An archipelago is a cluster of islands frequently formed by volcanic activity. Examples can be found in the open sea as well as close to large land masses. The former is usually more common than the latter.
In some cases, an archipelago forms because it is above an ocean hotspot. There are about 50 identified hotspots that are defined as having an extended period of active volcanism. One famous cluster is the Hawaiian Islands, which sits atop an ocean hotspot.
Both erosion and sediment deposits can influence how an archipelago forms. For example, when the islands occur near a larger land mass, they may have formerly been connected to that mass. Both erosion and shifting of tectonic plates may have resulted in parts of the land mass drifting away or being separated. This is generally a very lengthy process.
Due to volcanic activity, islands in an archipelago may either reduce or increase in size. If volcanic activity remains fairly constant, deposition of volcanic materials may actually expand an island. Since the island is surrounded by water, it is also subject to relatively continuous erosion. Fierce volcanic activity can completely decimate an island in an archipelago, too. Climate can certainly have an effect on the islands as well. Those located in tropical regions are subject to fierce hurricanes.
There are many archipelagos in the world, and the definition can refer not only to a group of islands, but also any sea that has a number of small islands. The Aegean Sea is considered an archipelago because of its many land masses. An island group doesn’t necessarily have to be small, nor is it always unstable. The British Isles, for example, are a large and stable cluster, and the Florida Keys includes at least 1,700 small islands extending from the southern tip of Florida. Another large archipelago is the islands that make up New Zealand and New Guinea.