Currently, areas of oceanic upwelling cause the formation of phosphates. AMCG plutonism is generally associated with asthenospheric upwelling under thinned lithosphere. Coastal zones provide nitrogen from runoff and upwelling occurs readily along the coast. The Pitcairn Islands were formed by a centre of upwelling magma called the Pitcairn hotspot. The most common occurrence of phosphorites is related to strong marine upwelling of sediments. The isentropic upwelling solid mantle material exceeds the solidus temperature and melts. It formed as a result of the upwelling of magma from within the Earth's crust. Natural causes include coastal upwelling and changes in wind and water circulation patterns. Rainfall is very high away from cold ocean current upwelling zones, from per year. Nonetheless, some diffusive upwelling does probably occur. Changes in bathymetry can affect the strength of an upwelling. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. It was suggested that the cause was mantle upwelling. This prevents nutrient-rich upwelling which has reduced productivity. Nitrate is depleted in near-surface water except in upwelling regions.