Our Ancient Hot Spring Setting

So what was it like at Rhynie at the time of the hot springs? The climate was arid, almost desert-like, with small ponds and lakes forming after seasonal, heavy rains. The land was flat, though to the north lay a mountainous region. The whole area lay inland of the long-since-gone continent known as Laurussia, and it sat about 28o south of the equator. Plants had just colonised the land, and insects were the first animals that had also begun to do so too. In other words, it was a very different place from today!

At Yellowstone today, the landscape is different – it has been heavily glaciated, and sits in a giant volcanic caldera. But it is the hot spring system and the microscopic organisms that it supports that are similar.
Geysers are one of the most inspiring features at Yellowstone. The tallest – Steamboat Geyser – can reach up to 90m in height, and Giantess Geyser can erupt for about 43 hours, causing the ground underfoot to rumble and shake! It is believed that at Rhynie, there was at least one geyser. The evidence for this is the presence in the chert of geyserite, a silica rich deposit found around the edge of geyser pools, formed in a similar way to sinter. This would have formed when the waters splashed over the edge of the pool.

The hot springs at Yellowstone can also be very colourful. The pools vary in temperature and pH (acid – alkali), even across individual pools. Different types of bacteria – cyanobacteria – thrive in different conditions, and cause the different colours. Fossilised cyanobacteria has been found in the Rhynie Chert, so we can presume that the hot springs at Rhynie were just as colourful.

The cyanobacteria in the Morning Glory pool at Lower Geyser Basin Yellowstone causes the intense colours.
Photo © Lyall Anderson, National Museums of Scotland.

In some of the cool springs at Yellowstone, there are green algal mats which float on the surface of the water. These act as a trap for the dissolved silica when the water evaporates, and so white silica crusts form on the water surface. Again, we find evidence of this having also happened at Rhynie.