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54 CHAPTER 4

Arnamurchan

N

Mull Sound of
Mull

Lorne
Staffa

of
Iona
Firth

Fig. 4.14 The Mull and Ardnamurchan
10 km
Tertiary intrusive complexes on the
west coast of Scotland. The rather
Tertiary basalt equant shapes of these complexes
Mesozoic sediments Dalradian schists
lavas
are similar to the shapes implied by
geophysical studies for modern magma
Lower Old Red chambers. (Modified from fig. 6 in
Tertiary igneous Moine schists
Sandstone lavas
rocks and gneisses Richey, J.E. (1961) Scotland: the
and sediments
Tertiary Volcanic Districts. HMSO,
Edinburgh. Reproduced courtesy
Caledonian granite Lewisian gneiss of the British Geological Survey.
IPR/92-27C.)
decades to centuries depending mainly on the Fig. 4.17 shows an example of a dike in which fresh
thickness of the intrusion and the temperature magma has been emplaced through the middle of
contrast between the magma and country rocks an earlier dike at a stage when the magma in the
(Fig. 4.16). An initial intrusion can develop into dike is cool enough to have almost solidified. In
a long-lived magma chamber only if the magma other cases fresh pulses of magma are injected into
within it is prevented from solidifying. This can be dikes which have experienced only small amounts
achieved only by the repeated input of heat to the of cooling and solidification.
proto-chamber in the form of fresh magma. The The injection of fresh magma from deeper levels
most likely way for this to happen is if the initial heats the magma already in the sill and the sur-
feeder dike is reused by fresh magma traveling rounding rocks and in doing so reduces the cooling
upwards from deeper levels. Reuse of dikes by rate and lengthens the time that the enlarged sill
fresh batches of magma is a well-documented will take to solidify. At this stage in the evolution
phenomenon. This reuse sometimes occurs when of the sill it is still highly susceptible to cooling and
the dike is almost totally solidified. For example, solidification. With each fresh pulse of magma which