One day in Chemistry 11 class Last school year I mentioned
an earthcache I would like to find – Holey Moley! http://www.geocaching.com/seek/cache_details.aspx?guid=23a88c4a-698b-4399-a753-0294dfbbe5f5
This cache is located in
Scenic Canyon Regional Park not too far from Okanagan Adventist Academy, and
features some interesting geology – the canyon was formed by water flowing
quickly downstream when Glacial Lake Penticton drained. The rock banks contain
cobbles imbedded in a matrix of calcite or quartz (Okanagan Geology, MA Roed
& JD Greenough, eds., 2004, pp. 92, 184-185).
Our class discussion revolved around the chemical reactions
which we would expect calcium carbonate, CaCO3 to undergo. Calcite
is one form of CaCO3, and students remembered that limestone is also
made of CaCO3. We discussed stalactite and stalagmite formation in
caves, and then moved on to predict a reversible chemical reaction. We first
came up with a word equation:
Calcium carbonate in
the presence of dissolved acid reacts to form the most common oxide of carbon
and water.
and then filled in the chemical symbols they had learned:
CaCO3(s) + H+(aq)
-> CO2(g) + H2O(l)
When students attempted to
balance this equation they realized that they needed to account for the calcium
ion:
CaCO3(s) + H+(aq)
-> CO2(g) + Ca2+ (aq) + H2O(l)
They also needed to balance
the hydrogen ions on each side of the equation and to document the observation
that this reaction is reversible:
CaCO3(s) + 2H+(aq)
<-> CO2(g) + Ca2+ (aq) + H2O(l)
I was quite happy with the
way our discussion had helped students make some real world connections to the
work we were doing in class. However, the connections don’t end there. One of
my students, W, came to me some time later to tell me that he had been to the
earthcache site, and was disappointed to find out that the nearby rocks didn’t
bubble like he thought they would when he moistened them with acid. I wasn’t
sure why and had to search through one of my resource books (Okanagan Geology, p.184)
to discover that the rock matrix which isn’t soluble in acid is quartz.
This last week I finally
found time to go to ground zero myself, and was able to fulfill the
requirements given by the cache owner:
1)
Take a
picture of you and your gps at the site of the cache. Instead of including a picture of myself or
my GPS I took a picture of my bottle of acid.
A) Why do you think there is more calcite in the rock at this area in particular and what does this tell you?
The rocks around this area in particular contain calcite that either crystalized from calcium rich lava or was deposited there by water with a high mineral content. Once this area became exposed to weathering after the river cut through the ground creating the canyon, exposing the rock to a totally new environment, acid rain dissolved the calcite within the rocks and they would simply crumble and fall from the rock face into the creek causing the unique holes you see now. This is known as dissolution due to chemical weathering. Chemical weathering is most common in areas that have large concentrations of limestone and acidic water whether acidic water from pollution or naturally occurring. This is because limestone is largely composed of calcite which is a reactive compound.
B) Chemical weathering is not the only process at work that is carving this rock face, name one other process you think is breaking the rock face down.
You are able to see veins and deposits of calcite in many of the rocks you can pick up from the ground. This is testable by applying a drop of hydrochloric acid and seeing the calcite within the rock effervesce (not that you carry acid around with you...).
Oh, but I do carry 0.1M HCl(aq) with me, at least when I’m forewarned!
When I posted pictures and an abbreviated story of my adventure on Facebook W commented that he had just found a chunk of crystal clear calcite. Besides dissolving it with acid how could he know?
The connections continue. . .the opportunities for adventure multiply. . . I just love my job!
Oh, one more thing. . . what caused the layers on Layer Cake Mountain?
Well, two more things . . . why are the deposits by Angel Warm Springs orange? Is that like the orange color at Yellowstone?
