Page 37 - Fundamentals of Physical Volcanology

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14 CHAPTER 1

Fig. 1.20 Maar crater in Death
Valley, California, USA, formed during
phreatomagmatic explosive activity.
The crater is approximately 800 m in
diameter. (Photograph by Elisabeth
Parfitt.)

second (East) maar took 7 days to form and was eruption took place ∼50 Ma ago, and so the surface
300 m wide and 70 m deep. Both centers exhibited deposits from such eruptions are not well pre-
intermittent explosions which produced eruption served, though we assume that because the mag-
columns a few kilometers high, the maximum height mas forming the kimberlites contained large amounts
being ∼6.5 km recorded for an explosion from the of gas the eruptions were very explosive and pos-
East Maar. These eruption clouds deposited ash as sibly similar to Plinian eruptions. Diatreme forma-
much as 160 km downwind, though the bulk of the tion is in many ways the least-well understood of all
ash was deposited within 2–3 km of the vents. Base the types of volcanic activity, but these features are
surges were observed during one violent explosion economically very important. The reason is that the
from the East Maar. Although the eruptions were magmas coming to the surface in these eruptions
hydromagmatic throughout, there was a gradual accidentally carry with them various minerals from
transition to less violent explosions and activity the source regions deep in the mantle, and one of
which was more nearly Strombolian in character as these minerals is the crystalline form of carbon

the eruption progressed. This was thought to be called diamond.
due to the gradual depletion of the aquifer supply-
ing water to the eruption.
1.3 Volcanic systems

1.2.9 Diatreme-forming eruptions
The descriptions of volcanic eruptions given in the
Diatremes are conical to elongate zones of shat- previous section are intended to give an indication
tered rocks extending downward from the surface, of the diversity of volcanic activity which occurs on
often to depths of at least many hundreds of meters. Earth and for the types of deposits that eruptions
They contain fragmental volcanic rocks called produce. Because we can observe eruptions as they
kimberlites. The mineralogy of most kimberlites occur and map and analyze the deposits that erup-
implies that their parent magmas left source regions tions produce, it is inevitable that we know more
deep in the mantle containing large amounts of about the mechanisms of volcanic eruptions than
carbon dioxide. The violent release of this gas as the we do about the processes which occur beneath
magma reached the surface caused the intense the ground before an eruption starts. From the
shattering of crustal rocks that characterizes the point of view of a physical volcanologist, however,
diatreme, and also rapidly fragmented and chilled a volcanic eruption is an end-point, the culmination
the magma. The most recent diatreme-forming of a sequence of processes which have their origin