Page 33 - Fundamentals of Physical Volcanology

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ments: in deep marine settings as a volcano grows
on the ocean floor; in shallow water; between lava
and water as lava enters a lake, river or the sea;
between lava and ice where an eruption occurs
beneath a glacier; or where magma comes into
contact with groundwater before erupting (in
which case the term phreatomagmatic is often
used). The range of ways in which magma can inter-
act with external water is reflected in the great
diversity of the types of eruption which can occur.
The following descriptions are designed to give
some feel for this diversity.

DEEP MARINE ENVIRONMENTS
It is estimated that 70–80% of the annual volcanic
output on Earth occurs at mid-ocean ridges(MORs).
This means that the styles of volcanic activity which
occur in this deep marine environment are, in fact,
the dominant eruption styles on Earth. The inacces-
sibility of these regions, however, means that they
are far harder to study than subaerial eruptions.
Studies using a range of techniques, including sonar
surveys and submersible dives, show that MOR
volcanism is basaltic and effusive. There are two
Fig. 1.16 Coarse pyroclasts deposited by a pyroclastic
density current formed during the January 1974 eruption of reasons why the eruptions are effusive rather than
Ngauruhoe volcano, New Zealand. (Photograph by Lionel explosive. One is that when lava is erupted on the
Wilson.) sea floor the pressure of the overlying water limits
the formation of gas bubbles within the lava and
thus suppresses the explosivity of the magma. In

traveled down the flanks of the volcano to distances other words an eruption which would have gen-
of up to ∼2 km (Fig. 1.16). Vulcanian eruptions vary erated Hawaiian activity on land will consist of
widely in the proportion of juvenile (i.e., derived passive effusion at a MOR. Interactions between
directly from the magma) and nonjuvenile (i.e., magma and water can also generate explosions in
incorporated from the surrounding rocks) material some settings, but at a MOR the interaction between
that they eject. The volumes of material produced lava and water is generally nonexplosive. When
by individual Vulcanian explosions are far greater lava oozes out onto the sea floor it is rapidly cooled
than those of Strombolian explosions. At and quenched by the surrounding water. Pillow
Ngauruhoe the individual explosions produced lavas are a distinctive form of basaltic lava flow
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up to 10 m of ejecta; larger volumes have been which form because of the rapid cooling of the
inferred for other Vulcanian explosions, such as the outer skin of the lava flow (Fig. 1.17).
1968 eruption of Arenal in Costa Rica.
LAVA FLOWS ENTERING WATER
1.2.8 Hydromagmatic eruptions
A different type of behavior occurs when lava enters
Hydromagmatic eruptions involve interaction water rather than being erupted beneath it. For
between magma or lava and external water. Such example, Kilauea volcano in Hawai’I frequently
interactions can occur in a wide range of environ- generates lava flows which are long enough to reach