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VOLCANISM ON OTHER PLANETS 193

Table 13.1 The consequences of erupting a basaltic magma containing 1 wt% water on the bodies and under the
conditions specified.

Location of eruption Ambient Percentage of Nature of
pressure gas released eruption

Europa (deep ocean always present) 130 MPa 0 Lava, no gas bubbles
Earth, under deep ocean 40 MPa 0 Lava, no gas bubbles
Earth, under shallow ocean 20 MPa 12.3 Lava, 9% vesicles
Mars, under deep ancient ocean 15 MPa 28.3 Lava, 24% vesicles
Venus (no ocean ever present) 9 MPa 49.8 Lava, 48% vesicles
Mars, under shallow ancient ocean 7.5 MPa 56.3 Lava, 56% vesicles
Earth, on land at sea level 0.1 MPa 97.9 Explosive eruption, ejecta
speed 220 m s −1
Mars (no oceans for most of history) 500 Pa 99.9 Explosive eruption, ejecta
speed 347 m s −1
Mercury, Moon, Io (no oceans ever) ∼0 100.0 Explosive eruption, ejecta
speed 484 m s −1

6 and show that the lower the external pressure the pressure increase on a low-gravity planet, reser-
more vigorous the explosion. voirs on all of the other planets are expected to be
The second major factor controlling eruption deeper inside volcanoes than on Earth. Second, the
conditions is gravity. Here the influence is subtle. acceleration due to gravity influences the vertical
The lithostatic pressure, P, at any given depth D sizes of magma reservoirs. It is currently thought
below the surface of a planet where the accelera- that reservoirs grow until they reach a vertical
tion due to gravity is g is given by P = ρgD, where height H such that the stress across the walls due to
ρ is the mean density of the overlying rock mass. the difference, ∆ρ, between the densities of the
This statement neglects the fact that, especially magma inside and the solid country rock outside is
near the surface of a planet, rocks can support equal to the strength of the walls. This stress is pro-
stresses of several megapascals before fracturing or portional to (∆ρgH). However, the strengths of all
deforming slowly in a viscous fashion, but even so it volcanic rocks are similar, so the value of (∆ρgH)

gives a good approximation to the typical pressure should be similar on all planets. Also the density dif-
at a given depth, and the approximation gets bet- ferences between the solid and liquid states of all
ter as the depth increases. The implication of this volcanic rocks are also rather similar, so the value of
relationship is that on a low-gravity planet (and the ∆ρ should be similar on all planets. The only way
Earth has the largest acceleration due to gravity of that both requirements can be satisfied is if H is
all the bodies we have to consider) one must go inversely proportional to g: the lower the gravity,
to a greater depth below the surface to reach any the greater the vertical extent of the magma reser-
given pressure. voir. Third, the acceleration due to gravity influ-
The gravity influences volcanic structures in ences the sizes of dikes and sills. This is really just an
three ways. First, it controls the depths at which extension of the second effect, because the lengths
magma reservoirs are likely to be found. In Chapter of the long axes of dikes are determined mainly by
4 it was shown that the density variation with the tensile strengths of rocks and magma density
depth, and hence the level of neutral buoyancy, in a differences in the same way as the vertical extents
growing volcano was controlled by the progressive of magma reservoirs, and to a first approximation
crushing of pore spaces as what was once a surface the thicknesses of dikes and sills, are proportional
layer of vesicular lava or ash was buried ever deeper. to the lengths in a way governed by the elastic prop-
The crushing process depends on pressure, and erties of the rocks (again the same for rocks on all
since a greater depth is needed to cause a given planets) and not the gravity. So dikes and sills are