On a global scale, shifts in sea level recorded by stratigraphers and on seismic profiles stem from one of two main processes: changes in land-ice volume and the volume of the ocean basins. The latter most often results from changing rates of sea-floor spreading, so that when it is rapid a greater volume of the lithosphere near spreading centres retains sufficient buoyancy to displace the oceans onto continental margins. During slow spreading, cooling of the lithosphere and an increase in its density enlarges the deep abyssal plains, so that the oceans withdraw to low levels. The mid-Cretaceous saw vast outpourings of plume-related lavas onto the floor of the West Pacific. So large, that they reduced the volume of the Pacific basin enough to result in continental flooding that was unprecedented in the Phanerozoic Eon.
On a local scale, changes in sea level recorded by the stratigraphic record include those due to local processes, generally ascribed to tectonic events at continental margins, which involved rising continental lithosphere. However, one of the greatest forces for local change in the continental freeboard is changing density of the lithosphere due to thermal effects. Anywhere once affected by major igneous events should record relative falls in sea level during the acme of magmatism, and rises when activity waned. The British Tertiary Igneous Province, a precursor to the eventual rifting of the North Atlantic under the influence of the Iceland plume is a good candidate for charting magma-sea level connections. The central volcanic complexes of the Hebrides, and their enveloping flood basalt piles formed at the start of the Palaeocene (~60 Ma). Around that time, much of the British Isles underwent several kilometres of vertical uplift and exhumation, whose effects remain today. In the surrounding marine basins, this event is recorded by Palaeogene sandstone bodies, presumable derived by erosion of the uplifted crust. Yet local Palaeogene sediments also record episodes of rising sea level. John Maclennan and Brian Lovell of the French Institut de Physique du Globe and Cambridge University have modelled the likely effect on sea levels around the British Isles by crustal underplating of magmas formed during the BTIP magmatism (Maclennan, J. & Lovell, B. 2002. Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of the Earth’s crust. Geology, v. 30, p. 675-678).
Up to 8 km of mafic igneous rocks seem to have ponded at the base of the British Isles’ crust while the BTIP was active. This estimate stems from the fact that the lavas of the province evidence high-pressure fractional crystallization. Calculations of the percentage of cumulates needed to generate the bulk chemistry of the BTIP lavas suggest that their volume far outweighs that of the volcanic part of the province. Given estimates of the volume of underplated cumulates, modelling boils down to examining the consequences for lithospheric density of initial heating and its subsequent relaxation. The Palaeogene sedimentary record provides good support for the model, with massive uplift from 60-56 Ma (the period when the BTIP was forming). Sudden sea-level rise at the end of this period never reached the level prior to magmatism; in fact it amounts to one half the estimated uplift. That is precisely in line with the underplating model.