Page 35 - Fundamentals of Physical Volcanology

9780632054435_4_001.qxd 12/10/2007 12:17PM Page 12

12 CHAPTER 1

Fig. 1.19 Image of cock’s tail
plumes, transient explosive tephra
jets generated when sea water and
magma interact, produced during
the eruption of Surtsey that began off
the south coast of Iceland in 1963. The
plume is approximately 500 m high.
Photograph by Sigurdur Thorarinsson,
used by permission of Sven
Sigurdsson.

forms in a ring around the base of the eruption col- tion started at 2 a.m. on September 28, 1965 with
umn and spreads out radially away from it at high a Strombolian phase but changed dramatically in
speed (the term was originally coined for a similar style after ∼90 minutes. The eruption began to pro-
phenomenon seen in some surface tests of nuclear duce large, ash-laden eruption clouds which rose to
weapons). Base surges are a common feature of heights of 15–20 km and large base surges which
many hydromagmatic eruptions. spread out from the base of the eruption column
As tephra was constantly being deposited around to distances as great as 6 km. The speeds of these
the vents at Surtsey, the easy access of seawater to surges were estimated to be as great as 100 m s −1
the vents would be stopped at times and then a more (i.e., ∼220 miles per hour). The surges appear to

continuous eruption would occur. These eruptions have been cold, as there is no evidence that they
often lasted several hours at a time and produced scorched or burned anything in their paths, but
a more stable eruption plume, which was dark in they knocked down all the trees within about 1 km
color and rose to heights of ∼2 km. Incandescent of the vent and sand-blasted trees in the area
clasts were common within the eruption plume beyond this. The explosions occurred at irregular
and fell out close to the vent. By April 1964 the vent intervals from a number of vents for a period of
had grown to the point where seawater could no about 7 hours. The change in character of the erup-
longer gain access to it, and the styles of activity tion from the initial Strombolian phase to the vio-
changed to the more conventional forms of basaltic lent explosions seems to have occurred when the
eruptions – Hawaiian and Strombolian. lake water started to gain access to the active vents.
Eruptions through crater lakes are often similar After the initial 7 hours of violent activity the explo-
in style to those in shallow marine settings. A spec- sions gradually declined in vigor, and by 4 pm on
tacular example of this type of eruption occurred at September 30 the eruption was over. This eruption
Taal in the Philippines in 1965. The eruption occurred killed 190 people.
at Volcano Island, which sits within Lake Taal, the A further type of hydromagmatic activity is asso-
lake itself occupying Taal caldera (a caldera is a ciated with eruptions through lakes or shallow sea-
crater or surface depression formed by the collapse water. These are phreato-Plinian eruptions, in
of the ground above a magma chamber). The erup- which the fine-grained nature of the ash and other