Friday, September 10, 2010

Bedrock rivers and active deformation

Brian Yanites is a Post-doctoral Researcher in the Department of Geological Sciences at the University of Michigan. Brian did his PhD in Boulder at the University of Colorado with Greg Tucker.

Brian is interested in geomorphology - geologic research tries to understand the origin of the landforms (mountains, rivers etc) on Earth. On his web page he says he is trying to answer two fundamental questions: 1) How do local processes interact to form landforms and landscapes? and 2) How can we, as researchers, appropriately model and constrain these processes to explore the long-term evolution of the earth’s surface?

On Friday Brian will be talking about how to use river incision to understand deformation. Let's break that down. River incision refers to a river eroding downward through its riverbed which may be made of sediment or bedrock.The river begins at a higher elevation and incises or cuts or erodes downward through the bed it flows over. The river may leave its floodplain behind at a higher elevation of it may be lowered at the same time. Deformation refers to changes in the shape or volume of a rock body. Stress/pressure is often applied to rock bodies by plate tectonics for example. Rock bodies may bend in a ductile fashion (forming anticlines or synclines) or in a brittle fashion (faults that fracture rock bodies).

Rail lines outside Christchurch, New Zealand deformed in a ductile fashion in response of the earthquake of September 3rd 2010 (photo by Ian McGregor).

As the rock body under a river deforms, the river bed responds by becoming steeper (the elevation change down the river bed increases) and the width of the river bed becomes narrower (less distance between either side of the river bank). Both responses cause the river to become more erosive and the river will incise or cut down through its river bed faster. If the age of the beginning of the incision/cutting down is known, then the geomorphologist can calculate how quickly the river is eroding its bedrock.

Brian will be talking about rivers that drain off the Tibetan plateau. He has calculated how fast the rivers are incising the landscape by dating the time when incision began. Brian has used known earthquakes events as well as optically stimulated luminescence (OSL) dating techniques to calculate when the river sediments last saw sunlight to date the beginning of incision. Using his results Brian has helped us further understand the tectonics that are actively shaping our planet.

You can read more about these techniques at 'Rivers crossing growing folds' or in the Yanites folder in resources on Ctools.


  1. Sounds interesting. I'm slightly confused at the moment, but am excited for his talk. The post mentions that if the age of the beginning of the incision is know, then the rate of erosion can be calculated. How do you find the age of the beginning of the incision? And wouldn't the rate of erosion vary over time making it hard to calculate how quickly the river is eroding the bedrock?


  2. I felt Brian did a good job of clarifying his paper for people who are not as well-versed in geology as himself. His walk-through of the four-step evolution of rivers above and below a fault really helped - it was not clear in the paper. The figure of the electrical resistance surveys of the river bottom didn't seem to match the line he had drawn on it though. It worked for the right half, but in the left half the resistance surveys seemed to indicate shallower sediment cover than he indicated.

  3. I thought the lecture was well presented and an interesting diversion from the political/governmental public speaking structure I'm more accustomed to. Brian did an excellent job presenting his work in a way that was easily understood. While I was entirely lost for the details of how he reached his conclusions (especially on the math formulas) I don't think that was really the knowledge he wanted to convey. Walking away, though, both Megan and I had the same question, "Why should we care?" or "Now what". Brian never really said what one should do with this new information about Tectonic Plate's affect on river bedrock. For him to receive the grant funding he did, I'm sure Brian had to provide some sort of justification for his work - a reason why this research will improve something. I would have loved to hear that in his presentation.

  4. Brian’s lecture was very informative and also very interesting which is important when presenting a subject that is new to a lot of people (including me). I was scared at the beginning because of the inclusion of the formulas and I thought what he was presenting would go over my head. As the seminar went along, it was not difficult to sit attentively and the inclusion of his own pictures was very impressive. The picture of the canyon that did not exist 10 years ago was astounding and definitely put things into perspective. After this first lecture I am very excited to learn many more subjects in the field of Geology because it is so diverse and a very interesting application of science.

  5. In retrospect, this presentation was one of the more enjoyable ones. It was clear and informative, utilizing graphs somewhat comprehendable terminology. I didn't understand the last 1/3 of the presentation, but I understood a little about river formation.