Environmental Engineering Reference
In-Depth Information
Figure 3.4.
Geological section along Plinth (right bank) at Foz Do Areia Dam (Pinto et al., 1985).
Figure 3.5.
Perspective view of, and cross section through a lava flow, showing a lava tunnel developed
when the lava flows forward faster than the supply.
Thick lava flows sometimes contain “lava tunnels”, which are circular, ovoid or lentic-
ular in cross section, up to 20 m across and some kilometres in length. These are formed
when the supply of lava to the flow is exhausted, and the internal lava “stream” (Figure
3.5) drains away.
Figure 3.6
shows a small lava tunnel exposed in basalt forming the wall
of a 30 m diameter shaft at Hoppers Crossing, Victoria, Australia.
Most lava flows show a hexagonal columnar joint pattern, with the columns being
interrupted by near-planar or saucer-shaped cross joints. These joints have developed as a
result of shrinkage on cooling and are usually either slightly open or filled with secondary
minerals or alteration products.
Figure 3.7
shows columnar-jointed rhyolite exposed dur-
ing construction at High Island dam, in Hong Kong. The rhyolite has been intruded by a
younger, darker rock forming a dyke up to 1.5 m thick.
In some flows there are closely spaced joints parallel and near to the margins, appar-
ently caused by shearing associated with viscous flow. Where columnar joints in lava
flows are open, e.g. due to mechanical weathering close to steep valley sides, the rock
mass permeability can be high.
