Page 120 - Fundamentals of Physical Volcanology

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TRANSIENT VOLCANIC ERUPTIONS 97

800

700 10 MPa
Maximum speed of ejected clasts (m s –1 ) 500 1 MPa

600
3 MPa

400
Fig. 7.1 The maximum speed of the
solid rock fragments ejected in a 300 Vulcanian
transient explosion as a function of the
pressure which built up in the trapped
gas before the explosion (curves 200 Heimaey
labeled with pressure in MPa) and 0.3 MPa
the weight fraction which the gas 100
represents of all of the materials Stromboli
0.1 MPa
ejected. The ranges of conditions
observed in explosions at Stromboli, 0
1 3 10 30
Heimaey, and in various Vulcanian
explosions are indicated. Weight percentage of gas in ejecta

plastic and deformable prior to the explosion. the strength of solidified magma which is thought
Where the skin is still plastic the limiting factor on to be the controlling factor on the pressure gen-
its deformation and ultimate failure is the yield erated prior to eruption. The tensile strength of
strength of the magma. Observations on lava flows solidified magma varies greatly depending on how
and on lava cooling in lava lakes show that the yield fractured the rock is. The 10 MPa value used as
strength increases as the lava cools, with maximum an upper limit in these calculations represents the
recorded yield strengths for basaltic lavas being strength of pristine unfractured igneous rocks.
∼0.23 MPa. Thus the strengths estimated from In practice the vent plug is likely to contain frac-

Fig. 7.1 are consistent with the idea that it is the tures or, indeed, to be composed of unconsoli-
yield strength developed in a cooling but still dated material which has slumped back into the
plastic skin of lava which controls the pressure vent. In either case the tensile strength is likely to
developed prior to eruption and hence the veloc- be significantly less than the 10 MPa upper limit.
ities of the ejected clasts. The range of strengths estimated from Fig. 7.1 is
In the case of Vulcanian explosions observa- thus consistent with the likely strength of the vent-
tions show that typical ejecta velocities range from plugging material.
−1
200 to 400 m s . The weight percentage of gas in
these explosions is unknown but, assuming some
PREDICTING THE RANGE OF BALLISTIC CLASTS
segregation of gas occurs, it must exceed the ini-
tial magmatic gas content which is likely to be a Thus far the theoretical model has been used
few weight percent. These constraints show, as to look at the relationship between vent pressures
expected, that the plug strength in these eruptions and ejecta velocities in transient explosions. The
is greater than in Strombolian eruptions, but a wide ejection of large blocks in both types of transient
range of strengths is possible, from ∼0.3 MPa up explosion represents a significant hazard. In the
to the maximum value used in the calculations of case of Vulcanian explosions these blocks may land
10 MPa. In the case of Vulcanian explosions it is as much as 5 km from the vent! So from a hazard