Geology Reference
In-Depth Information
representative of the main phase of island formation. Also, even an accurate age for
the main phase of building an island gives only a lower bound on the age of the sea
floor upon which it is built. This is because the sea floor will have the same age as
the island if the island formed at a spreading centre, but it will be older if the island
formed away from the spreading centre.
Despite the limited data available at the time, this phase of Wilson's work led
him to his first seminal idea, which was a mechanism to explain age progressions
in island chains. Darwin had been the first to recognise an apparent age sequence
among Pacific islands, from an island with fringing reef, through an island with
barrier reef, to an atoll. However, it was Dana who first observed such a sequence
among adjacent islands, and he extended the sequence to include an initial vol-
canically active island ([7] p. 195). Wilson was aware that the Hawaiian island
chain was a classic example of such a sequence, though the ages of the islands
were known only qualitatively. With the idea of sea floor moving sideways, he
realised that Dana's inferred age sequence for the Hawaiian islands could be pro-
duced if there was a (relatively) stationary source of volcanism deep in the mantle
that had generated the islands successively as the sea floor passed over [25]. He
conjectured that this 'hotspot' source might be located near the slowly moving
centre of a convection 'cell'. This particular mechanism was soon superseded by
the idea of a mantle plume, which we will develop later in the topic, but the
idea of a relatively stationary deep-mantle source of volcanism is common to both
hypotheses. Accurate dating of the islands soon provided strong support for the idea
[26, 27].
Wilson was a physicist turned geologist with very broad interests in geology, but
he had a particular interest in large transcurrent faults. His recognition of the North
American equivalent (the Cabot fault) of Scotland's Great Glen fault fuelled his
interest in continental drift. In 1963, the same year his work on island chains was
published, he also published a very wide-ranging discussion of continental drift
[28]. In this, it is clear he had a comprehensive grasp not only of a large number
of geological observations but also of the arguments from isostasy, post-glacial
rebound, materials science and gravity observations over ocean trenches that the
mantle is deformable and plausibly undergoing convection, as will be discussed in
the next chapter.
Wilson was also puzzled by the great fracture zones that were being discovered
on the ocean floor, because they seemed to be transcurrent faults of large displace-
ment, but they stopped at the continental margin, with no equivalent expression on
the adjacent continent. His clinching insight was his recognition of the way these
great faults can connect consistently with mid-ocean ridges or with 'mountains'
(meaning island arcs or subduction zones) if pieces of the crust are moving relative
to each other as rigid blocks without having to conserve crust locally. This led him
