Page 34 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 29

Figure 2.6
One way to
enforce a
no-passing zone,
Hawaii. Basalt is a
dense, black,
fine-grained rock.
Light areas in the
photo still are
red-hot.

Many volcanic features occur in the United States. The Cascade Range includes
Mts. Shasta, Rainier, and St. Helens. The Hawaiin Islands are volcanoes, and
Alaska forms a margin of the ‘‘Ring of Fire.’’ Other examples are San Francisco
Mountain, Mt. Taylor, and the San Juan Mountains in Utah, Arizona, Colorado,
and New Mexico.

2.5.2 Shield Volcanoes and the Strange State of Hawaii

Lava that has a relatively low viscosity can flow out to form domes instead of the
cones. Hawaii has the largest assemblage of volcanoes having this shape, referred
to as shield volcanoes.

The island of Hawaii, or the ‘‘Big Island,’’ is the largest of the chain and the
newest that extends above sea level. Farther west the islands are progressively
smaller and older, finally culminating in a series of tiny islands called atolls.
This is explained by a deep, stationary lava source that occasionally cuts a hole
in an overlying Pacific oceanic plate to create a volcanic island. The next island
already is forming to the east of Hawaii and consists of a huge volcanic mountain
extruding deep under the sea. The rate of plate movement deduced from the
successive ages of the volcanoes is as much as 5 cm (2 in.) per year.

Volcanic eruptions associated with Yellowstone Park also indicate plate move-
ment over a stationary lava source. This started in what now is southeastern
Oregon about 16.5 million years ago. As the continent continued to move WSW,
eruptions occurred in southern Idaho and northwestern Wyoming. The last
explosive eruption, 2.1 million years ago, resulted in ash being carried as far
as western Iowa.

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.