Page 173 - Fundamentals of Physical Volcanology

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150 CHAPTER 10

variation in magma gas content or in magma rise vidual Strombolian explosions produce very small
speeds, that viscosity is probably the dominant fac- volumes of material, eject this material from the
tor controlling whether significant bubble coales- vent at relatively low speeds, and usually occur
cence can occur in different magmas. This can be more closely spaced in time than Vulcanian erup-
demonstrated by some simple calculations such as tions. Strombolian eruptions usually involve basaltic
those performed for basaltic magmas in section 5.5.3. magmas whereas Vulcanian eruptions are typically
Let us consider the case of bubble rise in an inter- associated with the eruption of intermediate mag-
mediate or very evolved high-silica magma. Say that mas. Thus, there seems to be a strong connection
in both cases magma exsolution begins at a depth of between the composition of the erupting magma
5 km beneath the surface. Typical rise speeds for and the character of the transient explosion which
intermediate and evolved magmas are thought to results. In what way is the chemical composition
−1
be in the range 0.001 to 0.015 m s . This means controlling the eruption dynamics?
5
6
that it takes 3.3 × 10 to 5 × 10 seconds for the Field observations and mathematical modeling
magma to rise from 5 km depth to the surface. of Strombolian and Vulcanian eruptions suggest
Whether bubble coalescence can occur during that the differing “violence” of the eruptions is
this ascent depends on the rise speed of bubbles related to the strength of the “cap” on the magma
through the rising magma (section 5.5.3). If we con- column prior to eruption (see section 7.2.1 and Fig.
sider the intermediate magma first and assume 7.1). In Strombolian eruptions, the gap between
3
that it has a magma viscosity of 10 Pa s then a bub- explosions is too short to allow much cooling of
ble 1 mm in radius will rise at a speed of ∼6.5 × the magma at the top of the magma column, and
−1
10 −6 ms and thus can rise a distance of between 2.1 the cooled “skin” that does develop tears easily as
and 32.5 m through the overlying magma during gas bubbles accumulate beneath it causing weak
the time it takes the magma to rise to the surface. If explosions (Figs 1.14 & 7.1). In Vulcanian explo-
the bubbles are larger, say 1 cm in radius, their sions the magma at the top of the magma column
−1
rise speed is ∼6.5 × 10 −4 ms (eqn 5.12) and the dis- cools much more between explosions forming a
tance traveled by the bubbles relative to the magma solid “cap” (Figs 7.1 & 7.2) and so the pressure
is correspondingly greater at 214 to 3250 m. By beneath it must build to considerably greater
contrast, in a more evolved magma with a viscosity levels prior to explosion. This is probably because
6
of 10 Pa s the rise speed of a bubble 1 mm in radius of the different viscosities of the magmas involved.
decreases to 6.5 × 10 −9 ms −1 and the distance trav- In basaltic magmas the low viscosity allows rela-

eled through the magma during ascent to the sur- tively rapid rise of gas bubbles through the magma
face is only 2.1 × 10 −3 to 0.03 m. Even at a bubble and this means that the surface of the magma col-
radius of 1 cm the bubble rise speed is only 6.5 × umn is disrupted and removed frequently so that
10 −7 ms −1 and the distance risen by the bubbles there is not time for a thick crust to develop, i.e.,
only 0.2 to 3.3 m. Thus in this simple example the rise speed of the bubbles controls the time avail-
bubble coalescence is likely to occur in the inter- able for the skin on the magma column to develop
mediate magma case but is very unlikely in the case and hence the violence of the resulting explosion.
of the high-silica magma even at very low magma Thus basaltic magmas commonly give rise to
rise speeds. The low rise speeds of gas bubbles in Strombolian eruptions. The rise speeds of bubbles
these magmas explains why transient explosions in intermediate magmas will be lower due to the
are commonly associated only with basaltic and higher viscosity of the magma (sections 5.5.3 and
intermediate magmas but not with evolved magmas. 10.4.1). The slower rise and accumulation rate of
gas at the top of the magma column then means
10.4.2 Chemical composition and transient that the top of the magma column has more time
explosive activity to cool than in the Strombolian case. This greater
We have seen that transient explosions occur when cooling means that the pressure necessary to
magma rise speeds are slow. The two main types of cause failure of the cap is greater. The brittle failure
transient eruption – Strombolian and Vulcanian – of this cap gives rise to Vulcanian explosions
differ from each other in a number of ways. Indi- (Fig. 1.16).