a major global morphostructure located between the underwater continental margin and the ocean floor. Typical transitional zones are encountered along the northern and western margins of the Pacific Ocean, in parts of the Caribbean and Scotia seas, and in the northeastern part of the Indian Ocean. They consist of the basins of marginal seas, island arcs, and deep trenches. Such zones are characterized by the most varied types of relief, ranging from trenches with depths of 11 km to volcanic peaks with elevations to 5–7 km on the island arcs.

Transitional zones are marked by sharp differences between the values of the magnetic and gravitational fields and heat flow values (greater values on the island arcs and values below the norm in the trenches) and by rapid and sharp differentiation in vertical movement of the earth’s crust. Under the basins of the deep-water seas the crust is suboceanic, whereas under the island arcs it is subcontinental and sometimes continental. In the oceanic trenches, the crust may be subcontinental, suboceanic, or oceanic. The transitional zone is an area of volcanic activity, intense seismic activity, and mountain formation.