This Photo-Essay Won the Physical Category of an Internal Year 11 Competition

The above photo takes me back to the summer of 2017 when I was on holiday in Yellowstone National Park. The photo is of the Grand Prismatic Spring which is the third largest hot spring in the world. What I remember most is the amazing spectrum of colours across the spring and the swirling steam constantly rising majestically out of the pool.

The spring is 110m wide and was created by heated water emerging through cracks in the Earth’s surface and chemically eroding surrounding rocks. The spring sits in a caldera which is 35 miles wide and 50 miles long, and which was created by a massive volcanic eruption approximately 640,000 years ago. The water is unusually hot because there is a hot spot, created from a mantle plume, under the caldera. This hot spot is slowly moving across the USA as the North American Plate drifts westward.

The 50m deep spring constantly steams because extremely hot water continually bubbles up in the middle and then gradually cools as it spreads before falling or flowing out of the spring. Grand Prismatic discharges an enormous 2,110 litres of 70°C water per minute. The upward flow is continuous because the cracks beneath the spring are not obstructed by mineral deposits unlike other geothermal features such as geysers which intermittently explode.

The rainbow spectrum of colours around the spring are from mats of microscopic organisms called thermophiles which have heat stable proteins. The different colours are from different communities of thermophiles which can tolerate different temperatures. For example: the yellow ring is created by Synechococcus, a cyanobacteria which tolerates temperatures up to 65°C and which contains phycobiliproteins that fluoresce yellow-orange in strong sunlight; the orange ring is created by Phormidium which tolerates temperatures up to 60°C and produces orange carotenoids to protect itself from extreme sunlight. In winter, when there is less intense sunlight, the thermophiles lose their vivid colours and the spring becomes ochre and brown around the edge.

The centre of the spring is around 87°C; too hot for any microbial growth. The blue colour in the centre is therefore a reflection of the blue sky. The patches of white seen furthest from the edge of the spring are siliceous sinter created from chemical erosion of the silica rich, rhyolitic bed rock and deposition when the water cools on the surface. Around the fringes of the spring grow lodgepole pines which are tolerant of poor soil.

I remember also hundreds of ephydrid flies which are the primary consumers of the microbial mats. The flies are eaten by secondary consumers such as spiders, beetles and dragon flies which are then fed on by birds such as killdeer and swallows.

Grand Prismatic’s radiating beauty stands out vividly from the surrounding vegetation… and as a bonus it does not stink of hydrogen sulphide unlike many other geothermal features in Yellowstone!

(Photo: © Tarquin Parry-Wingfield)

(Featured Image: © Jasper Sodha)