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
International Association for Engineering Geology and the Environment International Association for Engineering Geology and the Environment The International Association for Engineering Geology and the Environment (), formerly International Association for Engineering Geology, also known as IAEG (), is an international scientific society which was founded in 1964. It is affiliated with the International Union of Geological Sciences (IUGS) and has 3,798 members and 59 national groups all over the world. The association aims to promote and encourage the advancement of engineering geology through technological activities and research. At the same time it strives to improve teaching and training in engineering geology, and to collect, evaluate and disseminate the