Destructive (subduction) boundary: an oceanic plate moves towards a continental plate. The denser oceanic plate sinks beneath the continental plate into the mantle, where it melts. Magma rises through the crust and erupts, forming steep composite volcanoes; the edge of the continent is crumpled into fold mountains and a deep trench forms offshore. Example: the Nazca plate subducting under the South American plate, forming the Andes and Nevado del Ruiz in Colombia.

Constructive boundary: two plates move apart. Magma rises from the mantle to fill the gap and cools, forming new crust. Over time this builds a mid-ocean ridge with gentle eruptions of runny basalt lava. Example: the Mid-Atlantic Ridge, where the North American and Eurasian plates are separating through Iceland.