Oceanic crust has a crust made up of basalt and gabbro and the upper mantle consits of peridoite. Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is denser, having a mean density of about 2.9 grams per cubic centimeter.

Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling. Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers.

Ioánnis Fokás (Greek: Ιωάννης Φωκάς), better known by the Spanish transcription of his name, Juan de Fuca (born 1536 on the Ionian island of Cefalonia; died there 1602), was a Greek-born maritime pilot in the service of the king of Spain, Philip II. He is best known for his claim to have explored the Strait of Anián, now known as the Strait of Juan de Fuca, between Vancouver Island, now part of British Columbia, Canada, and northwestern Washington State, United States.

Fokás's grandfather, Emmanouíl Fokás (Gk: Εμμανουήλ Φωκάς), fled Constantinople at its fall in 1453, accompanied by his brother Andrónikos (Gk: Ανδρόνικος). The two settled first in the Peloponnese, where Andrónikos remained, but in 1470 Emmanouíl moved to the island of Cefalonia. Iákovos (Gk: Ιάκωβος), Ioánnis's father established himself in the village of Valerianos (Gk: Βαλεριάνος) on the island and came to be known as "the Valeriáno Fokás" (Gk: ο Φωκάς ο Βαλεριάνος) to distinguish him from his brothers.

It was in this village of Valeriáno that Fokás was born in 1536. Little to nothing is known about his life before he entered the service of Spain, some time around 1555.