Pyroclastic Rocks I.G.Kenyon

Pyroclastic Rocks Consist of fragmental volcanic material blown into the atmosphere by explosive activity Mainly associated with felsic volcanoes

Pyroclastic Rocks – 2 Main Groups Material ejected from the volcano as liquid globules which solidifies in the air and is deposited as solid particles Material ejected from the volcano as solid fragments, this solid material has been fractured by the explosive activity

Materials Ejected in a Liquid State Pelées Hair Volcanic Bombs Pumice Scoriae

Pelées Hair A fine mass of hair-like glass Formed by lava being exuded through a small orifice and blown about by the wind Resembles candy floss in appearance 1cm

Volcanic Bombs Larger masses of liquid lava thrown into the air They rotate and take on characteristic shapes Spindle-bombs and breadcrust bombs are most common-usually vesicular Vary in size from small droplets to several cubic metres

Volcanic Bombs Volcanic bombs are large fragments of molten lava up to 1m in diameter expelled during an eruption. Bombs develop a rounded or almond shape as they are twirled through the air. Some bombs have a characteristic breadcrust surface, others resemble cauliflowers or cowpats depending on the way they land and solidify.

Section through a Volcanic Bomb Highly vesicular interior Breadcrust exterior, finer grained and less vesicular due to more rapid cooling 5cm

Pumice Highly vesicular material derived from acid lavas Very high porosity and low density So light that it may float on water

Pumice Low density, high porosity, floats on water Microscopic grain size, very rapid cooling at the earth’s surface Mineralogy: quartz, feldspar and mica 2cm Volcanic, felsic igneous rock Specimen from Mt. Teide, Tenerife Vesicles up to 3cm in diameter

Scoriae Associated with mafic lavas Vesicular but denser than pumice Globules of lava are ejected and the exterior chills and solidifies Interior is still hot and molten Upon landing they are still soft and are flattened into pancake shapes

Section through Strombolian Scoriae Cone Bedding dips 32°SE The cone has been half excavated for use in the construction industry 2m Some layers rich in volcanic bombs Strombolian refers to the style of pyroclastic eruption in which fragments of incandescent, vesiculating basaltic magma are ejected to a moderate height, landing as solid scoria to form a cone

Strombolian Scoriae Cone Volcanic bombs occur up to 50cm in diameter Scoriae clasts range in size from 3 to 15cm The structure is very friable and has an unstable surface

Material Ejected in a Solid State Agglomerate-fragments >64mm in diameter Lapilli – fragments 64mm - 2mm in diameter Ash, Tuff & Dust – fragments <2mm in diameter

Agglomerate – Volcanic Breccia Derived from agglomero meaning ‘gather into a heap’ Formed of volcanic or country rock in the vent or as part of the cone Produced by explosive activity which often shatters the top of the cone Coarse material is ejected a relatively short distance before settling back to earth Comprises angular fragments >64mm surrounded by finer tuff and lapilli

Agglomerate – Volcanic Breccia Large angular fragments up to 10cm in diameter Large fragments surrounded by material of ash and lapilli size Vent Agglomerate 5cm

Agglomerate–Volcanic Breccia, Arico, Tenerife Chaotic mixture of boulders over 2m to ash <2mm in diameter

Lapilli – Particles 2 – 64mm Derived from lapillus meaning ‘a little stone’ Most commonly small pea to walnut sized

Tuff – Particles <2mm The lithified equivalent of volcanic ash Classified according to the nature of the pyroclastic fragments Crystal Tuffs – composed of mainly crystals Lithic Tuffs – composed of fragments of rock Vitric Tuffs – composed of glassy fragments Welded Tuffs (Ignimbrites) – hot fragments welded together in Nuées Ardentes eruptions

The Formation of Ignimbrites Associated with Pyroclastic Flows Nueés Ardentes style eruptions Glowing fireclouds °C Particles weld together on settling Activity on Augustine, Alaska, photograph by M.Krafft

The Chimiche Ignimbrite, Arico, Tenerife This deposit covers more than 150 km2 of the Chimiche- Arico part of the island. It is thought to represent the collapse of a km high plinian eruptive column It is unwelded as the particles were cool by the time they had fallen 10-15km through the atmosphere back to earth 15m

Volcanic Ash – Unconsolidated material <2mm in diameter Road cutting in the Guimar Valley, Tenerife

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