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STEADY EXPLOSIVE ERUPTIONS 83

the vent. We are concerned here with eruptions mixed into it. The width of the column would have
which go on for hours or days. These include to increase as a result of the addition of air, but the
Hawaiian eruptions (Fig. 1.1), subPlinian and expansion is enhanced by the fact that initially the
Plinian eruptions (Fig. 1.2) and ignimbrite-forming volcanic gas and clasts in the column are much hot-
eruptions (Fig. 1.11). Although the products of ter than the entrained air and so they heat it up
these eruptions differ greatly from each other (cooling themselves in the process) and thus cause
(Chapters 8 and 10), they are all dynamically simi- it to expand as it mixes (Fig. 6.5). The amount of
lar. Here the basic physics which such eruptions entrainment which occurs depends primarily on
have in common is considered; the differences the exit velocity of the gas–magma mixture and the
between these eruption styles are discussed in radius of the vent (see sections 6.3 and 6.4).
Chapter 8. Initially we will look mainly at what One effect of entrainment is that as the material
happens to the stream or jet of gas once it exits the in the eruption column rises, the velocity at which
vent and largely ignore the magma clasts within it. it rises progressively decreases. This can be under-
The clasts are not critical to the overall dynamics stood by considering the principle of conserva-
and are discussed later. tion of momentum which states that

m v = m v (6.6)
6.5.1 Plume rise 1 1 2 2
The first thing that happens when the stream of gas where m is the initial mass in the eruption jet and
1
and clasts exits the vent is that the jet starts to incor- v is the initial upward velocity (i.e., the exit velo-
1
porate air from the surrounding atmosphere in city) and m and v are the mass and upward velo-
2 2
a process known as entrainment. As the gas jet city at some height above the vent. As the eruption
streams upward through the air it causes turbulent column ascends and entrains air, the mass of the
mixing between the air and the edge of the jet and column increases and, therefore, conservation of
so air is mixed in and added to the eruption jet or momentum requires the rise velocity to decrease.
column. The giant, turbulent convection cells in Mention of momentum should make you recall
which this entrainment occurs are what give the eqns 6.4 and 6.5 for the motion of magma below
edges of rising eruption columns their characteris- the ground; all of the same considerations apply to
tic “cauliflower cloud” appearance (Fig. 6.4). The motion above ground, with the added complication
further the column rises the more air becomes that instead of a fixed edge to the magma – the dike

Fig. 6.4 The turbulent convection
cells at the edge of the eruption
plume from the May 18, 1980,
eruption of Mount St Helens forming
the characteristic “cauliflower cloud”
patterns. (Photograph by Donald
A. Swanson, courtesy of Cascades
Volcano Observatory, U.S. Geological
Survey.)