6 January 2024

This morning I was reading through the geology subs on reddit and came across something that I thought might be rhyolite or volcanic tuff, and then started wondering what the distinction between them was. I started searching, and soon my quest turned into more of a ramble. Here are some of the things I learned.

• Rhyolite vs. Tuff. As it turns out the difference between rhyolite and tuff is that rhyolite is has a very fine-grained aphanitic texture, where as tuffs generally have a coarser and more varied texture, and may show some sorting. Rhyolite may have mineral grains (e.g. small quartz crystals) within it, but they are embedded in a ground mass of fine-grained material.

• Welded tuff occurs when the ash is more than 600° C (1100° F). Essentially glass and pumice fragments adhere, “necking at point contacts,” and deform and compact together.
  
• Unwelded tuff is relatively unconsolidated, but if it contains a lot of volcanic glass (a thermodynamically unstable mineral) it will lithify rapidly in the presence of water, which leaches alkali metals and calcium and forms new minerals (zeolites, clay, calcite) that cement the tuff. 
  
• Tuff Rheology. Tuffs may range from well-sorted, when produced by ash fall, to poorly sorted, produced by pyroclastic flows and surges — the latter my sometimes exhibit sedimentary structures such as dunes and anti-dunes produced by high velocity flows. In flows of tuff, the bottom will often be unwelded and poorly consolidated, due to compact with the cold surface.
  
• Anti-dunes are flow structures in which material accumulates on the ‘upstream’ side of the structure (whereas with dunes material accumulates on the ‘downstream’ or lee side). Anti-dunes migrate ‘upstream,’ and grow rapidly as they move counter-current until they collapse. 
  
• Fiamme are lens-shapes, mm to cm, usually seen in volcanoclastic  rocks like tuffs.

Somehow, I also got into ways in which granite can alter, but I’m not sure how

• Miarolitic cavity — a crystal-line cavity that can be found in granitic pegmatites. Often formed by volatile portions of magma excluded from the crystal phases — eventually the volatiles will form cavities. These cavities, in turn, often contain unusual or rare minerals that are incompatible with silicate granite mineralogy.
  
• Griesen is a highly altered granitic rock or pegmatite. Griesen is formed by self-generated alteration of granitic magma taking place at moderate to high temperatures; it involves hydrothermal-magmatic alteration and is often related to the release of volatiles in the later stages of solidification. Griesens exhibit variable alteration and range from coarse crystalline granite with miarolitic cavities to rocks rich quartz and muscovite. 
  
• Stockwork. A structural system of structurally controlled or randomly oriented veins. 

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