GEOLOGY

The Formation of Palagonite Tuffs

The basalt glass of the Surtsey tephra is easily altered, and the rock formed is called palagonite tuff (móberg). The conversion causes the tephra particles to stick together. This alteration process is called palagonitization.

The first sign of palagonite tuff on the surface was in November 1969. It had formed inside the hydrothermal area at a temperature of 60-70°C over a period of approximately 2.5 years. Since then the formation of palagonite tuff on the surface has been closely observed, and research has also been done on the drilling core from 1979.

The palagonitization is primarily dependent on the temperature of the water in the rock. At a temperature of 100°C and above, the Surtsey tephra has consolidated into palagonite tuff in approximately one year or even only a few months. This is a much more rapid palagonization than was expected. At lower temperatures, the rate of conversion is slower. At 40-50°C, for example, it has taken 4-8 years. At present, most of the tephra above sea level has consolidated into hard palagonite tuff. It is also believed that submarine palagonization has been considerable.

Complex chemical reactions take place when basalt glass transforms into palagonite. Cations are released from the glass and in their place the glass binds to water causing the iron to oxidize. The cations, particularly Si, Al, Ca, Na and Mg, can then form cavity fillings in the rock. A total of ten new kinds of newly formed minerals have been discovered in the palagonite tuffs in Surtsey. The most common are analsime, phillipsite, tobermotite, smektite and anhydrite.

The Surtsey eruption has led to a better understanding of how the palagonite tuff mountains were formed in the Ice Age. Research on the alteration of tephra has yielded important information on palagonization. In this regard, Surtsey can be compared to a gigantic laboratory. The latest discovery in these research projects is from a study of microorganisms in the rock. Results show that certain bacteria species dissolve the basalt glass in the tephra, thereby increasing the rate of palagonite formation.

See a slideshow on palagonite formation in Surtsey

(author: Sveinn. P. Jakobsson – sjak@ni.is)

- last updated 06-May-2007