Volcanoes in our Backyard
by Tara A. Spears
Travel inland from coastal Jaltemba Bay and in just a couple of hours you will view majestic volcanic mountains whose raw power contrasts with the farmlands and villages along the highway. While these giants can cause major catastrophes, there are beneficial effects for the peoples who live near the volcanoes.
Pumice is a type of extrusive volcanic rock, produced when lava with a very high content of water and gases is thrown out of a volcano. As the gas bubbles escape from the lava, it becomes frothy. When this lava cools and hardens, the result is a very light rock material filled with tiny bubbles of gas. Pumice is the only rock that floats on water, although it will eventually become waterlogged and sink. It is usually light-colored, indicating that it is a volcanic rock high in silica content. If the lava hardens quickly with few volatiles, the resulting rock is volcanic glass, or obsidian. Pumice and obsidian are often found together. Since pumice is a volcanic rock, and retains its useful properties only when it is young and unaltered, pumice deposits are found in areas with young volcanic fields. Worldwide, over 50 countries-including Mexico-produce pumice products.
This well-preserved central spine, known locally as la tetilla (“the nipple”) is quite unusual. It represents the hardened lava which cooled in the central vent of the volcano and which, solid and unyielding, was later pushed upwards by tremendous subterranean pressure. The Tequila spine is one of the best examples of this type formation that exist anywhere in the world.
The state of Jalisco derives its name from the numerous volcanoes within its boundaries. “Jal is sand pumice, ixco, plane or surface.” says Dr. Enrique Estrada Faudon, of the University of Guadalajara. “All the hills that we see around Guadalajara and Lake Chapala, (Tonalá, La Higuera, the Mexican, San Miguel, the Colli, and Copalita) are extinct volcanoes.” Jalisco is inside a volcanic belt that runs from the Gulf of Mexico toward the Pacific Ocean.
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