Page 61 - Fundamentals of Physical Volcanology

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38 CHAPTER 3

Table 3.1 The influences of the densities of magmas and the tectonic setting, which controls the density structure of
the crust and mantle, on how close to the surface magma can rise as a function of the depth from which it starts.
(a) Oceanic island volcano.
Depth of base of continuous Depth below surface of shallowest dike top (km)
magma pathway (km)
Magma density 3000 kg m −3 Magma density 2700 kg m −3
9.0 1.24 0.38
10.0 1.14 0.16
10.7 1.07 0.00
15.0 0.64 Magma
20.0 0.14 reaches
21.4 0.00 surface

(b) Typical continental crust.
Depth of base of continuous Depth below surface of shallowest dike top (km)
magma pathway (km)
Magma density 3000 kg m −3 Magma density 2700 kg m −3
35 2.3 Magma
40 1.8 always
45 1.3 reaches
50 0.8 surface

Then subtracting z from the total source depth base of the crust but can rise into it, although this
magma
(z + z ) gives the depth below the surface of magma can never reach the surface.
crust source
the top of the dike containing the magma.
Table 3.1 shows the result of carrying out this There is a final point to be made here. In addition
calculation for the density profiles shown in Fig. to all of the above issues, we should also apply
3.3. For the oceanic island case, dikes containing the considerations of the previous section and

the dense magma D can rise above the neutral check that for every scenario in Table 3.1 the
buoyancy level at the base of the crust but are still stress intensity at the tip of the dike exceeds the
trapped below the surface for all melt source likely fracture toughness of the crustal rocks. This
depths down to about 21.4 km. Deeper sources involves carrying out a numerical integration of the
allow this magma to reach the surface in an erup- effects of all of the local density differences, and
tion. Table 3.1 shows that the lighter magma L, not when this is done we find that it is only in the case
surprisingly, finds it even easier to reach the surface of the dense magma D rising into the continental
in an eruption as a result of the compensating effect crust that the stress traps are more important than
of its positive buoyancy in the mantle, and it is only the density traps. But in that case the effect is very
if melting is confined to the very top of the mantle important. Thus for a melt source at a depth of
that dikes are trapped below the surface. For the 35 km, even though on density grounds alone the
continental case, the magma density has a more magma should be able to reach within 2.3 km of the
clear-cut influence on what happens. Dikes con- surface, it is found that the dike tip ceases to be able
taining the lighter magma cannot only reach the to fracture the crustal rocks after penetrating only
surface, as expected because they are buoyant at all a short distance into the crust. For a melt source at
depths, but will always erupt vigorously because of 40 km the magma apparently should be able to rise
the large excess pressure provided by the buoy- to a depth of 1.8 km as shown in Table 3.1, but in
ancy. The denser magma is no longer trapped at the practice the dike tip ceases to propagate at a depth