CALCULATED EFFECT OF VESICLES ON THE THERMAL-PROPERTIES OF COOLING BASALTIC LAVA FLOWS

Authors
Citation
L. Keszthelyi, CALCULATED EFFECT OF VESICLES ON THE THERMAL-PROPERTIES OF COOLING BASALTIC LAVA FLOWS, Journal of volcanology and geothermal research, 63(3-4), 1994, pp. 257-266
Citations number
36
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geology
ISSN journal
0377-0273
Volume
63
Issue
3-4
Year of publication
1994
Pages
257 - 266
Database
ISI
SICI code
0377-0273(1994)63:3-4<257:CEOVOT>2.0.ZU;2-T
Abstract
This paper investigates the effects of porosity, in the form of vesicl es and bubbles, on the transfer of heat within basaltic lava. This stu dy was undertaken to provide input for realistic cooling models of bas altic lava flows and to help explain some recent field measurements by Jones (1992, 1993). These field observations indicate that the surfac es of vesicular pahoehoe flows cool significantly more rapidly than th at of dense flows. It has been suggested that thermal radiation across vesicles is responsible for this enhanced cooling rate. It is shown h ere that, for vesicles in typical pahoehoe flows, radiation across ves icles may enhance the effective thermal conductivity and thermal diffu sivity, but only at high temperatures (T > 800-degrees-C) and high ves icularities (pi > 40%). It is also found that convection of the gas ph ase within bubbles will not occur unless the cavities are larger than about 1 cm. Furthermore, previous work has demonstrated that porosity greatly reduces the thermal conductivity and thermal inertia of cold l ava. It is shown here that this should also be true at high temperatur es. Examining only radiation across vesicles severely understates the effect of porosity on the cooling of the surface of lava flows. Therma l inertia, developed to describe diurnal surface temperature variation s, is the most appropriate thermal parameter in this case. Thermal ine rtia is a measure of how quickly surface temperature responds to heati ng or cooling, with low thermal inertia materials responding more quic kly. It is shown here that porosity greatly reduces thermal inertia at all temperatures and that this provides a more general explanation fo r the field observation.