An extensive cluster of islands formed tectonically is called an archipelago.

Here's why:

* Tectonic Formation: Archipelagos are often formed by tectonic activity, specifically through volcanic eruptions or the movement of tectonic plates.

* Extensive Cluster: An archipelago is defined as a group or chain of islands, usually close together and often with a shared geological origin.

Examples of Archipelagos:

* The Philippines: Formed by the subduction of the Eurasian Plate under the Philippine Sea Plate.

* The Hawaiian Islands: Formed by a hotspot in the Pacific Plate.

* The Canary Islands: Formed by volcanic activity related to the African Plate.

Let me know if you have any other questions about tectonic processes or island formation!