Why are some islands volcanic?
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Answer: Tectonic hotspots or subduction
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.)
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).