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192 CHAPTER 13

less in diameter) bodies that escaped being incor- they have done so under very deep water. Even
porated into any of the larger planets and satellites. when only eruptions into a gaseous atmosphere are
It is known, by examining the meteorites that reach involved the pressure range we have to anticipate is
Earth after being broken off these asteroids, that at enormous: from a maximum of 9 MPa on Venus
least several tens of them formed quickly and very down to a hard vacuum, i.e., essentially zero pres-
early in the history of the Solar System. This allowed sure, on the Moon, Mercury, and Jupiter’s volcan-
them to incorporate some short-lived radioactive ically active satellite Io. The pressure under water
elements that acted as strong heat sources, and on Earth is about 40 MPa in many deep parts of
these asteroids were able to warm up to the point the ocean, and reaches about 110 MPa at the 11 km
where they differentiated into iron cores and sili- depth of the deepest oceanic subduction-zone
cate mantles. Furthermore, there is also evidence, trench. On Mars the maximum water depths may
again from meteorites, that at least several of these have been 3 to 4 km in the ocean that many scien-
asteroids formed basaltic crusts as a result of vol- tists think filled Mars’ northern lowlands in the
canic activity. early history of the planet, but the acceleration due
to gravity is about 38% of that on Earth so the high-
est pressure would have been only ∼15 MPa. The
13.3 The effects of environmental global ocean on Europa may be as much as 100 km
conditions on volcanic processes deep and, although the acceleration due to gravity
is only ∼13% that of Earth, this makes the pressure
Previous chapters have discussed what is currently typically a record 130 MPa.
understood about the physical processes which The general effects of this wide pressure range
control the character of volcanic activity. We took on eruption conditions can be illustrated by consid-
it for granted in doing so that a particular value ering the eruption of a basaltic magma containing
for the acceleration due to gravity applies to the 1 wt% water. Water is the commonest volatile in the
Earth, and that eruptions on land take place under Solar System, and 1 wt% is a reasonably common
a particular atmospheric pressure. However, when value on Earth. As shown in earlier chapters, the
eruptions occur on the Earth’s ocean floors, the first thing than can happen as the magma rises
weight of the overlying water exerts a sufficiently toward the surface is that it becomes supersatu-
great pressure to greatly reduce the amount of rated in the volatile and starts to form gas bubbles.
gas that can exsolve from the magma, and this These expand as the pressure decreases (and new

suppresses explosive eruptions except in gas-rich ones form). If the pressure becomes low enough
magmas or under circumstances where gas can be that the gas bubble volume fraction exceeds about
concentrated into a small part of the magma. So in 75%, the magma will be disrupted into pyroclasts
thinking about volcanism on other bodies it is nec- and an explosive eruption will happen. If not,
essary to consider how the differing environmen- a vesicular lava flow will be erupted. Table 13.1
tal conditions might affect the volcanic activity shows what will happen to this typical basaltic
which occurs. This section considers in a general magma on various bodies.
way how different environmental conditions are The variations are striking: this basaltic magma
likely to influence volcanism. would not exsolve any gas at all under the Europa
The above comparison suggests that the first fac- ocean and in the deeper parts of the Earth’s oceans.
tor to consider is atmospheric pressure, or more It would produce vesicular lava flows in shallow
exactly “the external pressure at the point where oceans on Earth, under all likely oceans on Mars,
magma emerges at a planetary surface”. The words and anywhere on the waterless surface of Venus;
are chosen carefully to take account of the fact that and it would only erupt explosively on land on
eruptions take place into both air and water on Earth and Mars, and anywhere on the atmosphere-
Earth. Later in this chapter it will be seen that they less (and waterless) surfaces of Mercury, the Moon,
have almost certainly sometimes done both on and Io. The eruption speeds given in Table 13.1 are
Mars, and that if eruptions have occurred on Europa, calculated using the methods described in Chapter