on November 20, 2007
Yellowstone is the product of vast volcanic action, but in recorded history, the only volcanic action is thethermals. The thermals are created by rain and melting snow percolating down into the ground until theyhit hot magma-- rock so hot it has melted into a slow moving liquid– where the water is heated to boilingtemperatures, causing it to return to the surface, either in an occasional explosive outburst– geysers– or ina continuous flow– Mammoth Hot Springs, the Grand Prismatic Pool, the Mud Volcano, the FountainPaint Pots, and Morning Glory Pool. Almost all the geysers– Old Faithful Basin, Norris Basin, Upper and Lower Basins, and West ThumbBasin along with a handful of solitary geysers– are located inside the west and south rim oft the vast Y*caldera. Old Faithful, which gets its name from regularly erupting every 92 minutes for decades, is the parks’ mostfamous geyser. Everybody goes to see Old Faithful blow, and it’s a good show– there are signs around thearea giving the estimated next time of an eruption so you can be on hand, but be 10-15 minutes earlybecause it is not an exact schedule. Old Faithful used to keep a more faithful schedule, but an earthquakesome years ago knocks its clock a bit out of whack.Don’t limit your visit to Old Faithful. At least walk through the rest of the Old Faithful Geyser Basin, andtry to find time to see the others. We especially like Norris, where, should you be lucky to see one of the 3-4 times a year eruptions of Norris’ biggest geysers, you will be impressed. Old Faithful is but a trickle bycomparison. The star of the other type of thermals, Mammoth Hot Springs is erratic. From 1997-2004, it was runningat a good clip, and was our favorite spot in Yellowstone. Then it largely dried up. The difference betweenOn and Off is the growth of multi-colored algae in the hot water. When Off, the springs are a series ofterraces of white travertine, the rock formed when mineral laden water evaporates on the surface. WhenOn, algae growth colors the rock red, yellow, blue and green– spectacular to see.The other slow flow termals are always running. Grand Prismatic Spring, the largest; the Fountain PaintPots; Morning Glory Pool and show a variety of vivid colors. Mud Volcano is a pit of bubbling mud. Sulfur Caldron is notable for its stinking smell. The high altitude makes its many lakes too cold for swimming, but you can swim in the Firehole River. The river, created by runoff from the thermals, is almost warm enough for year round swimming.
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