Geoscience Reference
In-Depth Information
(a)
(b)
i
i
ii
ii
iii
iii
iv
2
v
iv
2.0
3.0
Ma
10 km
v
0
10
Ma
50 km
Figure 3.9.
(a) A cross section through the upper part of the oceanic crust, showing
the magnetized layer generated by extrusion of lava randomly in both time and
space within an emplacement zone centred on the ridge axis. The half-spreading
rate is 1 cm yr
−1
. Black, normally magnetized rock; white, reversely magnetized rock.
(b) Theoretical magnetic anomalies generated by this random-extrusion model. In
both (a) and (b), (i), (ii) and (iii) are for an emplacement zone 10 km across, (iv) is for
an emplacement zone 2 km across and (v) is for the block model in which the
emplacement zone has zero width. (From Schouten and Denham (1979).)
anomalies calculated for the Juan de Fuca Ridge and the East Pacific Rise using
this 1966 reversal timescale. These theoretical anomalies match the actual anoma-
lies very well and were used to confirm the
Vine-Matthews hypothesis
.Figure
3.11 shows a more recent (and therefore more detailed) determination of the
reversal sequence for the last 4 Ma.
A geomagnetic timescale extending back 80 Ma was first established in 1968
by assuming that the spreading rate in the South Atlantic had remained constant
from 80 Ma until now. The observed anomaly pattern was matched with theoret-
ical profiles computed for a sequence of normal and reversed magnetized blocks
symmetrical about the Mid-Atlantic Ridge axis (Fig. 3.12). The timescale derived
