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In-Depth Information
process starts by continental rifting? Heat accumulation
takes 30-80 Ma to reach the point of rifting, during which
time light continental lithosphere is replaced by rising,
denser asthenosphere in the plume. Isostatic adjustments
cause this new crust to 'float' at a lower level on the mantle
as the continent rifts apart, flooding 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 geo-
chemistry, architecture, associated volcanic activity and
the duration of the oceanic stage of the Wilson cycle are
described in
Chapters 11
and
12.
Mid-ocean ridges
The
thermal welt
or dome over a rising mantle plume
stretches, thins and weakens the lithosphere. The conse-
quent fall in
overburden pressure
lowers the melting point
of asthenosphere, which rises faster than it cools. Rock-
forming processes are dealt with in detail in
Chapter 12.
It is sufficient here to appreciate that the fractionation of
different rocks is most intense in the lithosphere and that
a more buoyant
gabbro
-basalt mixture segregates and
accelerates away from its denser, parent asthenosphere
peridotite
at depths of 15-25 km to form subsurface
magma reservoirs. Magma creates new layered oceanic
crust where it penetrates the lithosphere and inevitably
leaves behind depleted peridotite. Gabbro cools to form
a subsurface
intrusive
layer 4-6 km thick, whilst the basalt
continues to the surface, forming an
extrusive
layer of lava
1-2·5 km thick. Volcanic activity is also associated with
mid-ocean ridges and is seen best where the ridge surfaces
in the Atlantic Ocean at the volcanic islands of Iceland,
the Azores, Ascension and Tristan da Cunha.
The focus of this activity forms a topographic rise or
ridge
1-3 km high in the sea bed (
Figure 10.8
).
Extension
faulting along its axis heart triggers shallow seismic
activity and may create a central rift in slow-spreading
ridges. Since the asthenosphere feeds the magma reser-
voir and thereby the continuous formation of oceanic
lithosphere, during the lifetime of the cycle the ridge
system eventually achieves widths of 10
3-4
km. Why do we
speak of oceanic crust and mid-ocean ridges when the
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
Active volcano
Paired palaeomagnetic
bands
TRANSFORM
FAULT
Seamount
OCEAN
WATER
COLUMN
0
BASALT
LAVA
S
e
a
-fl
oo
r
s
pr
e
ad
in
g
2.5
INTRUSIVE
GABBRO
5.0
GA
B
B
R
O
-B
A
SA
L
T
M
AG
M
A
R
E
SE
R
V
O
IR
7.5
PERIDOTITE
Figure 10.8
ocean ridge, drawn at right-
angles to the ridge axis.
Source: After Kearey and Vine (1996)
10.0
D
E
PL
E
TE
D
P
E
R
ID
O
TI
TE
2
0
km
RIDGE AXIS
