Early Earth's internal structure. Artwork of a hemispherical cross-section through the early Earth, with thin oceanic crust and formation of the first continental landmasses. The outer and inner core are at lower right. Convection in the mantle (circular arrows) drove volcanic activity and crustal spreading. The first continents arose where oceanic crust accumulated and thickened over areas of mantle down-welling. Stable continents first formed around 2.5 billion years ago, 2 billion years after the Earth formed. It would be another 2 billion years before complex marine and land life would evolve, with plate tectonics continually reforming the shape of the continents.