Page 35 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
30 Geotechnical Engineering

2.5.3 Atolls
Atolls are doughnut-like remnants of volcanic islands. Their origin was hypo-
thesized by Charles Darwin: after a volcano dies and the island slowly sinks
into the sea floor, a ring of coral growing on the flanks of the volcano continues
to grow upward until all that is left is the atoll of living coral.

2.5.4 Craters and Calderas

Extrusion of lava relieved internal pressure in a volcano, and as cooling begins
the vent area at the apex of a volcanic cone tends to subside and form a
crater. Calderas are similar areas of subsidence but on a much larger scale, some-
times extending kilometers in diameter. Calderas may be a location of continuing
volcanic activity, as in Hawaii. Crater Lake in Oregon actually is a caldera.

2.5.5 Basalt
Basalt, which comprises the bulk of most volcanoes, is an extrusive rock, meaning
that it has solidified after having been extruded at the ground surface. Extrusion
and rapid cooling freezes the rock into a fine crystal structure, and very rapid
cooling creates a volcanic glass called obsidian. Lava that flows into the sea is
quenched and may shatter and contribute to a black beach sand composed
of obsidian.

One consequence of extrusion is a sharp reduction in fluid pressure that results
in exsolution of gases, creating round bubble holes in the rock. The resulting
vesicular basalt sometimes is used for landscaping. An extreme example of bub-
bling is pumice or rock froth, which is so vesicular it may float on water. Pumice
can be pulverized to form a fine sanding agent for furniture and an ingredient
in mechanic’s soap.

2.5.6 Some Geomorphic Features of Lava Flows
Slow cooling of a flow after it has stopped moving can result in a hexagonal array
of shrinkage cracks that create columnar joining, Fig. 2.7. If a surface crust cools
sufficiently to solidify while the lava underneath is moving, the result is a harshly
jumbled surface of jagged rocks. Near the borders of flows, molten lava
underneath may run out, leaving a lava tube. After the lava has solidified, lava
flows typically are very permeable so there is little or no surface runoff of
rainwater, which drains through the tubes. Lava tubes exiting at sea level can
collect waves and create dramatic waterspouts in the sea cliff.

2.5.7 Volcanic Necks

Erosion of volcanic cones gradually takes away the cone and leaves only hard rock
comprising the neck as a monument. Examples are prominent in Western movies.

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