Deep beneath Yellowstone is a mysterious place, where water cannot penetrate and rocks flow like silly putty. Strange conditions exist in the deepest parts of Yellowstone’s hydrothermal system!
Let’s Get into Hot Water
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dan Dzurisin, emeritus geologist with the U.S. Geological Survey.
It’s nearly impossible to wrap your head around the way things are in the infinitesimally small realm of quantum mechanics, where objects can pop in and out of existence and be in two places at once. Even the nature of “objects” is ambiguous: Electrons and light both share properties of waves and particles. Huh? Strange, but true. If it weren’t, the sun wouldn’t shine and there would be no lasers or cell phones.
These ideas seem incongruous to us because we don’t consciously experience quantum effects in our daily lives. We don’t “live” in the quantum realm, where distances are measured in terms of the Planck length, which is 0.000000000000000000000000000000000016 meters. No need to express that in feet. Suffice it to say it’s REALLY small.
Rather than straining to understand the weirdness of the quantum realm, let’s tackle something easier and closer to our everyday experience—hydrothermal systems. You’re reading this, so you’re probably already familiar with them at Yellowstone or elsewhere. But understanding some of their peculiarities still presents a challenge. Consider that active hydrothermal systems, like quantum effects, are mostly inaccessible to our direct experience. We can marvel at the spectacular surface features produced by hydrothermal activity in Yellowstone, but we never encounter the conditions that exist near the roots of the hydrothermal system. Nor would you want to. Temperatures and pressures there can exceed 400 degrees Celsius (750 degrees Fahrenheit) and 1,000 bars (15,000 pounds per square inch, about 1,000 times atmospheric pressure at sea level).
Under those conditions, things get a little strange, too. For example, rocks at those temperatures are generally too hot to break. Unless they are stressed very quickly, they tend to flow like pulled taffy. In fact, rocks at those temperatures behave a little like silly putty…. slowly stretching unless they are pulled suddenly. That’s why few earthquakes occur below about 5 kilometers (3 miles) depth under Yellowstone Caldera. Starting at that depth, rocks transition from brittle (breaking) to plastic (like silly putty) behavior. They don’t break easily, so there aren’t many earthquakes, even if the rocks are stressed beyond what would be their breaking point at lower temperatures.
As a student of Yellowstone, you might be aware that the depth of the brittle-plastic transition coincides with deepest part of the hydrothermal system. Coincidence? Not at all. In fact, the two things are directly related. The “hydro” part of hydrothermal systems refers to water, and most of that water makes its way downward from the surface through permeable soils and rock. Permeability refers to interconnected fractures and other openings that provide passageways for down-going water. You might be surprised to learn that surface water can percolate as deep as 10 km (6 miles) into the crust in many places.
Not so at Yellowstone. That’s because temperature at about 5 km (3 miles) depth is high enough (~400°C) for rocks to flow like silly putty, closing any open spaces and making the rocks nearly impermeable to water flow. As a result, the hydrothermal system “bottoms out” near the brittle-plastic transition.
Rocks that flow aren’t the only oddity in deeper parts of Yellowstone’s hydrothermal system. In a zone not far above its base, temperatures are just low enough for fractures to form and fluid pressures are high enough for fluids, mostly water, to squeeze into (intrude) fractures and keep them open. The stress regime is such that the preferred orientation of new fractures is horizontal, so fluids in fractures means that fluid pressure had to be high enough to lift the weight of overlying rock (almost 5 km or 3 miles of it!) for the fluids to squeeze into the fractures. So even small increases in fluid pressure in the deep hydrothermal system can cause surface uplift, and small decreases that allow fluid-filled cracks to collapse can cause subsidence.
Active hydrothermal systems are mostly inaccessible to us, so you might wonder if there is any direct evidence that rocks can flow like silly putty and watery fluids can lift kilometers-thick sections of the crust. The answer is yes, and it comes from magmatic and hydrothermal systems that are long since dead.
Either by direct observation of ancient magma bodies (batholiths) exposed at the surface by uplift and erosion, or by drilling into them, geologists have found evidence of impermeable self-sealed zones at what must have been the brittle-plastic transition at one time, and mineral veins that can only have formed by crystallization in a crack or other open space. The mineralization process had to occur where temperatures were low enough for fractures to form, and in a zone accessible to fluids. The fluids might have come from above (surface water with dissolved minerals) or from magma below as it cooled (the sealed zone at the base of the hydrothermal system isn’t completely unbreachable; under some conditions, fluids can pass through it). In either case, the self-sealed zone formed because rocks behaved like silly putty, and the fractures opened because fluid pressure was high enough to lift the entire column of overlying crust.
Strange, but true.