An island formed by ancient rock from the ocean floor is called an oceanic island.

Here's why:

* Oceanic Islands: These islands are formed through volcanic activity, where magma from the Earth's mantle erupts through the ocean floor, creating mountains that eventually rise above the water's surface. The rock that forms these islands is basalt, a dark-colored volcanic rock.

* Continental Islands: These islands are formed when parts of continents break off and become isolated. They are composed of the same types of rocks as the mainland they came from.

So, while both types of islands are formed from ancient rock, the difference lies in the origin of that rock. Oceanic islands are formed from volcanic rock that originated from the Earth's mantle, while continental islands are formed from rocks that were once part of a continent.