Complex sentence: As the Pacific Plate moved northwest over the hot spot, these volcanoes gradually eroded and subsided, evolving from high islands to atolls (or seamounts north of the Darwin Point).
Split into simpler sentences: As the Pacific Plate moved north and later northwest over the hot spot, volcanic eruptions built up islands. The isolated land masses gradually eroded and subsided, evolving from high islands to atolls (or seamounts north of the Darwin Point).