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   Short course I NTRODUCTION TO PHYSICAL VOLCANOLOGY AND VOLCANIC TEXTURES   Prof. Dr. Christoph Breitkreuz Centre for Volcanic Textures   Institut für Geologie/Paläontologie TU Bergakademie Freiberg Freiberg, Oct. 3 rd  – 7 th , 2005   2 Background and intention of the course Based on experience in the investigation of fossil and (sub-)recent volcanic and volcanoclastic successions this short course puts emphasis on the recognition of volcanic textures in outcrop, rock slab and thin section, and on the potentials and limits of their genetic interpretation. The course neglects other important aspects of volcanology such as magmatic petrology, geochemistry, isotope geochemistry, geotectonic setting, and volcanic hazards. The following text emphasises the processes which lead to the formation of volcanic textures. The textures itself will be presented and discussed during the short course. Contents 1. Some physical characteristics of magma and lava 3 2. Types of eruption 7 3. Volcanic forms 11 4. Volcanic fragmentation and fragments 18 Volcanic glass and its alteration 20 5. Volcanic transport and deposition: Lava and pyroclastics 22 Lava flow 22 Pyroclastic transport and deposition 27 Magma mingling and mixing 36 6. Emplacement, cooling and alteration textures in SiO 2 -rich lava and high-grade ignimbrites 37 Concept of carapace and core facies 42 7. References 43   3 1. Some physical characteristics of magma and lava In order to understand processes of magma ascent, eruption and fragmentation, some physical properties and behaviours of silicate melts are considered here. These are also relevant for the understanding of nucleation and growth of bubbles and crystals, and of lava flow. The viscosity  of silicate melts depends strongly on composition and, to a lesser degree, on temperature, pressure, and on the content of bubbles, crystals and lithics (Fig. 1.1). SiO 2  and Al 2 O 3  occur in silicate melts in the form of tetrahedrons which tend to connect at their corners with each other forming chains or networks (framework builder). Therefore SiO 2 -rich magmas are extremely viscous. All other components (MgO, FeO, CaO, K 2 O, Na 2 O etc.) and especially volatiles like H 2 O and CO 2  interrupt these tetrahedron structures and therefore lower the viscosity (framework modifier). Figure 1.2 shows that in rhyolitic melts, the extraction of a few % of H 2 O results in an dramatic increase of viscosity. Fig. 1.1 Relationship between viscosityand temperature for some magmas. Therhyolite was glassy or liquid through theentire temperature range (From Cas &Wright 1987, after Murase & McBirney1973). Table 1.1   Estimates of eruption temperatures for some common magmas (After Cas & Wright   1987).     4 Temperature estimates of some common terrestrial magmas during eruption are given in Table 1.1. Higher temperature lowers the viscosity of silicate melts because the stronger vibration of the components prevents network formation and existing bonds are less stable. Silicate melts have also large differences in density  (Fig. 1.3). These are mainly related to the content of mafic elements like Fe and Mg. Melts are less dense than their product after cooling (the magmatic rock). E.g. granitic melt has about 2.2 g cm -3  whereas granite has about 2.7 g cm -3 . Apart from some very low-viscosity lavas (e.g. alkaline basaltic) most flow processes of magma Fig. 1.2 The effect of H 2 O on the viscosityof (a) granitic and (b) basaltic melts atvarying temperatures (From Cas & Wright1987, after Murase 1962). Fig. 1.3 Densities of some molten volcanicrocks with varying temperature at atmospheric pressure (From Cas & Wright 1987, after Murase & McBirney 1973).
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