Hotspots are sites of upwelling of relatively hot mantle, possibly associated with mantle plumes, that cause partial melting of the asthenosphere. This type of magmatism forms volcanic seamounts or oceanic islands when they become emergent. Over short geological timescales the hotspots appear to be fixed relative to one another, forming a reference frame against which plate motions can be measured. As tectonic plates move relative to a hotspot, the location of magmatic activity on the plate shifts, causing the development of time-progressive chains of volcanoes such as the Hawaiian-Emperor seamount chain. The main product of hotspot volcanoes are Ocean Island Basalts (OIB), which are distinct from MORB and IAB type basalts.