Crust (geology) centers, where partial-melting of the underlying mantle yields basaltic magmas and new ocean crust forms. This "ridge push" is one of the driving forces of plate tectonics, and it is constantly creating new ocean crust. That means that old crust must be destroyed somewhere, so, opposite a spreading center, there is usually a subduction zone: a trench where an ocean plate is being shoved back into the mantle. This constant process of creating new ocean crust and destroying old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old. In contrast, the
Bulletin of Engineering Geology and the Environment and groundwater conditions of geological formations; characterization of the mineralogical, physico-geomechanical, chemical and hydraulic properties of earth materials involved in construction, resource recovery and environmental change; assessment of the mechanical and hydrological behavior of soil and rock masses; prediction of changes to the above properties with time; determination of the parameters to be considered in analyzing the stability of engineering works and earth masses; maintenance of the environmental condition and properties of the terrain. Bulletin of Engineering Geology and the Environment The Bulletin of Engineering Geology and the Environment is a peer-reviewed scientific journal covering research in geoscience. It is