Page 241 - Fundamentals of Physical Volcanology

9780632054435_5_end.qxd 12/10/2007 12:35PM Page 218

218 ANSWERS TO QUESTIONS

heights. First, the silicate particles act as nuclei on atmospheric gas, and generally a greater dispersal
which the vapor condenses and the sulfur dioxide of pyroclastic products.
then dissolves and oxidizes to form aerosol droplets 2 There are two main effects. The first is that the
of sulfuric acid that reflect sunlight and cause cool- lower the acceleration due to gravity the smaller
ing. Second, the water vapor causes changes in the will be the pressure or stress at a given depth due
equilibrium of chemical reactions and can influence to the weight of the overlying rocks. This leads to
ozone stability. wider dikes, thicker sills and more vertically exten-
3 Although the eruption plumes from small-scale sive magma chambers. The second effect concerns
basaltic eruptions do not rise very high, the plumes the distribution of pyroclasts. Clasts of a given size
from high eruption-rate flood-basalt eruptions and density incorporated into an eruption cloud
probably do penetrate the stratosphere, and these will have smaller weights, and so can be carried up
eruptions release more sulfur dioxide than typical to greater heights and blown further in a given
evolved magmas and also continue for much wind. Similarly, clasts ejected more nearly ballisti-
longer. cally from a vent can travel to greater ranges.
3 We have only actually observed volcanic erup-
tions on the Earth and on Jupiter’s satellite Io. Some
CHAPTER 13
of the lava flows on Venus and Mars are very fresh in
1 The lower the atmospheric pressure, the greater appearance and we think that they were erupted
the amount of volatiles released from a magma geologically recently. This idea is supported by the-
reaching the surface (volatile solubility is pressure- oretical arguments as well as observations, and so
dependent). Also, the lower the atmospheric we expect to see further eruptions on these planets
pressure, the greater the amount of expansion of some time in the future. In contrast, both theory
whatever volatiles are released from the magma. and observation suggest that our Moon and the
This leads to a higher eruption speed in the material planet Mercury are too small to retain significant
leaving the vent, more efficient entrainment of heat sources and are volcanically dead.