Geology of Oregon Series: Part I: Hot Springs

Sarah here –

In 2022, I took a road trip around Oregon on the west coast of USA to see all the incredible geology there is to see there. Oregon is an incredible natural geologic laboratory because there are so many different processes at play across different environments: you can see hydrology in action through massive waterfalls, naturally heated bodies of water from geothermal energy, the movement of sands in desert environments, and more- all in a single state! I’ll be writing a series of articles on the geology that I saw, so that I can share with you a small part of the incredible beauty that this Earth has to offer. 

My journey started near Eugene, Oregon, a few hours inland from the Pacific coast, with a friend of mine from college. Our first stop was to go swimming at the Terwilliger hot springs in the Willamette National Forest. A hot spring is a naturally occurring feature caused by geothermal (geo meaning Earth, thermal meaning heat) activity- Earth processes cause the water in a spring to be far warmer than we would expect a typical body of water on the surface of Earth! This geologic phenomenon does not occur everywhere on Earth by any means. 

So where does this geothermal activity come from and why is it restricted to certain locations? It comes from areas with volcanoes, both active and dormant (like Iceland, Hawaii, and Oregon!). The magma (lava that’s still underground) of the volcanic system is in contact with rocks closer to the surface of Earth- that heat is passed to water that is in contact with the rock (Fig. 1).

A diagram of how water is affected by volcanic activity. Starting from the bottom: a layer of magma in chamber is in contact with porous rock above it- heat is rising. There is water in that porous rock that is heated from the magma. The water can rise to the surface and form a few different things. It can stay in the ground as steam, it can come out as a hot spring, or it can erupt as a geyser. The top of the diagram shows a hot spring as a small pool with steam rising and a geyser with a large fountain of hot water bursting from the Earth. The water will eventually return to the ground and begin that cycle again.
Figure 1. A diagram of how magma below the surface affects the temperature of ground water- as the magma chamber heats the porous rock above it, water that is in that rock is also heated and rises to the surface- in this case, it is a hot spring, but the water can also come to the surface in different ways, like geysers!

The temperature of the water can vary from pleasantly warm to extremely hot- meaning, some areas are safe to swim, and others are not (so if you’re in an area where hot springs exist, always check local safety guidelines!). Typically, areas with active volcanism (meaning, they’ve erupted in recent history, as opposed to dormant, where they have not erupted for some time, but have the possibility to erupt in the future) will have higher temperatures associated with their hot springs. At Terwilliger (Fig. 2), the water ranges from 112˙F to about 85˙F, so it felt a lot like a hot tub! The hottest water is closest to where the water begins to flow- so, the water closest to the heat source- as it travels downstream, it cools. 

The view of the hot springs from the most uphill portion- it's in a forest, surrounded by trees and faint views of mountains in the background. there is a wooden platform off to the side. Directly in front is a pool of water about 8 feet across and a few feet deep (maybe 3)- the rocks make a circle around this pool, and water overflows over some of the rocks to continue to trickle downstream to pools outside the view of this image
Figure 2. The hot springs, from an upstream view. The pool directly in view is the warmest- as the water in it travels downstream, it cools a bit.

Due to the nature of hot springs, the water there often contains a high amount of dissolved minerals, and the minerals present in them can range drastically, as can the pH of the water. Often, you’ll find that the water can appear very different in color and clarity across different hot springs, and that’s why- the dissolved minerals. In Teriwilliger, the water has a lot of sodium, calcium, magnesium, iron, aluminum, silica, and sulfates present in it. 

Despite the high temperatures of the water, life still thrives in this environment, too- while this hot spring is not among the warmest, it is still a difficult environment for many different organisms to survive in. However, certain species of blue-green algae, or cyanobacteria, have adapted to be able to thrive in these extreme freshwater environments (species that can live in extreme environments are called extremophiles), where most other species cannot. These cyanobacteria can be seen on the rocks closer to the edges of the pool (Fig. 3) and it can be very slippery if you step on it- so be careful! I wanted to highlight these cyanobacteria because cyanobacteria represent some of the earliest complex life on Earth, with their fossil record extending billions of years- we can thank them for providing a lot of the oxygen we breathe today! Biology and geology are intertwined with one another, so by studying both, we can get a fuller picture of the world around us.

A close up of the hot springs pool. The water is a distinct greenish shade and on the edges, you can see a blue-green shade to the rocks that is actually the algae. it stands out because the other rocks, not coated in algae, are mostly basalt, so they are dark gray in color (lighter gray if they have been weathered more)
Figure 3. A close-up image of one of the pools at the hot springs- note the distinct color of the water and the blue-green algae (cyanobacteria) that coats the rocks. It’s tough to see the cyanobacteria in the deeper water, but toward the edges, you can see a colorful sheen- that’s the blue-green algae! A limited number of organisms on Earth thrive in extremely warm waters, but those that have adapted to these extreme environments can really thrive there!

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