Việc chuyển tấm thạch quyển tạo ra lớp vỏ mới đại dương trong một chu kỳ liên tục trẻ hóa của vỏ trái đất. Thạch quyển subducting lưu thông qua lớp vỏ và reemerges macma ở hàng chục hay rặng núi để midocean trên khắp thế giới, tạo ra hơn một nửa của vỏ trái đất. | MARINE GEOLOgy THE MIDOCEAN RIDGES The shifting lithospheric plates create new oceanic crust in a continuous cycle of crustal rejuvenation. The subducting lithosphere circulates through the mantle and reemerges as magma at a dozen or so midocean ridges around the world generating more than half of Earth s addition of new basalt on the ocean floor is responsible for the growth of the lithospheric plates upon which the continents ride. A large part of this activity takes place in the middle of the Atlantic Ocean where molten rock welling up from the upper mantle generates new sections of oceanic floor of the Atlantic acts like two opposing conveyor rollers are convection loops in the upper mantle transporting oceanic crust in opposite directions outward from its point of origin at the Mid-Atlantic Ridge. The spreading ridge system runs from Bovet Island about 1 000 miles north of Antarctica to Iceland itself a surface expression of the Mid-Atlantic Ridge Fig. 64 . Extensive volcanism gives rise to volcanic islands such as Surt-sey Fig. 65 located 7 miles to the midocean ridge is a string of volcanic seamounts created by molten magma upwelling from within the mantle. Running down the middle of the ridge crest is a deep trough as though a giant crack were carved in the ocean s crust. This trough reaches 4 miles in depth and up to 15 miles wide making it the greatest chasm on Earth. The submerged mountains and undersea ridges form a continuous chain 46 000 miles long Fig. 66 . It is by far the largest structure on the planet surpassing in scale all mountain ranges on land. The mountainous belt is several hundred miles wide and up to 10 000 feet above the ocean floor. When starting from the Arctic Ocean the ridge spans southward across the Atlantic basin continues around Africa Asia and Australia runs under the Pacific Ocean and terminates at the west coast of North America. Figure 64 Iceland straddles the Mid-Atlantic Ridge. 88 .
