840 Encyclopedia of the Solar System
excellent example of how the rise speed, gas content, and
viscosity of a magma are critical in determining the style of
explosive activity that occurs. While the magma as a whole
is ascending through a fracture in the planetary crust, bub-
bles of exsolved gas are rising through the liquid at a finite
speed determined by the liquid viscosity and the bubble
sizes. If the magma rise speed is negligible, for example,
when magma is trapped in a shallow reservoir or a shal-
low intrusion, and if its viscosity is low, as in the case of a
basalt, there may be enough time for gas bubbles to rise
completely through the magma and escape into overlying
fractures that convey the gas to the surface, where it escapes
or is added to the atmosphere if there is one. Subsequent
eruption of the residual liquid will be essentially perfectly
effusive. If a low-viscosity magma is rising to the surface
at a slow enough speed, most of the gas will still escape as
bubbles rise to the liquid surface and burst. Because rel-
atively large bubbles (those that nucleated first and have
decompressed most) will rise faster through the liquid than
very small bubbles, it is common in some magmas, espe-
cially basalts, for large bubbles to overtake and coalesce
with small ones. The even larger bubbles produced in this
way rise even faster and overtake additional smaller bub-
bles. In many cases, a runaway situation develops in which a
single large bubble completely fills the diameter of the vent
system apart from a thin film of magma lining the walls
of the fracture. In extreme cases the bubble may have a
much greater vertical extent than its width, in which case
it is called a slug of gas. As this body of gas emerges at the
surface of the slowly rising liquid magma column, it bursts,
and a discrete layer of magma forming the upper “skin” of
the bubble or slug disintegrates into clots and droplets up to
tens of centimeters in size. These are blown outward by the
expanding gas (Fig. 9). The pyroclasts produced accumu-
late around the vent to form a cinder cone that can be up to
several tens of meters in size. The time interval between theFIGURE 9 Jets of hot gas and entrained
incandescent basaltic pyroclasts ejected from a
transient Strombolian explosion on the volcano
Stromboli in Italy. (Photograph by L. Wilson.)