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 a thinned piece of crust from another continent called the Meguma terrane. There are five units in the stack. At the lowest level are high pressure, low temperature metamorphosed rocks. Second is an ophiolite. Third is the lower part of a continental crust which has been metamorphosed to high temperature with high pressure. Fourth is a layer of sediments derived from weathering land with low-grade metamorphism. There is also an underlying Ediacaran and early Paleozoic layer called the autochthenous sequence. Metamorphism of the allochthenous nappe occurred 390–380 Ma in the Middle Devonian. This is possibly from the Rheic Ocean. Finally,