An active plume of molten rock from the earth’s mantle reaches up to the crust just below the Yellowstone area making it a super volcanic zone. As the local continental plate slid over this hot spot in a south westerly direction, major volcanic eruptions have taken place 2.1 million years, 1.3 million and the latest one some 630 thousand years back.
With the accumulated molten magma forcibly evicted by the last of these major volcanic outbursts, the top crust caved in into the emptied space below under its own weight to create the Yellowstone Caldera that was an elliptical depression some 72 kilometres long by 48 kilometres wide.
Sporadic minor eruptions and also the more gradual oozing out of molten lava to the surface thereafter, the latest episode being around 70 thousand years ago, filled up the caldera somewhat to result in the relatively flat current day landscape of the Yellowstone plateau at an average height of 8,000 feet above the mean sea level.
The presence of the super heated hot spot some 10 kilometres below Yellowstone combined with the water from rain and snow melt percolating down cracks and other faults in the rocky ground, through their dynamic interaction, have created more than 10,000 hydrothermal features in the Yellowstone area that together constitute almost half of such features found world over.
The hottest thermal features of the area are the hissing and whistling fumaroles, found mainly in the North West section of Yellowstone. They emit steam alone through vents in the ground as the scanty ground water in that area is entirely converted into steam such as the Black Growler in the Norris Geyser Basin. The effect of the subterranean hot spot is, however, more often seen in the form of colourful hot springs dotted all over.
Cool surface water being heavier descends into the ground till it comes in contact with the super heated rocks below and then rises back to the surface through gaping holes in the ground as hot water springs. Heat is dissipated on the surface and the water as it cools down sinks in again. This convection current set in motion keeps the hot spring from building up heat large enough to erupt.
The colour to the edges of the hot water springs is provided by microscopic heat loving thermophile bacteria that prosper in the super heated water, each colour band representing different colonies of bacteria, each of which prefer their own temperature zone. On the edges of the stunning Prismatic Spring, the yellow band represents bacteria that prefer higher temperature than the ones that form the rust or brown bands.
The deep blue colour of the central parts of other hot springs such as the Sapphire Pool is on account of the optics of white light penetrating the water wherein the blue end of the spectrum gets dispersed more than the red end. Typically, these dark blue pools are very deep with a clear wide column of water for the laws of optics to operate in that manner.
The North West corner of Yellowstone has extensive sedimentary deposits of limestone left behind by evaporation of sea water some 350 million years back. Hot water passing through this porous soft rock dissolves the calcium carbonate in it and once the water emerges on the surface the dissolved carbon dioxide is released and the residual calcium carbonate deposited as travertine terraces.
The travertine terraces of Mammoth Hot Spring, looking like ‘frozen waterfalls’, can grow at the rate of 5 mm a day or one meter in a year. The microbial mats formed by communities of thermophile bacteria provide colour to the terraces and also slow down the flow of the water to enhance the rate of deposit accumulation. The flow of the water changes direction frequently on account of possibly the underground cavities caving in under the ever increasing weight of the rapidly growing terraces above. The portions rendered devoid of water revert to the chalky white coloured travertine that hardens over time to form a softer version of marble and is mined for making counter tops.
If the underground plumbing system at a particular spot is not clear but has a constriction, usually close to the surface, then the heated water below the constriction tends to get super heated under increasing pressure. Beyond a point, the pressure can no longer be held back by the constriction and the super heated water forcefully pushes the water above the constriction through an eruption of steam and scalding water, visible as a geyser above the surface. Consequent release of pressure and the built up heat makes the geyser subside, usually within a few minutes, till the next cycle of pressure build up repeats the process.
The iconic Old Faithful Geyser is known for the regularity of its eruptions on account of its hydrothermal system not being connected with any other neighbouring feature. Over decades the gap between successive eruptions have increased and the current pattern is of an eruption after an average of 65 minutes, if the duration of the preceding eruption was less than two and a half minutes and an average of 92 minutes if the preceding eruption was for more than two and a half minutes.
In certain areas surface water collects in shallow basins that are heated up by steam and hot hydrogen sulphide gas, notorious for the rotten egg smell, seeping up from deep underground. Some bacteria, by using the hydrogen sulphide gas to extract energy, help convert it into hydrochloric acid that dissolves the surrounding rocks. Inadequate water flow in these locations results in a gooey mass called mud pots through which the steam and gas continue to bubble through.
The Yellowstone area in general still remains highly volatile with the floor of the caldera pushed up by almost 25 centimetres between 2004 and 2008, though the uplift has slowed down since then. The cause of this relatively sudden upheaval was the additional infusion of the molten rocks into the magma chamber 10 kilometres below the surface of the earth.
Researchers from Montana University studied the rate of mineral crystal growth from the samples found in the lava and ash from earlier major super volcanic eruptions and have discovered that the local infusion of sufficient quantity of magma to create a major volcanic eruption took, not thousands of years as hypothesised earlier, but a few decades alone.
The rapid speed at which adequate magma infusion could happen created a degree of public concern over an impending volcanic outburst in Yellowstone. The authorities have been quick to step in to allay such fears. The official position is that super volcanic eruptions are neither regular nor predictable and though another eruption may take place, but only after thousands of years, as the quantum of magma necessary for an explosion is simply not in place at the moment.
For the moment, the Yellowstone geology and its physical changes in real time are being very closely monitored by geologists and other scientists. This natural museum, where diverting thermal features for human use have been consciously avoided, will hopefully retain its character for many more centuries and continue to provide a window into the dynamics of the core of our planet.