Flux in geochemical cycles is the movement of material between the deep Earth and the surface reservoirs. This occurs through two different processes: volcanism and subduction of tectonic plates.
Subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge. This leads to the sinking of one plate into the mantle which creates a broad range of geochemical transformations or cycling.
Volcanism is the process that takes place at divergent boundaries by which one tectonic plate separates from another creating a rift in which molten rock (magma) erupts onto the surface of the Earth. This molten rock magma then cools and crystallizes, forming igneous rocks. If crystallization occurs at the Earths surface, extrusive igneous rocks are formed; if crystallization occurs within the Earths lithosphere, intrusive igneous rocks are formed which can then be brought to Earth's surface by denudation