At times, Mastiff has frequent boiling and splashing "eruptions" These "eruptions" are usually very small but sometimes reach 5 feet. For the most part, this activity signifies little and goes virtually unnoticed. What is noticed though is Mastiff's role in Giant's Hot Periods and in Giant's Mastiff Function eruptions.
Mastiff Geyser plays a key role in the observation of a Giant Hot Period. From the boardwalk, it is possible to see a short distance into Mastiff's front vent. This vantage point makes it possible to see if water is rising during a Bijou pause. As the water rises, it forms a slowly rising pool in Mastiff. This pool is easily distinguished from the splashing that is sometimes seen in Mastiff's front vent. If at any time, the pool drops and does not rebound, the hot period is probably doomed to end without an eruption from Giant. As long as the pool stays up, the eruption is possible. As the water rises, if this is going to be a hot period, eventually, some of the platform vents will start overflowing and at least one will start erupting. The water in Mastiff may continue to rise and start to overflow. Depending on the strength of the hot period and Mastiff's contribution to it, Mastiff may continue to rise. The overflow can become very large. Mastiff may start surging from its two vents. These surges can reach the height of Giant's cone, about 12 feet and yet the hot period may still end without an eruption. On the other hand, if this is a Mastiff Function eruption of Giant, Mastiff will eventually start to erupt. Mastiff's two vent erupt differently. One erupts in a massive vertical column while the other erupts less massively at an angle. Surprisingly, considering the two vents can be seen to be connected a short distance below ground, the two vents do not always erupt together. Mastiff's eruption reaches over 35 feet in height and has been known to reach a very impressive 125 feet. Mastiff usually lasts less then 5 minutes and ends just before Giant reaches its full height. A Mastiff Function eruption of Giant is one of the most spectacular eruptions to be seen in Yellowstone National Park.