Geology Reference
In-Depth Information
Amer.
Africa
EPR
MAR
MIR
(a)
(b)
Internal heating
Figure 3.1. (a) Heezen's conception of the convection cells underlying the mid-
ocean ridge system. EPR, East Pacific Rise; MAR, Mid-Atlantic Ridge; MIR,
Mid-Indian Ridge. (b) Internally heated convection has active downwellings, but
no active upwellings. Ridges are therefore free to move.
the form of the textbook-style 'cells' sketched in Figure 3.1(a). This form occurs
only when the fluid is heated from below, thus generating hot upwellings, and only
if the heating is not too strong. If the heating is stronger, upwellings and down-
wellings tend to move about. More importantly here, if the fluid is heated internally,
which is a more accurate picture of the mantle case, there are no hot upwellings
(Figure 3.1(b)). There must be upwelling between the active downwellings, but it
is simply the fluid being passively displaced by the downwellings. If there are no
hot, active upwellings, then a spreading centre can be located anywhere, or can
move about. We will look at this again in a later chapter.
3.2 Wilson, plumes and plates
Tuzo Wilson cut through the prevailing confusion when he followed the logic of
surface features, without worrying about what might be happening underneath [23].
The seafloor spreading papers of Hess and Dietz led Wilson to change his ideas.
Until then he had been a 'fixist', believing that continents did not move, but those
papers convinced him that they must move. He set about finding more evidence.
This led him to not one but two seminal ideas, both central to understanding mantle
convection. The first idea did involve thinking about what is below the surface, but
the second did not.
Wilson first thought of using ocean islands as probes of the sea floor, reasoning
that islands should be progressively older at greater distances from spreading cen-
tres [24]. The idea was good, but the data were scattered and somewhat misleading,
since some islands include fragments of continental crust, and other ages were not
