I never had a geology course where this might have been taught. What exactly is the “continental shelf”? And, if it might be thought of as a land’s perimeter (submerged below sea level) after which you drop off into deep ocean…then, what are islands? Are they the equivalent of mesas in the ocean? Do they have a shelf, or is it not much to speak of a mile out from the beach, let’s say? - Jinx
The continental shelf is the extended perimeter of each continent, which is covered during interglacial periods such as the current epoch by relatively shallow seas (known as shelf seas) and gulfs. The shelf usually ends at a point of increasing slope (called the shelf break).
Lots more details here:
Islands are (obviously) dry land smaller than continents sticking out of the ocean (or lakes, etc.). But beyond that, there is no single standard or definition for them. Some of them are on extended shelves, with shallow water between them, and/or between them and the nearest continent. Examples: the Bahamas, western Indonesia, the British Isles. Some of them are projections from the deep ocean floor (e.g., Hawaii, the Society Islands, Tonga). Some of them are parts of mid-ocean ridges (Iceland, Ascension). Some are “island arcs” at plate convergences (Japan, New Zealand).
There is no good definition save size between island and continent. Typically, a continent has two cordilleras (one of which may be relatively quite low relief, e.g., the Brazilian highlands) with a cratonic plain/plateau area between them. But one really needs to stretch that definition to make it fit every continent, and by it Greenland qualifies as continent rather than island. FWIW, Australia has something on the order of 2.5x the land area of Greenland, they being the two land masses that border the divide.