Why are some islands volcanic?
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Answer: Tectonic hotspots or subduction
Underwater pressure creates lava — Wrong. Pressure doesn't create lava — higher pressure actually keeps mantle rock solid. Rock melts when its melting point is lowered (water added at subduction zones) or when hot mantle rises and decompresses (hotspots).
Ocean water heats up magma — Wrong. Seawater doesn't heat the magma. Water does matter at subduction zones, but in the opposite way: it lowers the melting point of the hot mantle rock so it melts more easily — it's not the water getting hot that makes magma.
Tectonic hotspots or subduction ✓ — Correct! Volcanic islands form two main ways. (1) Subduction: an oceanic plate sinks beneath another plate and, at about 100-120 km depth, releases water into the hot mantle wedge above, lowering its melting point (flux melting). That magma rises to build a chain of volcanic islands — an island arc like Japan or the Philippines. (2) Hotspots: a fixed mantle plume melts through a moving oceanic plate, leaving a trail of islands like Hawaii. (Islands like Britain or Madagascar aren't volcanic — they're broken-off continental fragments.)