tectonic plates) is essentially floating on the earth’s mantle. As the earth’s crust moves, the location of the hotspot shifts relative to the crust, so you see new islands instead of one giant island. Since plates tend to move in one direction, we see lines of islands.
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