plate at a subduction zone drags the rest of the plate along behind it. The slab pull mechanism is considered to be contributing more than the ridge push. A process previously proposed to contribute to plate motion and the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" due to deep convection (see image). However, some studies have shown that the upper mantle (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along. Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from seismic tomography and observations of the seismic discontinuity in the upper mantle at about 400 km (250 mi). On the other hand, some of the world's largest tectonic plates such as the North American Plate and South American plate are in motion, yet only are being subducted in restricted locations such as the Lesser Antilles Arc and Scotia Arc, pointing to action by the ridge push body force on these plates. Computer modeling of the plates and mantle motions suggest that plate motion and mantle convection are not connected, and the main plate driving force is slab pull. Impact on global sea level Increased rates of seafloor spreading (i.e. the rate of expansion of the mid-ocean ridge) have caused the global (eustatic) sea level to rise over very long timescales (millions of years). Increased seafloor spreading means that the mid-ocean ridge will then expand and form a broader ridge with decreased average depth, taking up more space in the ocean basin. This displaces the overlying ocean and causes sea levels to rise. Sealevel change can be attributed to other factors (thermal expansion, ice melting, and mantle convection creating dynamic topography). Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges. The 100 to 170 meters higher sea level of the Cretaceous Period (144–65 Ma) is partly attributed to plate tectonics because thermal expansion and the absence of ice sheets only account for some of the extra sea level. Impact on seawater chemistry and carbonate deposition Seafloor spreading on mid-ocean ridges is a global scale ion-exchange system. Hydrothermal vents at spreading centers introduce various amounts of iron, sulfur, manganese, silicon, and other elements