Hotspot tracks are age-progressive chains of volcanoes or volcanic islands and seamounts that are believed to form as tectonic plates drift over more-or-less stationary plumes of upwelling mantle rocks. Age progressive means that only the volcano at the end of the track is active and the extinct volcanoes get older the farther they are from the active hotspot. The old volcanoes formed while they were over the mantle plume but have slowly (speed that your fingernail grows) moved away, as the plate slowly drifted away from its midocean ridge.
Mantle Plume: Partial melting of the upwelling mantle peridotite occurs as it approaches the surface due to the reduction of pressure on the hot rocks. This partial melting (decompression melting) produces mafic magma from the ultramafic mantle peridotite rock.