Page 38 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 33

2.5.11 Weathering of Basalt
Even though the geological origins of the Columbia Plateau in the U.S. and the
Deccan Plateau in India are similar, the soils developed by weathering in these
two areas are very different because of the marked differences in climate and
weathering. Lava flows in dry climates are not highly weathered. The contrast
due to weathering is exemplified by the island of Hawaii, where lava flows on
the climatically dry side of the island remain as jagged rock, while those that are
younger but occur on the wet side have weathered and support lush vegetation.

Weathering of basalt is aided by its composition of high-temperature minerals
and its small crystalline grain size. Generally, minerals that crystallize at high
temperatures, such as those in basalt, are more readily weathered than those
that crystallize at lower temperatures, such as those in granite. Quartz, as a low-
temperature mineral in granite, survives and is concentrated in sands.

2.5.12 Dikes

Dikes are wall-like masses of igneous rock that have intruded into fractures
in older rock. Dikes associated with volcanoes often form a radiating array
extending outward from the volcanic neck. Dikes also are associated with granite
intrusions (see below) and may extend through cracks in sedimentary rock layers,
in which case they usually consist of the last minerals to be crystallized, quartz,
feldspar, and mica, sometimes enriched with rare minerals including gold. Because
rock in a dike often is harder than the rock into which it is intruded, erosion
causes dikes to stand above the surrounding ground level.

2.6 OCCURRENCES OF GRANITE

2.6.1 Intrusive Igneous Rocks
Whereas volcanoes are composed of extrusive rocks, meaning that they formed
from magma that was extruded at the ground surface, another category of igneous
rocks is called intrusive, meaning that the magma has cooled more slowly under
a cover of older rocks. With more time for crystallization the individual grains
become larger, so individual crystals are visible with the unaided eye.

2.6.2 Occurrences of Granite
Granite and closely related rocks are the most abundant intrusive igneous rocks,
making up the cores of most mountain ranges, where the granitic mass is called
a batholith, which literally translates to ‘‘deep rock.’’ As geological erosion works
to pare down a mountain range, the removal of load allows the earth’s crust to
‘‘float’’ back up, with consequent shifting and rubbing of earth crustal layers that

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.