Environmental Engineering Reference
In-Depth Information
(Coats 1962). The genesis of this new science is detailed
in Le Grand (1988), Oreskes (1999 and 2001), and
D. M. Lawrence (2002), and its advances are reviewed
in Bebout et al. (1996), Richards, Gordon, and Hilst
(2000), Stein and Freymueller (2002), and Eiler (2003).
The Earth's lithosphere consists of irregularly shaped,
semirigid oceanic and continental plates that are sepa-
rated by divergent (constructive), convergent (destruc-
tive), and transform boundaries, which meet at triple
junctions. The seven largest plates cover 94% of the
Earth's surface, but there are twenty plates in total, an
equal number of broad zones of deformation, and areas
with accreted terranes (crustal blocks whose composition
is different from their surroundings). The African plate
may actually consist of two blocks, the Nubian and the
Somalian, which are splitting along the East African Rift
(Djibouti to Mozambique). Continental plates are thick
(typically 10 km and up to 200 km) and nearly station-
ary, with parts composed of rocks more than 3 Ga old.
Relatively thin (@10-15 km) oceanic plates are created
at the ocean ridges as basaltic magma rises from the man-
tle and as plate divergence (spreading) produces new
ocean floor.
Fast-spreading ridges (8-18 cm/a) have low axial
highs, slow-spreading ridges (
<
5.5 cm/a) have deep rift
valleys, and some 20,000 km of the roughly 55,000 km
global ridge system belong to an ultraslow-spreading
class with annual divergence of less than 2 cm/a (Dick,
Lian, and Schouten 2003). Annual extrusion of oceanic
basalts amounts to nearly 20 km
3
(@56 Gt), and because
their latent heat of fusion is almost 400 kJ/kg, their pro-
duction requires only about 700 GW of heat. An average
spreading rate of 3 cm/a creates nearly 2 km
2
of new
ocean floor, sufficient to form the current floor of about
310 million km
2
no sea floor older than about 200 Ma because the ocean
crust
is
repeatedly recycled by subduction into the
mantle.
Plate collisions result in massive deformation as well as
in subduction (fig. 2.9). Major mountain ranges (Hima-
layas, Andes, Tianshan) are the most spectacular results
of continental deformation. The Himalayas, the highest
chain, began to form about 55 Ma ago when India col-
lided with Asia, and this process also gave rise, in a step-
wise manner, to the high Tibetan plateau (Tapponnier
et al. 2001). Deep ocean trenches (Mariana, Kuril-Japan,
Tonga, Sunda, Alaska) mark the most active subduction
zones where cooler and denser slabs penetrate deep into
mantle, and there are also zones of diffuse oceanic trans-
formation (mainly in the Indian Ocean) and relatively
small areas of ridge-transform systems, above all, along
the mid-Atlantic Ridge and the Pacific-Antarctic Rise.
According to Kreemer, Holt, and Haines (2002), 63%
of the minimum tectonic moment rate within all plate
boundary zones (totaling 7
10
21
Nm/a) are associated
with subduction, 14% with continental transformation,
19% are expected for ridge-transform systems, and just
4% for diffuse ocean zones.
Compression keeps the plates together, and once the
stress changes, plates may get reconfigured (D. L. Ander-
son 2002). Every new grand tectonic cycle begins with
the breakup of a supercontinent (as the mantle heat
trapped beneath it eventually rises to the surface in the
form of massive injections of basaltic magma that pro-
duce juvenile crust) and ends with renewed continental
amalgamation (Murphy and Nance 2004). During
the last billion years the Earth has experienced the forma-
tion and breakup of three supercontinents: Rodinia
(assembled@1.1 Ga ago, broke up@760 Ma ago), Pan-
notia (formed @650 Ma ago, broke up @540 Ma ago),
in only about 175 Ma. Indeed, there is
