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
Geology of the Iberian Peninsula in the study of geology in Spain. In 1972 the MAGNA plan was created to produce 1:50000 scale maps of Spain (this work began in Portugal in 1952). Between 1986 and 1987 a Spanish-French collaboration called ECORS-Pirineos made a seismic vertical reflection profile 250 km long through the Pyrenees. By 2000 there were six seismic profiles through the Pyrenees which provided a wealth of information of the thickness of the crust and the internal structure of the orogen. Between 1852 and 1857, Carlos Ribeiro made a geological map at 1:480 000 scale of the Portuguese region between the Douro and