Hot water is much, much more effective at dissolving silica than cool water. With some materials, like table salt (NaCl) the change in solubility doesn't change with temperature as much as one might expect. With others, such as washing borax and silica, it's much greater than one expects. With silica, the solubility continues to increase beyond what we think of as "boiling," which is dependent on pressure. If there's significant confining pressure on a volume of water beyond that of the atmosphere, it's boiling temperature goes up- and so does the amount of silica that water can dissolve.
So when pressurized hot water is relieved from that pressure and cools down, it's often carrying more silica in solution than it can continue doing. The silica precipitates, generally as hydrous, amorphous silica, known as opal. Sinter has a characteristic "foamy" texture, like styrofoam. I suspect the circular voids are spots where gas bubbles prevented water from occupying that space, thus preventing silica from precipitating there.