Page 64 - Fundamentals of Physical Volcanology

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MAGMA MIGRATION 41

Table 3.2 The average widths of the dikes containing the columns of magma described in Table 3.1.
(a) Oceanic island volcano.

Depth of base of continuous Average width of dike (m)
magma pathway (km)
Magma density 3000 kg m −3 Magma density 2700 kg m −3
9.0 5.8 1.6
10.0 7.7 5.1
10.7 9.1 7.6
15.0 17.8 24.0
20.0 29.1 45.1
21.4 32.4 51.4
(b) Typical continental crust.

Depth of base of continuous Average width of dike (m)
magma pathway (km)
Magma density 3000 kg m −3 Magma density 2700 kg m −3

35 103 28
40 143 102
45 184 179
50 226 257

• Diapirs slow down as they rise to shallower depends on the tensile strength of the rock, the
depths because decreasing temperature makes compressive load due to the weight of overlying
the host rocks more viscous. Diapirs cease to rise rocks, and the presence of any gas, released from
when they cease to be buoyant, which com- the magma, that has accumulated in the tip of the
monly happens when they reach the base of the growing fracture.
crust, where the rock density is much less than • Dikes can become trapped, i.e., cease to grow,
in the mantle. when the stress at the dike tip becomes less than
• The rise speed and size of a diapir control the rate the local fracture toughness. If a dike ceases to
at which it strains the host rocks as they move grow in this way and there is still magma avail-
aside to make way for its passage. If the strain rate able in the source region, a new dike may pro-
becomes too high, the response of the host rocks pagate from the source region.
changes from viscous to elastic and they fracture, • The sizes and shapes of dikes propagating from
allowing a dike to start to grow. This will tend deep magma sources are controlled by the distri-
to happen at a greater depth for a large mantle bution of stress and strength in the rocks through
plume diapir than for a smaller, slower-rising which they pass, and many configurations of the
subduction zone diapir. end products are possible. In oceanic environ-
• The rate at which melt can drain from a partially ments these include the very localized feeding
molten region into a rapidly growing dike is lim- of mid-ocean ridge volcanoes and the forma-
ited by the viscosity of the magma rather than the tion of chains of shield volcanoes above mantle
potentially very rapid growth rate of the dike. plumes well away from ridges. In continental
The ability of a dike to start to grow, and the final environments dikes may be trapped to form giant
limit on its growth, are controlled by the appar- dike swarms or may erupt to form flood basalt
ent fracture toughness of the host rock, which provinces.