Page 42 - Fundamentals of Physical Volcanology

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MAGMA GENERATION AND SEGREGATION 19

T (°C) and the USA, through Central America, and finally
0 500 1000 1500 down the west coast of South America to Deception
0
Island at the tip of the Antarctic peninsula (Fig. 2.3).
The Ring of Fire is associated with many of the
Dry largest volume and most energetic volcanic erup-
tions to have occurred in human history, including
1
Water- the largest eruptions of the 20th century – the 1912
saturated eruption of Katmai in Alaska and the 1991 eruption
P (GPa) 2 of Mount Pinatubo in the Philippines, and also the
1883 eruption of Krakatau and the 1815 eruption of
Tambora in Indonesia (the latter being the largest
eruption to have occurred in modern history).
Liquidus
Less obvious but just as important are the long
Solidus
3
Solidus
Liquidus
narrow chains of volcanoes lying beneath the Earth’s
oceans. Observations using sonar imaging systems
and manned and remote-controlled submersibles
Fig. 2.2 The solidus and liquidus curves, and the zones of show the existence of mid-ocean ridges (MORs),
partial melting (shaded) are compared for a mantle rock
actually volcanic mountain chains which mark
containing no water (dry) and containing abundant
the sites of repeated eruptive activity (Fig. 2.4). As
water (wet). The addition of water to a rock moves the
eruptions along the MORs occur at great depths
entire temperature range over which it melts to lower
temperatures and also increases the melting temperature beneath the ocean (typically 1–4 km) we are rarely
range. Thus addition of water can allow a rock to melt even aware of activity there and even less often able to
if its actual temperature and pressure do not change. (After observe it. In June 1993, however, a newly emplaced
fig. 2 in Lambert, I.B. and Wyllie, P.J. (1972) Melting of network of hydrophones detected seismic activ-
gabbro (quartz eclogite) with excess water to 35 kilobars,
ity along part of the Juan de Fuca ridge (a spreading
with geological applications. Journal of Geology, 80,
center located ∼400 km off the west coast of Oregon
692–708. Copyright University of Chicago Press.)
in the USA). Subsequent investigations using various
types of equipment showed that the seismic activ-
Next we look at the evidence for where melting ity had been the precursor to an eruption along the
occurs on Earth, how this relates to the structure of ridge which produced a basaltic lava flow 3.8 km

the planet, and what kinds of melts are produced long and up to 500 m wide.
in different settings. Volcanism in these long narrow zones is inti-
mately associated with the large-scale structure of
the Earth, specifically the fact that the outermost
2.3 Volcanism and plate tectonics layer of the planet consists of a series of separate
slabs called plates. The study of the relationships
Some fundamental information comes from look- between these plates and the deeper interior of the
ing at a simple map of the locations of volcanic Earth is called plate tectonics, and the narrow
activity on the Earth (Fig. 2.3). Volcanoes are not volcanic zones mark some of the boundaries where
distributed randomly around the planet but instead the plates meet. In some cases these are places
occur in well-defined zones. The most famous of where two plates are moving apart. These are
these is the Pacific Ring of Fire – this is a narrow known as divergent margins or spreading cen-
band or ring of volcanic centers which circles the ters and are the sites of MOR volcanism (Fig. 2.4).
Pacific, running from New Zealand, up through the The fact that plates are moving apart at these
Tonga and Solomon islands and the New Hebrides locations and new material is reaching the surface
to the Philippines, through Japan and the Kamchatka makes it clear that some slow, upward movement
peninsula in Russia, through the Aleutian islands, of deeper material in the mantle must be taking
then through the Cascades in western Canada place here.