Page 27 - Fundamentals of Physical Volcanology

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4 CHAPTER 1

Fig. 1.7 Tephra blanket from the Pu’u Puai vent near the
Fig. 1.5 Lava fountain forming lava flows at the Pu’u ‘O’o summit caldera of Kilauea volcano, Hawai’I. The blanket
vent on the East Rift Zone of Kilauea volcano, Hawai’I. Hot extends into the forest from the edge of the cinder cone
clots of magma falling within the cone have formed a lava at the bottom right of the image. (Photograph by Pete
pond that is overflowing from the lowest point on the rim Mouginis-Mark, University of Hawai’I.)
of the cinder cone to form a lava flow. On the right side
of the cone, clots are coalescing as they land to form a
rootless lava flow. (Photograph taken on June 30, 1984 by
low viscosity. Lavas erupted at Kilauea volcano in
J.D. Griggs, courtesy of Hawaiian Volcano Observatory.)
Hawai’I, for example, commonly emerge with tem-
peratures of ∼1140–1150°C and viscosities of 50–
100 Pa s. Basaltic magmas come directly from zones
of melting in the mantle with very little interaction
with other rock types on their way to the surface.

1.2.3 Flood basalt eruptions
Another type of basaltic lava-forming eruption is
the flood basalt eruption. Humans have yet to
witness a flood basalt eruption because the most
recent one occurred ∼20 million years ago, but their
deposits have been mapped out in many parts of
the world (Fig. 1.8). These are eruptions which
Fig. 1.6 Spatter ramparts along either side of a fissure vent generate enormous volumes of basaltic lava. They
on the East Rift Zone of Kilauea volcano, Hawai’I. Figure is
occur in sequences, so that the volume of an entire
standing at the location of the fissure itself. (Photograph by
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flood basalt province can be as great as 10 km .
Lionel Wilson.)
Individual lava flows in such a province can be
more than 600 km long and 100 m thick with vol-
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that it is carried downwind from the vent forming a umes as great as 2000 km . There is considerable
tephra blanket (Fig. 1.7). Hawaiian eruptions are debate about the exact character of these eruptions
sustained eruptions which can last for hours or days but they appear to be similar to Hawaiian eruptions,
– in some cases for years. The magmas involved in though with individual events producing far larger
Hawaiian eruptions are usually hot magmas called volumes of lava and with the lava being erupted far
basalts. The combination of their chemical com- more rapidly.
position (especially the relatively low silica content) The closest equivalent to a flood basalt eruption
and high temperature gives these magmas a relatively yet observed by humans is the Laki or “Skaftár Fires”