Yellowstone Caldera Chronicles is a weekly chronicle written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Jeff Havig, a researcher in earth and planetary sciences, and plant and microbial biology, at the University of Minnesota.
When visiting Yellowstone National Park, you may spend some time exploring places where the pH-neutral to alkaline hot springs (those with pH ranges from about 7 – or neutral – to near 10 – about bleach value) are dominant features, precipitating silica (SiO2) to produce agglomeration deposits. This includes the Upper Geyser Basin (where Old Faithful and dozens of his geysing compatriots reside), the Midway Geyser Basin (home of Grand Prismatic Spring), the Lower Geyser Basin, and the West Thumb area, to name a few. -some of the most popular. .
While the charismatic geysers and bubbling hot springs may immediately draw the eye and consume your concentration, take a moment to gaze at the bleach-white siliceous frits that surround the springs and geysers. The agglomerates form due to the dissolved solids in the hot spring water which precipitate out as the hot spring waters cool. To the naked eye they may not look so exciting, but when we use imaging techniques to zoom in and take a closer look, they become much more fascinating.
First, a little history is in order. Hydrothermal water that comes out as geysers and hot springs began as snow and/or rain that seeped into groundwater. In Yellowstone, cold groundwater can flow to the magma chamber, where it is heated to very high temperatures, and this hot water can dissolve the rock it comes in contact with.
Specifically, the rhyolitic volcanic rocks of Yellowstone are rich in SiO2, present as the mineral quartz or aluminum silicates like feldspar. Thus, the hot water dissolves the silica present in the volcanic rock. If hot water reaches the surface, we consider it hydrothermal features, such as hot springs and geysers. But as this water cools, the dissolved silica becomes supersaturated and precipitates. Perhaps you have dissolved large amounts of salt in hot water and then watched salt crystals precipitate as the water cooled and evaporated. It’s the same principle.
Let’s go back to the siliceous frits. Using a Scanning Electron Microscope (or SEM for short) – an instrument that uses electrons to essentially take pictures of microscopic features – textures of frits are revealed that you might not have expected to see. . Using SEM at the University of Minnesota Department of Earth and Environmental Sciences Continental Scientific Drilling Facility, it is clear that some Yellowstone agglomerates consist of very small string-like features, almost looking like a pile of cooked spaghetti on a plate, only here the strings were the diameter of a human hair. Zooming in further, it appears that these “strings” are actually hollow tubes. This is an important clue to understanding how this siliceous agglomerate precipitated. So let’s take a look at the context.
As you walk the walks of the Geyser Hill Group, the Castle Group, and others in the Old Faithful area of the Upper Geyser Basin, you may first notice the more spectacular features, such as Castle Geyser. But you’ll also likely notice the brilliant colors in the geyser and hot spring outlet channels, like the orange pool in the Tortoise Shell Spring outlet next to Castle Geyser. The yellows, greens, oranges, reds and browns that you see in these hot springs flows (or in colder temperature hot springs pools) are pigments produced by photosynthetic bacteria such as cyanobacteria, chloroflexi and chlorobi (as well as algal photosynthetic bacteria in cooler areas). These photosynthetic microorganisms thrive at temperatures that would cause second to third degree burns with only a few seconds of exposure to humans. But they also act as nucleation points for dissolved silica in hydrothermal water.
An SEM image of an algae filament shows that a silica sheath (the round outer part) has rushed around the algae. So now we know what formed these little tubes. While algal filaments can be 5 to 10 micrometers in diameter, bacteria are smaller – typically around 1 micrometer in diameter for cyanobacteria – so bacterial filaments living in a hot spring flow channel had a precipitation of silica around them. Then, when the flow channel changed direction, the photosynthetic microbial community died and decomposed, but the silica sheaths remained as a siliceous agglomerate.
These physical characteristics are what scientists call a biosignature – something formed by life that, in this case, can be preserved in rocks. This is an example of a type of biosignature, or sign of past life, that the Perseverance Rover on Mars is currently looking for in a place called Jezero Crater. These are also the types of biosignatures we can use to search for evidence of early life forms that existed over 3.5 billion years ago on Earth.
So, I hope that when you enjoy a visit to the hydrothermal areas of Yellowstone National Park, or marvel at photos or videos of the spectacular geyser eruptions, you will take a moment to appreciate the siliceous sinter under your feet, as well as the geological processes and the billions of bacteria that contributed to its formation.