Earth’s Interior and Plate Tectonics Vocabulary List Plate Tectonics-the theory that explains how large pieces of Earth’s outermost layer, called tectonic plates, move and change shape. Convection Current-the movement of matter within earth’s mantle and asthenosphere due to differences in density. Transform Plate Boundary-The boundary between tectonic plates that are sliding past each other horizontally. Convergent Plate boundary- the boundary between tectonic plates that are colliding or coming together. Divergent Plate Boundary-the boundary between two tectonic plates that moving away from each other. Subduction Zone-A convergent plate boundary where a more dense oceanic plate slides underneath a less dense continental plate. Crust-the thin and solid outermost layer of Earth above the mantle. Mantel-The layer of rock between Earth’s crust and core. Lithosphere-The solid upper layer of Earth that consists of the crust and rigid upper mantle. Asthenosphere-The soft layer of the mantel. The layer that the plates float on. Core-The central part of earth below the mantel. Inner Core-the sold, innermost part of the core. Outer Core-the liquid, outer part of Earth’s core. Density-how compact an object is. The mass of an object compared to its volume.