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?
In: Other
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.
Aren’t they mostly volcanic islands?
The majority of isolated Pacific islands result from so called “hot spots” of volcanic activity that push through the tectonic plate underneath the Pacific ocean.
The Hawaiian archipelago is a great example of how this works, with the Big Island the youngest geologically sitting on top of an active volcanic spout, and the smaller islands having been moved off this volcanic hot spot by plate movement.
The magma gets pushed up through the plate, cools into solid rock, and over a few million years becomes the islands we know today.
Most of the other Pacific archipelagos were formed in the same way.
Mountains don’t necessarily have to be above ground.
They can be underwater as well.
Actually the longest mountain range in the world, is the Mid-Oceanic Ridge, which is underwater.
If you can get directly to their summits, by boat, you’ll have found an island.
Hawaii for example is the top of a mountain range/mountain.
Often they over volcanic hotspots so they are huge mountains if you measure from the seafloor. Hawaii the hot spot is moving (or the crust is) resulting in a chain of islands.
Volcanoes that erupt on ocean floor many times over millions of years until the piling up of hardened lava eventually climbs above sea level.
Volcano gonna grow wherever the volcano decide to go.
Once it gets tall water can’t cover it all.
Latest Answers