Page 26 - Fundamentals of Physical Volcanology

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VOLCANIC SYSTEMS 3

Fig. 1.3 An approximately 160 m
high, 400 m wide lava dome slowly
growing in the vent of the May 18,
1980 eruption of Mount St Helens
volcano, Washington State, USA.
(Photograph taken on August 22,
1981 by Lyn Topinka, courtesy of U.S.
Geological Survey/Cascades Volcano
Observatory.)

escape of most of the gas from a substantial volume
of magma stored at shallow depth. That magma is
then erupted every few years as lava flows.

1.2.2 Hawaiian-style eruptions
The Hawaiian eruption style is named after the
predominant style of activity observed at the cur-
rently active volcanoes of the Hawaiian Island
chain. The term Hawaiian can be applied, though,
to any eruption exhibiting this same style regard-
less of where in the world it occurs. Hawaiian erup-
tions are characterized by their lava fountains
Fig. 1.4 Part of a dense, sheet-like lava flow erupted on the
(Fig. 1.1). These are composed of hot, incandescent
ocean floor, where the high pressure suppresses gas release,
minimizing explosive activity and the formation of gas clots of magma (often up to 1–2 m in diameter)
bubbles in lavas. (Image courtesy of Monterey Bay which are ejected from the vent at speeds of ∼100
−1
Aquarium Research Institute, © 2001 MBARI.) ms and typically rise to heights of only a few tens
to hundreds of meters above the vent before falling
instance, after major explosive eruptions have fin- back to the ground. The majority of the clots of
ished their explosive phase it is common for vis- magma fall close to the vent and are still very hot
cous lava to ooze from the eruptive vent to form a upon landing (∼1135°C), hot enough that the clots
lava dome (Fig. 1.3). In deep submarine eruptions, coalesce on the ground forming fluid lava flows
where the pressure of the overlying water is great which may travel several kilometers or even tens of
enough to suppress the release of gas from the kilometers from the vent (Fig. 1.5). Some of the
erupting magma, the dominant mode of eruption is clots and smaller clasts landing close to the vent are
effusion (Fig. 1.4). In other cases lava may effuse cooled enough during flight and after landing that
from a vent because the lava has previously lost the they are too cool to form lava flows but instead
gas which was initially dissolved in it. This happens, weld together forming a spatter cone or spatter
for instance, at Stromboli, where repeated small rampart around the vent (Fig. 1.6). A small amount
explosions every few tens of minutes allow the of the erupted material is sufficiently fine grained