Continental and Oceanic Island Formation

Geologists divide the roughly quarter of a million islands scattered across the planet into two fundamentally different categories based on the type of crust beneath them. Continental islands sit on continental crust, granitic rock roughly 2.7 grams per cubic centimeter in density, and were once physically joined to a larger landmass before being separated by rising seas, rifting, or erosion; Great Britain, Ireland, Sri Lanka, Newfoundland, Borneo, and Madagascar all fall into this category, sharing geology, and often plant and animal life, with the neighboring continent. Oceanic islands, by contrast, sit on denser basaltic oceanic crust, roughly 3.0 grams per cubic centimeter, and were built up from the seafloor itself, almost always through volcanic activity at a hotspot, a mid-ocean ridge, or a subduction-zone arc, so that they never had any land connection to a continent; Hawaii, Iceland, and the Azores are the textbook cases. This distinction matters enormously for biology as well as geology: continental islands typically inherit a subset of the mainland's plants and animals at the moment of separation, while oceanic islands must be colonized from scratch by whatever seeds, spores, and animals can cross open ocean by wind, water, or wing, producing the strikingly impoverished but highly distinctive faunas, rich in flightless birds and giant tortoises, associated with places such as the Galapagos and pre-human New Zealand.