Story highlights. 1  A pregnancy previously was considered term at 37 weeks. 2  Recent research prompted the change.  3 Early term now begins at 37 weeks of pregnancy.
Subduction Zones. Where two tectonic plates converge, if one or both of the plates is oceanic lithosphere, a subduction zone will form. An oceanic plate will sink back into the mantle. Remember, oceanic plates are formed from mantle material at midocean ridges. Young oceanic lithosphere is hot and buoyant (low density) when it forms at a midocean ridge. But as it spreads away from the ridge and cools and contracts (becomse denser) it is able to sink into the hotter underlying mantle.
Oceanic trench. Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches. Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lit