Encyclopedia of Volcanos.pdf

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FREEDOM PALESTINE

First Edition, 2007

ISBN 978 81 89940 54 6

Published by:

Global Media

1819, Bhagirath Palace,

Chandni Chowk, Delhi-110 006

Email: globalmedia@dkpd.com

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Table of Contents

1. Columnar Jointing

2. Ophsiolite

3. Types of Volcanoes

4. Volcanic Mineral Deposits

5. Current Volcanic Activity

6. Latest SI & USGS Reports

7. Things to Know

8. Volcanic and Geologic Terms

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Columnar Jointing

Columnar jointing forms in lava flows, sills, dikes, ignimbrites

(ashflow tuffs), and shallow intrusions of all compositions. Most

columns are straight with parallel sides and diameters from a few

centimeters to 3 m. Some columns are curved and vary in width.

Columns can reach heights of 30 m.

The columns

may form sets.

Straight, regular

columns are

called

colonnade.

Irregular,

fractures

columns are

called

entablature.

From Spry

(1962).

Most columns tend to have 5 or 6 sides but have as few as 3 and as

many as 7 sides.

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The columns form due to stress as the lava cools (Mallet, 1875; Iddings, 1886, 1909;

Spry, 1962). The lava contracts as it cools, forming cracks. Once the crack develops it

continues to grow. The growth is perpendicular to the surface of the flow. Entablature is

probably the result of cooling caused by fresh lava being covered by water. The flood

basalts probably damned rivers. When the rivers returned the water seeped down the

cracks in the cooling lava and caused rapid cooling from the surface downward (Long

and Wood, 1986). The division of colonnade and entablature is the result of slow cooling

from the base upward and rapid cooling from the top downward.

1931 postcard of Devils Tower, Wyoming, a

shallow intrusion that formed columnar jointing as

it cooled.

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