Question

Deep-sea hydrothermal vents form along mid-ocean ridges where tectonic plates pull apart, allowing seawater to penetrate the Earth's crust, heat up, and dissolve minerals from the surrounding rock. As this superheated fluid, often exceeding $350^\circ\text{C}$, is expelled back into the ocean, it immediately encounters ambient seawater, which is near-freezing at approximately $2^\circ\text{C}$. This abrupt temperature drop reduces the solubility of dissolved minerals, causing metal sulfides to precipitate rapidly out of the solution. The precipitated minerals accumulate around the vent opening, gradually building the towering chimney structures known as "black smokers." While the physical structure of the chimney is formed by this precipitation, its growth rate is directly influenced by the velocity of the exiting fluid. A faster flow rate prevents the cold seawater from entraining deep within the vent opening, which preserves the high temperature of the core fluid and projects the zone of precipitation outward, resulting in wider but less stable walls. Conversely, a slower flow rate allows cold seawater to seep into the conduit, cooling the fluid prematurely and causing mineral deposition to occur internally, which constricts the flow channel and ultimately limits the height of the chimney.

According to the passage, a slower flow rate of the exiting fluid leads to the constriction of the chimney's flow channel by doing which of the following?