The movement of mantle rock through the cracks between tectonic plates
Our Earth is not a static planet. The crust, the part where we live, is above a more fluid componet, the mantle. Where the crust thin out, it is possible that the mantle comes to the surface, causing a series of phenomenon like volcano activities and earthquake. This is particularty visible in the Ring of Fire, because there a lot of those cracks in this zone.