408 EARTH/Crust Figure The Earth’s crust floats on a dense mantle that be haves as a viscous liquid If we place an ice cap on it, enough mantle material is displaced sideways to equal the additional crustal load, and a low broad welt is raised peripheral to the ice cap The mantle adjustment is very slow, so that depression and rebound after removal of the ice cap are delayed The situ ation at present in Greenland is shown in the lower part of the figure, and the delayed rebound is being experienced in the Baltic From Van Andel TJ (1994) New Views on an Old Planet Cambridge: Cambridge University Press ice-sheet which has sagged below sea-level in places (Figure 5), and also in the rebound occurring now in the Baltic after the ice-sheet has gone – the renowned Swedish geologist, Harry von Eckerman, used to go to the Alnoă Island carbonatite complex yearly to see what new rocks had surfaced due to this rebound Heat Flow to the Crustal Surface The continental surface heat flow is linearly related to the heat productivity of the near-surface granitic rocks Variability from region to region is mainly related to the distribution of near-surface radioactivity, derived from certain minerals in the granitic rocks Most models favour an exponential decrease in radioactive heat source with depth in the continental crust and a residual amount of heat rising from the upper mantle The weighted average heat flow from both continental and oceanic crust is 1.5 heat flow units (HFU) This equivalence is explained by the heat flow to the oceanic crust surface coming mainly from the mantle, whereas it comes mainly from radioactive minerals in the crust at the continental surface This requires increased radioactive sources in the mantle under the oceans, or that convective heat from the mantle under the oceans happens to equate to the radioactive-sourced heat from the continental crust, or that the mantle-derived Figure A sectional diagram of the French entry section to the Channel Tunnel showing tectonic deformation (folding, faulting) of the Cretaceous and Quaternary strata of the crust Extract from BREAKTHROUGH by Derek Wilson published by Century Hutch inson Reproduced from The Random House Group Limited convective heat is much the same under continental and oceanic lithosphere, but the thicker continental lithosphere is more depleted in radioactive heat sources than the oceanic lithosphere The latter is preferred because it allows equal movements of both continental and oceanic crust-dominated plates Crustal Deformation The crustal rocks of the Earth are subject to many and diverse deformation processes The most significant are ‘tectonic’ processes (folding and faulting; Figure 6), which act very slowly through long periods of geological time and are mostly related to the movement of tectonic plates (especially collision and subduction) Stress on faults is subject to long build-up, but may be relieved by abrupt, almost instantaneous dislocation (which may form a linear earthquake scarp on the surface; Figure 7), or prolonged dissipation by creep movements without any earthquake Earthquake foci are mostly situated within the crust, but some, especially those in margins of continental areas of plates, may have foci several hundred kilometres deep, within the mantle (see Tectonics: Earthquakes) In contrast with tectonic deformation, ‘superficial’ deformation of the crust, such as landsliding, sink-hole formation, and submarine slumping, occurs abruptly, within a matter of seconds, minutes, hours, or days (Figure 8) (see Sedimentary Processes: Landslides)