Tuesday, September 29, 2015

Ever wonder whether rock weathers?

When driving along Interstate 80 between Cheyenne and Laramie, you may have marveled at one of Wyoming's most colorful natural landmarks: Vedauwoo, a collection of plump pink granite rock hills.  Janet Dewey, a researcher in the Department of Geology and Geophysics, believes that their curious shape results in part from variable rates of chemical weathering of different rocks, and gradual erosion through rainfall, snowmelt, and temperature changes.  As she explains, "The rate at which different rocks weather governs the landscape, topographic expression, and the porosity in the rock."  Vedauwoo is one striking example of a phenomenon that shapes the intermountain west.

Rock varieties in Wyoming, including pink granite on the far left
(photo credit: Janet Dewey)
Since weathering affects the form and density of rocks, it also plays a major role in how water flows and where it is stored.  Weathering can change the chemical composition of rocks, as when weather leaches out elements like calcium.  When rocks and water come in contact, the chemical composition of both can change; this in turn changes the relationship between the two.  For example, if dissolving a portion of the rock changes the acidity of the water, then the rate of chemical weathering can also change.  Weathering and erosion can also include factors like slope and aspect.  One side of the mountain might be steeper, and one side might spend more time in direct sunlight.

Weathering also influences climate.  Weathering of silicate rock is "a pretty big sink" for carbon dioxide.  "Silicate minerals in the rock react with carbonic acid, consume carbon dioxide and produce weathering products that depend on the minerals present," explained Dewey.

Dewey and others on the weathering research team hope to identify what controls rates of weathering and erosion of rocks in the Laramie Range.  "Like many scientific problems, it's a scaling issue.  What we see at the micro scale is not necessarily what we see at the landscape scale."
Glass column reactors
(photo credit: Janet Dewey)

Their study near the Laramie area includes several different techniques.  First is the micro (or very small) scale which includes column experiments to test the weathering of different kinds of rock.  Dewey and her team have build glass column reactors containing three kinds of crushed rock, including the pink granite that is iconic to Vedauwoo.  Water continuously flows through the columns and is allowed to react with the minerals in the rock.  They test the water runoff for traces of elements like calcium, iron, and phosphorus.  You can see a picture of these columns at left.

Another step involves taking samples of rock at different locations and mapping weathering profiles.  Researchers can also look at thinly-sliced rock under a petrographic microscope to analyze the structure, mineralogy, and grain size of the various rock types and how those features might influence weathering.  The group also uses techniques that will help them determine the landscape-scale erosion rates.  These data will be combined with geophysical data to create a multilayered map of the region.

Graph showing different minerals found in column reactor runoff
(image credit: Cassie Nauer)
After this phase, the team increases the "suite," or set of rocks in the water column study.  The researchers are currently testing three types of rock, but will increase to eight types.  They will also test faster rates of water flow, to determine if there is a predictable relationship between flow and rate of chemical weathering.

Graph showing column reactor runoff rates over time
(image credit: Cassie Nauer)

Dewey's motivation to explore weathering comes from an interest in understanding the future and a curiosity about the past.  She says, "From my perspective, one of the biggest issues confronting us in the future is going to be water quantity and quality.  We need to know as much as we can about the relationship between water and the rock it passes through.  Without that understanding it's easy for us to make mistakes."

In considering the past, she added, "If you've ever gone out on the landscape at Veedauwoo and stood there by those rocks, you think: Why do they tower four stories above me and I'm standing on flat weathered rock that seems to be made of the same material?  Why does that beautiful landscape exist - that we take pictures of, and climb on, and love to visit year round?  If we are sitting on one giant granite batholith, why isn't it the same across the landscape?  That's a cool question."

by Jess White 

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