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ocean
trench
ocean
ridge
crust behaved like a giant conveyor belt,
rising at the ridges and moving side-
ways towards the deep-ocean trenches
where it descended (Figure 3.4). In other
words, both continents and oceans were
mobile rather than static. Wegener had
thought that the ocean ridges marked
the lines of separation of the conti-
nents, but palaeomagnetic dating of the
ocean floor of the Atlantic and Indian
oceans in the 1960s showed that the
ridges were the most recently formed
parts, and that the ocean floor became
older towards the continental margins.
The dating of the ocean crust relies
on the fact that new crust formed along
the ocean ridges becomes imprinted
with the contemporary magnetic field,
and this changes periodically by swap-
ping magnetic north and south poles.
Each change creates a long strip of
crust, parallel to the ridge axis, whose
magnetic character differs from the pre-
vious one, and as new strips are created,
older strips move away from the ridge
axis. This process creates a series of
strips (or
magnetic stripes
) on the
ocean floor, each representing a particu-
lar period of formation (Figure 3.5). The
stripe sequence can be calibrated with
reference to dated lava flows on land.
The age pattern of the oceanic crust
could then be used as evidence that
the Atlantic Ocean, for example, has
been formed by the creation of new
oceanic crust at the mid-Atlantic ridge,
as shown in Figure 3.6A. A uniform
pattern of ages is evident, becoming
younger towards the present ridge axis.
The oceanic crust adjacent to those con-
tinental margins that had moved apart
showed a 'concordant' age pattern, the
oldest ages being consistent with the
date of separation of the continents.
continent
ocean crust
A
destruction of
old ocean crust
creation of new
ocean crust
ocean
ridge
continent
continent
ocean crust
B
creation of new
ocean crust
Figure 3.4
The 'conveyor-belt' model. Schematic cross-section showing how ocean crust is created
and destroyed:
A.
Creation of new oceanic crust at an ocean ridge and destruction of oceanic crust
at an ocean trench;
B.
How continents move apart by creation of oceanic crust.
continental
margin
continental
margin
active
rift
P
P
3.4-2.5Ma
3.4-2.5Ma
A
normal
polarity
active
rift
Q
Q
2.5-
0.75
Ma
P
2.5-
0.75
Ma
P
B
reversed
polarity
active
rift
R
R
Q
Q
P
P
0.75-
0Ma
0.75-
0Ma
C
normal
polarity
Figure 3.5
Oceanic magnetic stripe pattern. Schematic representative cross-section of oceanic crust
resulting from an opening event at 3.4 million years (Ma) ago. New crust (P) is added at the central
rift on the ocean ridge from 3.4-2.5 Ma added during mainly normal magnetic polarity; new crust Q
added from 2.5-0.75 Ma is added during mainly reverse polarity; and from 0.75 Ma to the present,
new crust (R) is added during normal polarity.
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