Yellowstone National Park in northwest Wyoming has long been a vacation destination for those wanting to see the rather unique geographical features it offers. While the steep, colorful walls of the Grand Canyon of Yellowstone and the rushing waters of the Upper and Lower Falls are beautiful, they are not among the most unique features. The free roaming elk, bison, and other wildlife are certainly a draw as well, but it's the concentrated collection of hydrothermal features that make Yellowstone so very unique and surreal at times.
For those who have never visited Yellowstone, a bit of a description or explanation is in order. There are geysers and hotsprings elsewhere in the world of course, but it is estimated that over 25% of the all of the world's known geysers exist within 2 miles of Old Faithful, within the geyser basins at Yellowstone. The area as we now know it was formed by a massive volcanic eruption 640,000 years ago. It was so large that it is said to have spewed rock fragments as far away as the Gulf of Mexico. The 47 mile wide crater that the eruption created then filled with molten lava and ash. The resulting caldera is what is now Yellowstone.
While the earth's crust is general 25 to 30 miles deep, in Yellowstone National Park it is much less, only 2 to 3 miles deep. This means that hot magma is closer to the surface, and with abundant water and plenty of cracks and fissures, hydrothermal features form easily. These take a variety of forms. Geysers, hot springs, mud pots, and furmaroles are common.
Geysers are probably the most recognizable hydrothermal feature. They are easily differentiated from the other features because they periodically send plumes of water skyward. Some will shoot water over 200 feet in the air. Some erupt hourly, others only once every few months or years. Clearly some are more predictable than others. Geysers are constricted and build pressure over time, once pressure is sufficient, they erupt. Geysers, like other hydrothermal features change over time. They may even go dormant altogether. Old Faithful is one of the best known geysers in Yellowstone National Park, but it's not the largest.
Some geysers can make the ground tremble before an eruption. There are over 300 of them present in Yellowstone.
Furmaroles are also known as steam vents. They are similar to geysers but primarily emit only steam as they don't have the same constriction and pressure as a geyser. They can still warm an area nicely and create a very surreal effect. When driving through the park you will see them along the roadside and from a great distance. Steam vents can actually produce some of the hottest emissions. Steam from a furmarole can be as hot as 284 degrees Fahrenheit.
Hot springs are pools of water, sometimes beautifully colored. They too create steam as the hot water cools at the surface. Bacteria, algae, and minerals in the water can create a rainbow of colors; yellow, orange, blue, and even black. Hot springs may have limestone/travertine deposits which create terraces. These can grow and change every year. The Mammoth Hot Springs at Yellowstone for instance, is said to deposit up to two tons of sediment each day.
Mud pots are another common hydrothermal feature visitors will see at the park. These are essentially hot springs that have less water. Instead of the pretty aqua colored pools, they will look like boiling mud; a grey/brown color that emits a strong sulphur type smell.
Certainly there are many other wonderful features and sights to enjoy at Yellowstone National Park, but it's the thermal features that set it apart geologically.