Mt HEKLA located in Iceland is situated near a divergent plate boundary. Therefore, it should be classified as a shield volcano, which typically emits pahoehoe-type lava. In past eruptions, however, it has shown characteristics of a composite volcano. Ejecta from Hekla has contained rhyolitic material, the typical material found in volcanoes situated near convergent plate boundaries. Why is Hekla natured so if it is located near a divergent plate boundary? Why does it emit composite materials? I suspect the reasons are that, due to the time spans of eruptions here, Bowen's reaction series are allowed to continue, resulting in the formation of composite materials. I may be a bit off in my suspicions. Please let me know if I am right or not (I am stationed here in Iceland).

John Rudolph

Dear John,

I think you have this pretty well figured out. I might be able to provide some details. The composition of material erupted from Hekla changes progressively from andesite and dacite (58-63 weight percent silica) at the beginning of an eruption to basaltic andesite (54 weight percent silica) at the end of the eruption. The longer amount of time between eruptions the more silica rich the volcanic products are at the start of the next eruption.

Baldridge and others (1973) studied the chemistry of the minerals and rocks from the 1973 eruption. They found that the chemical changes in the rock could be produced by the formation and removal of crystals of olivine, augite. plagioclase, and magnetite from the magma.

Because the first volcanic products are silica rich they tend to erupt as tephra. As the silica content in the magma becomes lower and the viscosity decreases the gases escape more readily. At some point there is not enough gas to fragment the magma as it comes out of the vent and it pours out as lava.

Types of volcanoes are not restricted to specific types of plate boundaries. Processes, like the settling of crystals, can influence the composition of the rocks and exert some control on how a volcano erupts and the morphology of the volcano.

Steve Mattox, University of North Dakota

Sources of Information:
Baldridge, W.S., McGetchin, T.R., and Frey, F.A., 1973, Magmatic evolution of Hekla, Iceland: Contributions to Mineralogy and Petrology, v. 42, p. 245-258.

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