Environmental Engineering Reference
In-Depth Information
areas below the contemporary sea level. Continued sea-floor spreading gives birth to a
new ocean in which hot new crust reacts with sea water. This and other aspects of ocean
geochemistry, architecture, associated volcanic activity and the duration of the oceanic
stage of the Wilson cycle are described in Chapter 11.
SUBDUCTION ZONES
We know that as the cycle proceeds, and continental lithosphere is rolled away,
subduction must be induced elsewhere. It occurs through the cooling and thickening of
spreading oceanic lithosphere and is enhanced by renewed basal adhesion to the
asthenosphere, which itself spreads and cools where it is in contact with cold lithosphere.
Where continental and oceanic lithosphere converge, the greater density of the latter
ensures that it is always subducted, but cold oceanic crust will also be subducted beneath
oceanic crust where regional Earth stresses permit. The consequences are a unique global
landsystem and hydrothermally driven geochemical reactions on the
resorption
of
lithosphere. They may be oceanic or continental in style and location, and can be
differentiated further into primary mechanical/isostatic effects, or secondary geochemical
reworking of lithosphere and mantle. Indeed, the notion that subduction zones are
destructive
margins is only partially correct. It is not the original oceanic lithosphere
which is recycled but a version altered by hydrothermal reaction with sea water on its
emplacement, together with sea water itself, ocean sediments and fragments of adjacent
continental crust and asthenosphere. It perpetuates fractionation processes and is
responsible for a wide range of
constructive
materials which form new continental crust,
tectonic landforms such as mountain belts and the majority of surface volcanoes.
Subduction proceeds by the displacement of lithosphere with negative buoyancy,
acquired by densification and cooling, down a plane inclined away from the direction of
spreading oceanic lithosphere. Known as the
Wadati-Benioff
or, commonly,
Benioff
or
B
-
subduction
zone, it promotes deep seismic activity. Descending slab is heated by
conduction, on contact with hotter lithosphere, and friction in the narrow seismic zone. It
undergoes
metamorphism
or physico-chemical alteration and eventually melts, at a
depth set by the thermal and pressure environment of the subduction zone and its own
thickness and geochemistry. Melting would occur at greater depths than at constructive
margins, since the critical 1,400° C isotherm may be drawn down over 200 km below the
surface by the cold, descending slab. However, contamination by surface materials
reduces the initial melting temperature to less than 650° C. Ocean-wetted basaltic
lithosphere, in particular, begins to melt at only 80 km and the water driven off aids the
melting of peridotite in adjacent continental asthenosphere. Subduction eventually ceases
when thermal equilibrium is reached with the surrounding mantle at depths of 600-700
km. By then it has created the most unstable and complicated global surface architecture
of marine basins, volcanoes and mountains (Figure 10.7).
VOLCANIC ISLAND ARCS AND OCEAN TRENCHES
A glance at an atlas shows that most island systems form curved 'necklaces' strung out
towards ocean margins. An almost continuous string in the western Pacific extends from
the Aleutian Islands of Alaska, south through the Kuril Islands, Japan, the Marianas, the
