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.
Simplified plate tectonics cross section (A to B). Simplified plate tectonics cross-section showing how Santa Maria Volcano is located above a subduction zone formed where the Cocos and Caribbean plates collide.