Page 33 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
28 Geotechnical Engineering

Figure 2.5
Convection
current hypothesis
to explain
continental drift.
The driving force
may be heat
generated from
deep-seated
radioactivity.

decay, and one hypothesis, which is illustrated in Fig. 2.5, is that very slow
convection currents occur in soft, near-liquid rocks deep within the earth.

2.4.6 Earlier Episodes of Continental Drift

Mountain ranges such as the Appalachians, Urals, and Alps appear to have
formed during earlier cycles of plate separation and collision. The most stable
areas are located away from active plate margins in South America, Africa,
Australia, and Eurasia.

2.5 VOLCANIC ROCKS AND VOLCANOES

2.5.1 Occurrence of Volcanoes
As previously indicated, volcanoes cluster along lines of plate collisions, and
along lines of plate separations at mid-ocean ridges. Volcanoes outline oceanic
plates of the Pacific to define the famous ‘‘Ring of Fire’’ that includes the Aleutian
Islands and Japan. Such strings of volcanic islands are referred to as island arcs.
One of the largest eruptions in recent history was when the island of Katmai in
the Aleutians erupted violently in 1912 and blew out several cubic kilometers
of material. Probably the most famous volcanic explosion is that of Krakatoa
in the Strait of Java, which in 1883 shattered eardrums, blew away most of the
2300 ft (700 m) high island, created a huge tsunami (‘‘tidal’’ wave), and created
lasting red sunsets around the world with the dust (Winchester, 2001). (Tsunamis
also are created by fault-related earthquakes, as witness the devastating 2004
tsunami in the Indian Ocean.)

Mt. Vesuvius in Italy is perennially active and is over a subduction zone as
the African plate is moving northward at about one inch (2–3 cm) per year,
slowly closing the Mediterranean basin while being pushed underneath the
Eurasian plate.

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