Structure of the Earth obducted onto the continental crust and preserved as ophiolite sequences. Many rocks now making up Earth's crust formed less than 100 million (1) years ago; however, the oldest known mineral grains are about 4.4 billion (4.4) years old, indicating that Earth has had a solid crust for at least 4.4 billion years. Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. The mantle is divided into upper and lower mantle. The upper and lower mantle are separated by the transition zone. The lowest part of the mantle next to the core-mantle boundary is
International Association for Engineering Geology and the Environment of and the development of measures for prevention or remediation of geologic hazards. Engineering geology embraces: the definition of geomorphology, structure, stratigraphy, lithology and groundwater conditions of geological formations; the characterization of the mineralogical, physico-geomechanical, chemical and hydraulic properties of all earth materials involved in construction, resource recovery and environmental change; the assessment of the mechanical and hydrologic behaviour of soil and rock masses; the prediction of changes to the above properties with time; the determination of the parameters to be considered in the stability analysis of engineering works and of earth masses; and the improvement and maintenance of the