Page 19 - Global Tectonics

6 CHAPTER 1

Figure 1.4 The concept of convection as suggested by Holmes (1928), when it was believed that the oceanic crust was
.
a thick continuation of the continental “basaltic layer” (a) Currents ascending at A spread laterally, place a continent

under tension and split it, providing the obstruction of the old ocean floor can be overcome. This is accomplished by the
formation of eclogite at B and C, where sub-continental currents meet sub-oceanic currents and turn downwards. The
high density of the eclogite causes it to sink and make room for the continents to advance. (b) The foundering of
eclogite at B and C contributes to the main convective circulation. The eclogite melts at depth to form basaltic magma,
which rises in ascending currents at A, heals the gaps in the disrupted continent and forms new ocean floor. Local

swells, such as Iceland, would be formed from old sial left behind. Smaller current systems, initiated by the buoyancy of

the basaltic magma, ascend beneath the continents and feed flood basalts or, beneath “old” (Pacific) ocean floor, feed

the outpourings responsible for volcanic islands and seamounts (redrawn from Holmes, 1928).
sedimentation. But as J. Tuzo Wilson, a Canadian geo- ocean basins. However, it is only since World War II and
physicist, said, this is like looking at the deck of a ship notably since 1960 that sufficient data have been

to see if it is moving. obtained from the 60% of the Earth’s surface covered
by deep water for an understanding of the origin and
history of the ocean basins to have emerged. It tran-
spires that, in contrast to the continents, the oceanic
1.2 SEA FLOOR areas are very young geologically (probably no greater
than 200 Ma in age) and that horizontal, or lateral,
SPREADING AND movements have been all-important during their history
of formation.
In 1961, following intensive surveying of the sea
THE BIRTH OF floor during post-war years, R.S. Dietz proposed the

PLATE TECTONICS mechanism of “sea floor spreading” to explain conti-
nental drift. Although Dietz coined the term “sea fl oor
spreading,” the concept was conceived a year or two
earlier by H.H. Hess. He suggested that continents
If there is a possibility that the continental areas have move in response to the growth of ocean basins between
been rifted and drifted apart and together, then presum- them, and that oceanic crust is created from the Earth’s
ably there should be some record of this within the mantle at the crest of the mid-ocean ridge system, a