Earth's mantle convection. Diagram of a sectioned Earth, showing the convection (red arrows) in the mantle. Magma in the mantle is heated by the Earth's core (white). Hot magma reaches the outer layer of the upper mantle and spreads out, before finally sinking as it cools. Convective currents in the mantle drive plate tectonics and forms volcanoes on the Earth's crust. 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.