The five principal islands and four smaller ones are all volcanoes in various stages of erosion. The islands are hilly and mountainous, especially toward the east.
Coastlines are mostly rocky and rough.
Only Oahu and Niihau have much coastal plain. Surface streams are not abundant, because the ground is highly porous, composed as it is of lavas. Dense rainforest in Hawaii Volcanoes National Park. The surrounding ocean and highly persistent northeast trade winds maintain almost uniform climate throughout the year. At any given location, temperature and precipitation remain nearly constant year-round, but both vary greatly with altitude and exposure.