A Tectonic plate is a part of a planet's crust, they float on the mantle and are moved by convection currents. The movement of tectonic plates causes the formation of mountains and ocean trenches is responsible for volcanic activity and earthquakes. In 2370 when Odo was observing a group of twenty Horta he thought they looked rather like a museum diorama of tectonic plate activity. (DS9 novel: Devil in the Sky)