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
The Layer Monument Layer Quaternity correspond to the alchemical "deities" of Apollo, Luna, Mercurius and Vulcan as named in "Atalanta Fugiens" (1617) by the German alchemist-physician Michael Maier (Emblem XVII). Collectively the Layer Quaternity are a unique alchemical mandala. Through polarized symbolism they delineate essential coordinates associated with Mandala art, namely Space (Heaven and Earth) and Time (Young and Old). Utilizing variety and multiplicity, key attributes of Northern Mannerist art, they also represent fundamental aspects of the human condition, namely, gender, youth and age, pleasure and suffering. A fifth, uniting symbol, a skull, is located at the very centre of the monument. The