Environmental Engineering Reference
In-Depth Information
the following categories (González de Vallejo, l.i.
et al.,
2006):
- Discontinuities of thermal origin.
- Discontinuities of tectonic origin.
- Discontinuities created by intrusive structures.
- Discontinuities due to the contact of diferent
formations.
- Voids/cavities (see below).
Discontinuities of thermal origin are,
undoubtedly, one of the main characteristics of
lava flows. They are created by the retraction suf-
fered by volcanic materials when cooled. These dis-
continuities can present a great opening; their walls
are usually smooth and show little filling.
The most characteristic of these discontinuities
by the cooling of massive lavatic materials. These
columns usually present polygonal structures of
three to twelve sides, although usually limited to
between five and seven sides. They usually show
perpendicular fractures with an even spacing.
length of the columns can vary enormously from
one site to another, from 3 cm to 130 m (lyell
1871). Diameter can also vary, from more than
3 meters to 3 centimeters or less.
columns grow perpendicular to the lava cooling
surface, thus appearing usually vertical though
they can also be horizontal, leaning or even curved.
The orientation of columns can change from one
part of the lava flow to another.
columns can sometimes show a radial distribu-
tion that further hinders their study.
spheroid discontinuities appear when water
penetrates the inside of a lava flow. if the flow is of
great thickness, horizontal retraction planes may
appear, usually located at third of the thickness
closest to the base (González de Vallejo, l.i. et al.
2006).
Discontinuities created by intrusions can be of
importance in slope instability processes, mainly
due to their great continuity.
Discontinuities due to the contact of different
formations appear usually in the contact between
a lava flow and a pyroclastic deposit, being the
contact of erosive or depositional origin.
2.1.2
Voids
one of the main features of volcanic massifs is the
presence of voids generated during the eruptive
process, with sizes ranging from microns to several
kilometers.
These cavities can be of several different origins,
the main three being discussed below.
The succession of volcanic episodes in time can
produce an alternating of materials of different
compositions. Drossy or breccioid materials can
appear in between more or less massive lavatic
materials. in many occasions these materials can
give way to voids at the base and top of the lavatic
Magma contains great quantities of dissolved
gases. on cooling, these gases tend to separate
from the magma (degasification). Given enough
lava flow viscosity, the gases can be trapped,
forming vacuoles of different sizes. examples of
these structures can be found in vacuolar basalts.
in the cooling process of a very fluid lava flow,
areas in contact with open air are cooled rapidly,
creating a crust under which lava keeps on flowing,
due to the low thermal conductivity of basaltic
rocks (González de Vallejo, l.i.
et al.,
2006).
sometimes, these crusts remain after the flow has
left and become hollow structures that can run
for several kilometers, called lava tubes. examples
of these structures are: the cueva del Viento in
Tenerife, which runs for more than 17 kilometers;
the Túnel de la atlántida in lanzarote, with more
Figure 2. scoriaceus contact between pyroclastic deposits
and a basaltic flow with decimeter-sized cavities. Garrotxa
natural Park (spain). image courtesy of irene lópez.
Figure 1. columnar disjunction in castellfollit de la
Roca (spain). image courtesy of irene lópez.
