Get to Know Yellowstone’s Doublet Pool

Doublet Pool is located a short walk from Old Faithful. It lies on the upper boardwalk of Geyser Hill. This is one of the prettiest pools in the Upper Geyser Basin. True to its name, it is one pool below the surface, but appears as two pools at the surface connected by a narrow channel. With its lacy, scalloped edge and blue water, it’s a favorite of photographers and worthy in itself to take the short walk around the Geyser Hill loop. It is about 8 feet deep and has rare eruptions that reach 2-3 feet high. However, the most interesting thing about this thermal feature is that it thumps regularly.

The Thumps

The heating of Doublet Pool isn’t enough to cause a true eruption, but instead, it is enough to bounce the water and thump. About every 20-30 minutes, the water in Doublet Pool rises to the top of the scalloped edge. While at a “full pool,” you’ll see bubbles rise and waves on the water’s surface. Then the thumps begin, lifting the water in the entire pool about an inch. This lifting of the water coincides with the thumps. You can hear and feel them if nobody walks on the boardwalk or talks too loudly. You can feel the thumps better when sitting down on the bench or putting your hand on the boardwalk. These thumps last 5-10 minutes.

The thumps come from steam bubbles collapsing. You can see this same process when boiling a pot of water. At first, there are small bubbles on the bottom of the pan, but they keep disappearing because they collapse as they’re not large enough to overcome the weight of the water above them.

Doublet Pool “Eruptions”

Eruptions of Doublet Pool are incredibly rare and have been recorded only four times. Two of these times were in conjunction with Giantess Geyser, one was after the Hebgen Lake earthquake, and during unusual behavior in this section of Geyser Hill in the fall of 2018. They are not a typical eruption where water shoots into the air, but rather, an area of water boils up 2 feet or so (see photo below). These are sometimes called “bubble eruptions” and can also be seen in nearby Beach Spring.

In 2018, Doublet Pool had a few days of these rare eruptions when nearby Ear Spring had a powerful and very rare eruption – the first since 1957. There were many interesting changes to this area of Geyser Hill, and two new small geysers opened up. You can read the details about this here. With the 2018 Ear Spring eruption, Double Pool’s intervals between thumps sped up, but are back to about 20-30 minutes between thumps.

Doublet Pool Helping Us Understand Geysers

With its regular thumps, Doublet Pool is helping researchers understand more about what factors influence eruptions. They have looked at how wind can cool a pool’s surface enough to delay an eruption or what effect the barometric pressure may or may not have. Strong winds have been correlated with longer times between eruptions, and strong wind would logically affect Doublet Pool’s ability to heat enough to thump. 

Doublet Pool’s regular thumps also give researchers regular opportunities to study what’s below ground using seismometers, and were a help to find out more about Old Faithful’s plumbing system.

In Conclusion

It’s worth investing your vacation time in a walk around Geyser Hill. This will let you see this gorgeous hot spring as well as other thermal features. Bonus points if you sit and wait for the thumps of this “almost-a-geyser.”

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