The water in
Sponge Geyser often completely fills the raised cone, which is formed of grey-brown sinter with an uneven surface, resembling a sponge. Like
Pump Geyser on the opposite side of the trail, Sponge Geyser is generally very active, throwing water 1 or 2 feet in the air over a period of up to one minute, with a similar interval between eruptions. In recent years the spring has become less spectacular, however, the water rising and falling but not being ejected. The large cone partly obscures two other small geysers behind, Plate and Slot.
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map of the Upper Geyser Basin