Classification of Pyroclastic Rocks

While plutonic rocks look relatively alike (except their color), volcanic rocks can look vastly different from each other, depending on their origin. Volcanism creates different types of materials: pyroclastic rocks and solid rocks. Solid volcanic rocks form from magma (inside the volcano) or from lava (on the Earth’s surface). The outflowing of lava requires calm and volatile poor volcanic activity.
Pyroclastic rocks though are formed by highly energetic, volatile rich, explosive eruptions. The spontaneous volume increase of gas caused by decompression during ascend from the interior of the Earth literally tears apart the surrounding melt and catapult them away from the crater.
Because of their form of deposition they fall partly into the realm of sedimentary rocks.

For more information on volcanoes check out the post from @mirkon86 here

Pyroclastic rocks are subdivided by size:
• ash is < 2 mm in size
• lapilli are 2-64 mm in size
• bombs are > 64 mm in size
• Already solid particles from the volcano > 64 mm in size are termed blocks

^{Volcanic Bomb.
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^{Lapilli.
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^{Ash.
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^{Blocks.
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Based on the distribution of sizes, pyroclastic rocks can be classified using the following chart.

^{Classification of pyroclastic rocks using size abundances (after Fisher, 1966).
atlas-hornin.sk}

Other than during downfall after ejection, pyroclastic material can be deposited from a pyroclastic flow. Rocks deposited by such process are termed ignimbrite. Ignimbrites commonly include crystals, pumice (frothy magma blobs), lithic fragments (pre-existing, solidified parts of the volcano), and an ash rich matrix.

Pyroclastic rocks can be classified using the fragments that are embedded in the matrix:

^{Classification of pyroclastic rocks using fragment composition (afer Pettijohn, 1975)
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In magmas, gases are dissolved at high pressure, but exsolve as magma rise to the surface. Exolution of gas produces vesicles, or bubbles, in the magma. Mafic pyroclasts with abundant vesicles are called cinder or scoria, whereas felsic vesicular magmas are called pumice. The highly vesicular texture causes pumice to have a density below 1, which means they will float on water. In pumice, the linings of the bubble walls are glassy. Since pumice becomes abraded during transport in a pyroclastic flow, it is common to see triangle-shaped or hourglass-shaped glass shards in the ashy matrix. These are broken bubble walls. Glass shards can also be found in some mafic pyroclastic deposits.

^{Pumice with highly vesicular texture.
geology.com}

After deposition, the ignimbrite may compact, if it is hot and thick enough. This process is termed welding. Typically, thin ignimbrites, or the top and the base of thick ignimbrites are not compacted and are termed unwelded. In unwelded rocks, the pumice are still fat and frothy. As more pressure is exerted on the central parts of the flow, the pumice becomes flattened to form fiamme. The greater the pressure, the flatter the pumice become, and the rocks go from moderately to densely welded. In densely welded rocks, the fiamme may be remelted to form obsidian. The ashy matrix is also compacted during welding. However, crystals and lithic fragments are not flattened, because they are already solid when the ignimbrite is deposited, so either fiamme or ash matrix is commonly wrapped around lithic fragments.

^{Unwelded Tuff.
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^{Densely welded tuff with flattened pumice fragments (fiamme) and matrix wrapping around undeformed lithic fragment.
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^{Extremely densely welded tuff with recrystallized obsidian fiamme.
volcano.oregonstate.edu}

Sources

• Best, M. G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
• Markl, G. (2008). Minerale und Gesteine, 2. Auflage. Spektrum Akademischer Verlag
• Fisher, R.V. (1966). Rocks composed of volcanic fragments and their classification. Earth-Science Reviews 1, 4, 287-298
• Pettijohn, F.J. (1975). Sedimentary Rocks. Harper. 718 pages

https://volcanosareamazing.weebly.com/types-of-pyroclastic-material.html
http://www.atlas-hornin.sk/en/article/7/classification-principles
http://www.dplot.com/blog/2009/06/ternary-plot-options-for-rocks.html
http://www.sandatlas.org/ignimbrite/
http://volcano.oregonstate.edu/stop-30-filo-del-gado-ignimbrite
https://geology.com/rocks/pumice.shtml