Between a Rock and a Hard Plate: How to Save Your Metamorphic Sole

by Katie Lang, WWU Geology graduate student
  • A blueschist rock in outcrop along a logging road in Washington state. This rock  underwent intense heat and pressure  for millions of years and looks “flaky” as a result.
    A blueschist rock in outcrop along a logging road in Washington state. This rock underwent intense heat and pressure for millions of years and looks “flaky” as a result.

What’s Millions of years old, undergone a lot of stress, and turned noticeably blue?

Metamorphism – the word for change in geology – is a bit of a mysterious process. A lot of ingredients are required for metamorphism to happen, but with hot temperatures (more than 200º C), high pressures (greater than 3 kilobars), and the presence of water, any type of rock can become metamorphosed. No one has ever seen metamorphism happen with their own eyes. If we could, it would take millions of years to witness and would happen at depths and temperatures humans couldn’t withstand. So if we can’t experience it, why should we care?

A special type of rock, called blueschists, are the key to this question. Besides being vividly blue from the minerals that grew during metamorphism, they hold precious clues of information for what was happening inside a subduction zone, or the type of tectonic environment in which they were formed. A subduction zone occurs when two tectonic plates clash against one another, with the denser of the two plates being pulled underneath the more buoyant plate. The scraping and dragging movement of one descending plate can cause hazards such as earthquakes and volcanoes thousands of miles away from the place where the plates meet.

Yet, it is the place between the plates in a subduction zone that some of the most complex chemical and physical transformations take place on our planet. It is also the place that few ever get the chance to see remnants of, even after the subduction zone has long come and gone away. Perhaps most importantly, it is the only place where metamorphic soles, and their associated blueschist rocks, can be created.

It is this blue type of metamorphosed rock from a subduction zone that Katie Lang, a WWU Geology master’s degree student from Chesapeake, Virginia, is attempting to model. Lang, a first year graduate student, hopes to be able to publish her findings in the Journal of Metamorphic Petrology once she completes her analyses and field work.

“My research is focused on understanding how metamorphic soles, or the first little slivers of rock that are pulled down between two tectonic plates, are created and ultimately preserved,” said Lang. “Their story of being pulled down into the Earth is almost as fascinating as how they were then brought back up to the surface, after being buried as deep as 15 kilometers.”

But how can geologists even tell how deep these rocks really went? 

“We use the minerals within the rocks to tell us about the journey it’s been on,” Lang said.

Using techniques to figure out the chemical composition of the minerals, there are specific computer programs that can be modified to model the paths of the rocks throughout the earth’s interior. Lang will be relying on these programs to figure out how the metamorphic soles were changed over time, and how they were preserved as they made their way back up to the surface through tectonic activity.

During this time of exhumation, the blueschist rocks that once originated as ocean sediment before being metamorphosed can become folded and stretched. However, the thin metamorphic sole is often one of the first rocks to be destroyed. Therefore, figuring out how these rare blueschists within the metamorphic sole are preserved is important for understanding what was happening in ancient subduction zones.

“Metamorphism tells us about the important changes going on in our Earth and how the rocks changed in response to their surroundings,” Lang said. “But blueschists are like the poster-child for metamorphic rocks: they’re rare to find, can only occur in a few specific environments on Earth, and have such a beautiful blue color.

"They’ve truly seen the inside of the Earth and lived to tell their tale.”


Editor's note: Graduate and undergraduate students from Regina Barber DeGraaff and Melissa Rice's  Science Communication class were tasked this quarter with writing about their own research or about research that interests them. Western Today will be running some of their stories.




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Friday, April 26, 2019 - 12:21pm