Pisolitic Tuff

Lapilli are spheroid, teardrop, dumbbell or button-shaped droplets of molten or semi-molten lava ejected from a volcanic eruption that fall to earth while still at least partially molten. Lapilli tuffs are a very common form of volcanic rock typical of rhyolite, andesite and dacite pyroclastic eruptions. Here, thick layers of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and welding of semi-molten lapilli into what is known as a welded tuff.

The heat of the newly-deposited volcanic pile tends to cause the semi-molten material to flatten out as they become welded. Welded tuff textures are distinctive (termed eutaxitic), with flattened lapilli, fiamme, blocks and bombs forming oblate to discus-shaped forms within layers. Rounded tephra balls are called "accretionary lapilli" if they consist of volcanic ash particles. Accretionary lapilli are formed in an eruption column or cloud by moisture or electrostatic forces, with the volcanic ash nucleating on some object and then accreting to it in layers before the accretionary lapillus falls from the cloud. Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus.
The processes of collision, binding, and breakup of particles in moisture-bearing eruption plumes lead to the formation of aggregates of ash that in turn play an important role in the deposition of ash from phreatoplinian plumes. Ash aggregates fall with greater velocities than their component particles and so the process of aggregation results in the premature flushing of large quantities of fine ash and the simultaneous deposition of a wide range of grain sizes. This creates poorly sorted and fine-grained fall deposits even close to the vent.

The formation of aggregates is governed by factors such as the concentration, shapes, and size distributions of ash, the abundance and physical state of water in the plume, and the temperature and relative humidity of the plume and surrounding atmosphere. Phreatoplinian ash aggregates form a spectrum, which can be related, in the first instance, to the liquid content (water plus dissolved gas species) of the depositing pyroclast assemblage. The spectrum ranges from weak clusters of grains at the dry end to ash-free rainfall at the wet end. In between these two endmembers are accretionary lapilli of various types, sizes, and structures and mud rain. The dry end of the ash aggregation spectrum is represented by weak clusters of grains, which have been observed to fall from dry plinian plumes.

With condensation of water in an eruption plume, ash will accrete into denser clumps as accretionary lapilli. Recent experimental work shows that the likely origin of accretionary lapilli is through collision of liquid-coated ash particles in the plume. Ash particles in a water-rich plume act as condensation nucleii for water vapor and clasts and may be continually coated in acidic liquid or ice. When clasts collide, due to different fall velocities and convection in the plume, the potential for binding is enhanced because of the effect of strong, short-range surface tension forces. Electrostatic attraction is also important for attraction of particles and the fine-grained outer layers on many accretionary lapilli probably form this way.

Two morphologically different types are distin-guished:

(1) Rim-type lapilli are composed of a coarse-grained core surrounded by a fine-grained rim. Rims are internally graded or made up of several layers of alternating fine and very-fine grained ash.

(2) Core-type lapilli lack fine-grained rims. Field relationships, internal, and grain-size characteristics are specific to accretionary lapilli from different types of tephra deposits.


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Pisolitic tuff sample




Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• D’Amico C., Innocenti F. & Sassi F.P. (1987): Magmatismo e metamorfismo. UTET
• Innocenti F., Rocchi S. & Triglia R. (1999:) La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG.
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
lapilliaccrezzionali(16).jpg

Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
lapilliaccrezzionali(17).jpg

Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 10x (Field of view = 2mm)
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Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 10x (Field of view = 2mm)
lapilliaccrezzionali(28).jpg

Accretionary lapilli (rim-type) in a Pisolitic tuff from Roccamonfina. Note the layers with different grain size. PPL image, 10x (Field of view = 2mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
tufopisolitico2012(3).jpg

Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
tufopisolitico2012(4).jpg

Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
tufopisolitico2012(7).jpg

Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
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Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)
tufopisolitico2012(6).jpg

Accretionary lapilli in a Pisolitic tuff from Roccamonfina. PPL image, 2x (Field of view = 7mm)