6.1. Introduction
This Chapter describes the major physical characteristics of the rock samples used for triaxial and fracture toughness tests. In addition, in an attempt to explain the toughening effect of thermal microcracking observed in the fracture toughness results, detailed petrographical studies. X-ray diffraction analyses, and scanning electron microprobe and X-ray microprobe observations were undertaken. A detailed analysis on the rocks’ behaviour when subjected to thermal stresses is not only useful to understand and interpret the results, but also has applications in lava flow studies, where cooled lavas are re-heated by overlying fresh lava. The rock description also includes thermal cracking experiments, density, porosity and wave velocity measurements, all described in detail in this Chapter. In Section 6.5, I analyse the effect of the thermally induced damage with the specific purpose of explaining the toughening of our samples due to heat treatment (Chapter 7-8). In the discussion of the experimental results (Chapter 8) I will refer to this analysis, however it is considered as part of the sample description and preparation and therefore kept as a separate section in this Chapter.
This study involves the study of volcanic hazards of European volcanoes, as such, samples were collected from Mount Vesuvius and Mount Etna, in Italy. Both rocks were taken from aa basalt flows with less than 50wt% SiO] and represent typical basaltic rocks of effusive volcanoes. Table 6.1 lists the typical characteristics of basaltic flows compared to the andesitic flows. The samples chosen for our study fall within these characteristics both from the compositional and from a morphological point of view. Therefore, results of this study may be confidently applied to the study and modelling of other basaltic effusive volcanoes.
The majority of the experimental program was based on the Vesuvian rock, while Etnean rock was used as a comparison. The Vesuvian samples were collected from a quarry therefore a greater quantity of it was available. In addition, the samples were collected from the massive core of a flow and were well preserved, with only a few visible flaws. The Etnean samples on the other hand, were collected from the field and were chosen amongst other naturally broken parts of the flow. They contained larger flaws, and only a small block could be collected. Hence, the Vesuvian rock was chosen to be the primary testing sample. Further details of the samples, including the location of the original flows, are found below.
This study is primary concerned with the fracturing of the cooling crust o f a flow. It is therefore important to investigate on the mechanical behaviour o f the crustal part o f the flow compared to the core o f the flow, normally used in laboratory experiments. The main visible difference between the two lies in its heterogeneity and its much higher porosity with respect to the core. The rapid cooling of the crust and
C li(ij)ft'r 6- C liin tic tc r is d lio ii o f th e S a n ip lc P r o p e r lic s tlir o n p li ( l e o l o p i c i il T cc h n 'u p ie s
Strong longitudinal stresses across the surface of the flow, cause the top part of the flow to be rough and cindery. Stresses may also cause the crust to overturn before complete cooling has occurred, allowing the crust to seal to the underlying flow. High cooling rates of the crust mean that large vesicles may get trapped inside the rock as it cools, while the rapid movement, typical of aa flows, causes the vesicles to be deformed in shape. Due to the low thermal diffusivity of basalts (Lore et al, 2001; Kilbum, 2000), the core of the flow is well insulated beneath the crust, allowing time for larger crystals within the groundmass to form. On the other hand, the first few centimetres of crust cool within minutes and have generally higher glass content and have a very fine and glass rich groundmass. Figure 6.4b shows the mainly glassy nature of the crustal sample groundmass. All these characteristic differences between the crust and the core suggest that the mechanical properties of the two might vary considerably. A number of comparative mechanical tests on the crust of the flow were therefore needed to determine the importance o f such differences. A section o f the crust of the Etna flow was collected and used for our experiments. The crust of the Vesuvian flow was not available from the quarry.
Composition
Typical length (km)
Mean thickness (m)
Mean discharge rate (m^s‘) Flow volume (km^) <55% SiOz <10 (basalts to 50) 3-20 10-100 (basalts to 1000) 0.01-0.1 (basalts < 1-2)
Basalts, basaltic andésites. E.g. Kilauea and Mauna Loa, Hawaii; Etna and Vesuvius, Italy; Iceland;
Piton de la Fournaise, Reunion Is.; Lanzarote and Tenerife, Canary Is.; Arenal, Cost Rica; Paricutin, Mexico. >55% SiOz <5 (some to 15) 20-300 1-10 (andésites to 100) 0.01- 1.0
Andésites, dacites, trachytes. E.g. Loquimay, Chile; Nea Kameni, Santorini, Greece; Hibok-Hibok,
Philippines; Trident, Alaska.
Table 6.1- Characteristics of typical basaltic and non-basaltic lava flows (from Kilbum, 2000).
6.2. Rock Description
6.2.1, Etnean Basalt
Two blocks of basalt approximately 0.5 x 0.5 x 0.3m were collected from the 1983 flow of Mount Etna (Figure 6.3 and 6.4); one from the core and one from the crust o f the flow. They were collected from the same section of the flow, close to the Rifugio La Sapienza, south of the SE crater. Figure 6.1 shows the SE section of a geological map of Mount Etna indicating the sampling location. Figure 6.2 shows a geological map of the 1983 flow. One block was taken from an intact part o f the crust of the flow and the
C h a p te r 6 - ( h a r m t e r i s a t io n o f th e S a m p le P r o p e r tie s th r o u g h C e o lo g ie a l T e e h tiitp tc s
other was taken from the core of the flow. The samples were taken from this flow, because it was easily accessible, but also because it represents the archetypal basaltic flow both in its composition and its field characteristics shown in Table 6.2 (compared to standard values on Table 6.1).
While the core and the crust are very similar in composition, there are obvious clear physical differences between them, which might affect the mechanical properties. Chemically, both the core and the crust of the flow are typical porphyritic basaltic rocks. Photomicrographs of the core and the cmst (Figure 6.5a and b) imder cross-polarised light, show clinoyroxene, plagioclase feldspar and olivine crystals o f up to 0.5mm in size set in a fine glassy/cryptocrystalline ground mass with an average crystal size of approximately 50 microns. Results of our petrographical study of the samples are shown in Table 6.3. A large quantity of glass within the fine matrix is also present especially in the crust sample and large vesicles of up to 1mm in size can also be seen in both samples. The composition of the core and the cmst is very similar, with exception to the higher glass content in the groimdmass of the cmstal sample. Hence, any difference in the result of deformation experiments between the two samples is to be attributed to physical differences or to the glass content. The bulk composition of the 1983 flow has been analysed (Pinkerton and Norton, 1995) and is listed in Table 6.4, together with the bulk composition of the Vesuvian basalt. When plotted on a KoO + Na^ - SiO? the Etna 1983 flow sample falls within the trachybasalt field (Trigila et al., 1990).
Sampling location
Figure 6.1- Geological map of Mount Etna. The arrow indicates the location on the 1983 flow where the samples were collected. The scale bar represents 5km.