Page 36 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 31

Figure 2.7
Contraction during
cooling of a broad
lava flow can
create a
distinctive pattern
of vertical cracks
and columnar
jointing. (Author’s
photo from fishing
trip north of Lake
Superior in
Canada.)

2.5.8 Pyroclastic Rocks

The explosive nature of some volcanic eruptions creates rocks that are igneous in
origin and sedimentary in deposition, as materials are blown into the air and then
settle out near and far from the volcano. ‘‘Pyro’’ means fire; ‘‘clastic’’ means
fragmented. The greatest danger to life from volcanoes is not from lava flows,
which usually are slow enough that people can get out of the way, but violent
explosions that throw huge masses of incandescent debris, or pyroclastics, high into
the air. As the particles are quite heavy they settle and form a density current that
sweeps down the mountainside, leaving a burned-out trail of fire and devastation.

Cones
Volcanic cones consist of layers of relatively soft pyroclastics interpersed with
layers of hard lava. Pyroclastics are classified by size into bombs, cinders, ash, and
dust. Rarely, fine stringers of glass are formed called Pelle’s hair.

Bombs
Are incandescent masses of lava that are tossed into the air and come down with
a mushy thump close to the exploding crater, sometimes catching in the crotches
of trees.

Volcanic Cinders
Are much more abundant than bombs. They also fall close to the vent and
annihilate all life and vegetation. Cinders often comprise the bulk of volcanic
cone mountains, and small cones called cinder cones are almost entirely made up
of cinders.

Volcanic Ash
Being finer than cinders, can be carried tens of kilometers, and constitutes the
most lethal part of most eruptions. Deposits of ash may be meters thick,
particularly close to the erupting volcano. The ancient Roman towns of Pompeii
and Herculaneum were buried by volcanic ash.

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