Ice on Minihaha Creek, 2

11 January 2024

I returned to Minihaha Creek five days after my previous visit for a run, and a look at the ice. The snow, not very deep before, has receded, and the fallen leaves arch above its surface, leaving a myriad of little cave-lets speckling the surface. I wonder if there are any organisms — macro or micro — that are adapted to take advantage of these niches.

Theatre: ‘Art’ at the Guthrie

6 January 2024

We had a nice outing today. The main event was the play ‘Art’ at the Guthrie.

‘Arts,’ a play by Yasmina Reza, is not about art per se, but rather a satire on friendship, and the dynamics of the power structures that underlie, or perhaps constitute, friendships. The play begins with a purchase of a modern painting – all white – by one of the trio of friends, and the ensuing reactions of the other friends. While the art purchase is that catalyst, it quickly becomes clear that what is at stake is really the changing relations between the friends. Although there are a lot of funny moments, the deepening disagreements and the vehemence with which they were conducted brought ‘Who’s Afraid of Virginia Wolfe’ to mind. The acting was excellent, particularly that of Max Max Wojtanowicz who played Yvonne, the ‘buffoon’ of the group. The lighting and sound design was also excellent, in particular in signaling moments of solliquoy by the various characters.

A Geology Ramble: Tuff to Granitic Alteration

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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First Ice Crystals on Minihaha Creek, 2024

5 January 2024

We’ve had a remarkably long fall. Although we’ve had some chilly periods, the temperatures are still tending to edge above freezing during the day, and in the last month we’ve had days that make it into the forties, and even the fifties. We’ve had two light snows: one but a dusting, and the second perhaps an inch, enough to almost bury the downed leaves. So the world is mostly gray and brown: the dully gray winter sky arches overhead, and the grass, trees, and carpet of leaves is brown brown brown. Very dull.

I have been waiting, with anticipation, for the first ice crystals to form on Minihaha creek, where I do most of my runs. I enjoy looking at ice on the creek as the winter progresses. First, most of the water is open, and a crystalline fringe forms along the edges of still portions of the creek. Next, the fringes grow, meeting in a delicate and parlous surface in the middle. As the cold deepens, the ice thickens. In the early part of the winter the ice tends to be crystal clear — although depending on the way in which the crystals form the may make portions of the surface matte white.