Old Faithful

Yellowstone National Park is famous for its geothermal activity, evidenced by mud pots, bubbling pools, steam, and geysers, all fueled by the hot rocks of magma chambers that heat the water.  Half of the earth's geothermal features and two-thirds of the earth's geysers are concentrated in Yellowstone, according to Wikipedia.

Old Faithful was first named by the Washburn-Langford-Doane Expedition because the geyser erupted at regular intervals, approximately hourly.  Since then, this well-known geyser's activity has been disrupted by the effects of small quakes, and it is now less faithful.  Today, eruptions very anywhere from 45-125 minutes apart.

In the above photo, my grandson is plugging his ears as we wait for Old Faithful to do its thing at twilight.  He thought the geyser would be noisy, but the hot water eruption is not like an explosion,  just a tremendous upward force of hissing and gushing.
The eruptions are an average of 145 feet high and last 1.5 to 5 minutes. 

We also saw the Beehive Geyser erupt.  A ranger told us this one erupts unpredictably, sometimes "nunce" a day. Its 4 foot tall cone is shaped like a beehive.  Wikipedia says it erupts every 10-20 hours, and reaches a height of 200 feet.