During the few mornings we stayed at Gardiner, the north entrance to the park, we hiked up to the terrace at Mammoth Hot Springs before dawn to catch the first light. Standing on the brink of the pool, frigid, the light first hit the snow-capped hill behind us and gradually, the pool started to glow by reflecting the blue-white-ish sky. Before we knew it, the entire landscape in front of us was bathing in dazzling golden light. At Mammoth Hot Springs thousands of gallons of water well up each day, deposit large amounts of travertine, and build tier upon tier of cascading, terraced stone. The mist arose slowly from the terrace and shaped the sunlight in thousands of different ways. After a handful of pictures captured in as short as 10 minutes, we packed and headed to our transportation of the day.
Compared to an average of 25000 to 30000 people on any given summer day, the park only hosts 78 snow coaches and 318 snowmobiles each day during the winter. The traditional Bombardier snow coaches look like overgrown VW Beatles, and the more recent snow coaches are simply modern van placed onto a tracked undercarriage. Due to the lack of snow this December, however, we had to travel on bus to get to Snowlodge at Old Faithful where we spent a few days to explore the geyser basins.
Geologists theorize that a hotspot in the upper mantle of the Earth’s crust is responsible for the thermal and volcanic activity in Yellowstone. The hotspots are postulated to be fed by narrow streams of hot mantle rising from the Earth’s core-mantle boundary in a structure called mantle plume (this is different from the well-known island arc volcanoes which form over subduction zones at converging plate boundaries). The hotspot itself is stationary, but since the North America Plate is really moving about 1.8 inches per year in a southwesterly direction, the volcanic activity center appears to move in the opposite direction.
There were three super-eruptions in the past 2 million years. These eruptions were so enormous that the stores of magma were emptied and the overlying land collapsed to form the so-called caldera. Yellowstone’s thermal features are evidence of a continued heat source not far from the surface. There are 4 types of thermal features: hot springs, geysers, mud pots and fumaroles. Basically the surface water seeps down to meet the heat of the molten rock. The superheated water then rises back up to form a hot spring. If the water is constricted on its way up, the pressure builds and eventually an eruption happens to form a geyser. More than 55% of the world’s geysers reside in Yellowstone. Fumaroles, or steam vents, are features where the water is vaporized and expelled from holes in the ground. Mud pots are acidic hot springs with limited water supply. The sulfuric acid breaks down the rock into clay and various gases escape through the wet clay and cause it to bubble.