We investigate the formation and coalescence of surface relief structures in symmetric diblock copolymers films of poly(styrene-b-2-vinylpyridine) (PS-PVP) on chemically patterned surfaces. The substrates have patterns of self-assembled monolayers (SAMs) produced by microcontact printing HO-terminated (HO-) SAM stripes alternating with H3C-terminated (H3C-) SAM stripes. The PS-PVP lamellae over the H3C-SAM have a defect-rich morphology compared to the film above the MO-SAM. These film sections above the H3C-SAM attract excess PS-PVP that would normally form islands on a uniform substrate. In the early stages of annealing waves of thickness develop from the H3C/HO-SAM boundary and propagate into the film over the MO-SAM. For a 3 mu m wide MO-SAM stripe a single line of excess copolymer breaks up into a single line of islands with a well-defined spacing along the grating. At long times these islands dissolve by transport of copolymer to the H3C-SAM stripe. The well-defined 2D geometry allows us to analyze the transport leading to this dissolution quantitatively. The rate-controlling process during the island dissolution is two-dimensional pressure relaxation.