The driving forces for metamorphism are changes in temperature, pressure and composition of the environment. In metamorphism, new minerals are formed that are in equilibrium with the new environment and a new rock texture develops in response to the new growth of minerals. Subsystems of the hydrologic system include river systems, glacial systems, groundwater systems, shoreline systems, and eolian systems. Running water, although impressive during floods, is a minor agent of erosion over geologic time. Glaciers are systems of flowing ice that form where less snow accumulates each year than melts.
Continental glaciers are areally extensive ice sheets covering continents to a few meters depth.
The term magma refers to rock after it has ascended from great depth and solidified on the surface. Strombolian eruptions are characterized by convecting eruption clouds of volcanic ash or tephra. Hawaiian eruptions are characterized by continuous fire-fountaining. Mount St. Helens will never erupt again because the magma is all used up. Ophiolites are fragments of ancient oceanic crust that have been thrust up onto land.