Including Original "Paul H. Letters" Copyright © 1996-2017 Paul V. Heinrich - All rights reserved.

Sunday, 11 January 2015

Question - Lava

Question - Lava

On December 8, 2014 and in “Question” at ,
Axel wrote,

>It is true that iron content influences viscosity of
>lava in general, isn't it? (If not, I got to get out and
>drive some to kick a volcanologist in the proverbials...) LOL
>[Axel] I was indoctrinated (hmmm) that felsic lava
>has more silica in it, which forms chains upon cooling...
>hence high viscosity. Mafic lava (more Mg and Fe)
>takes higher temperature to melt but is more liquid.
>I logically derived that if Fe and Mg exsolve from
>the mix, the partial Si content would rise resulting in
>more Si-oxide chains and higher viscosity.

There are three major controls on viscosity of lava.

They are:

1. Temperature – The higher the temperature of a
lava, the less viscous it is and easier it flows. In
some cases, this is partly a function of phenocryst
content and size, which increases the viscosity of
lava with increasing phenocryst size and content
as the lava cools. Primarily, it is because viscosity
of liquids decrease with temperature.

2. Silica content – the lower the silica content, the
lower the viscosity. This is because the silica
polymerizes and the chains increase viscosity. As a
result, Mg and Fe-rich, mafic lavas are less viscous
than silicic lava. Carbonatite lava, which consists of
sodium and potassium carbonate, of Oldoinyo
Lengai has the lowest viscosity of any terrestrial
lavas because it has than three percent silica.

3. Gas content - the greater the gas (volatile) content,
the lower the viscosity.

Typically mafic lava is extruded at higher temperatures
(circa 1100°C) than silicic lava (circa 750°C). As a result
mafic lavas are less viscous than silicic lavas because
of both temperature and composition.

Some web pages.

World's Coolest Lava is in Africa

Practical volcanology - Lecture notes for understanding
volcanic rocks from field based studies By Martin, Ulrike; Nemeth, Karoly

It was also written,

>[The difference between aa and pahoehoe lava is
>related to cooling of the lava, and also loss of gasses,
> both of which increase the viscosity, turning pahoehoe
>into aa. Both can have the same chemical composition, …

This is true. However, flow rate can control whether
pahoehoe lava can form or not. For example the study of
historical geologic data from Kilauea and Mauna Loa
volcanoes found that only the rough-surfaced variety
of basalt lava called a'a / aa forms when lava flows at rates
greater than 5-10 m3 per second, and the smooth-surfaced
variety called pahoehoe forms at rates lower than 5-10 m3
per second. Of course, the maximum flow rate at which
pahoehoe forms varies with viscosity. There is another
form of lava called “toothpaste lava” which forms at the
transition from a'a to pahoehoe lava.

Go see,

Rowland, S. K., G. P. L. Walker, 1990, Pahoehoe and aa
in Hawaii: volumetric flow rate controls the lava structure.
Bulletin of Volcanology. vol. 52, no. 8, pp 615-628.

Rowland S. K., and G. P. L. Walker, 1987, Toothpaste
lava: characteristics and origin of a lava structural type
transitional between pahoehoe and a'a. Bulletin of
Volcanology. vol. 49, pp. 631-641.

Lava Flows


Paul H.

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