I love questions. Part of my enjoyment of the public hikes is listening to their questions. Good questions usually beget good answers.
We had begun walking again when this woman asked the question, and I couldn't hear Nancy's response. And I'd like to know!
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I've studied a lot of geology, and when you know a lot about something, it's harder to decide what to say in a 1 minute summary. I've been thinking about how I want to tell the story of limestone, since there are always many ways to tell a story.
I like that Nancy stops and talks about it at the first top of trail 3 - that she introduces it with "this is usually under our feet, but here it's exposed". It's important, I think, for visitors to see or hear the thing we're talking about. Otherwise, why actually be there? We could all sit around some beers and talk about it instead.
So - my limestone story begins to be explored: Limestone is a rock that was built by life. All of this you see is the remains of shells that tiny animals who lived in a warm, shallow sea made. They extracted calcium carbonate suspended in the ocean water, built up their shells with that and lived their lives inside. When they were eaten, their fleshy parts were digested, but their shells were pooped out, and sank to the bottom. Billions upon trillions of these sank to the bottom, and as more and more were added, the pressure of all those on top squeezed the water out of the ones on the bottom. More and more pressure gradually turned it into a solid mass we call rock - limestone - calcium carbonate. If you put some under a microscope, you can still see some of the shells intact. Further up trail 3, we'll see rock that was made up of bigger shells that we can see with our naked eye.
And, since a google search makes it apparent that you can't talk about limestone without referring to current climate change, here's this, which I wouldn't talk about unless someone asked:
The ocean acts as a carbon sink, meaning that it absorbs CO2 from the atmosphere. This causes ocean acidification: the ocean becomes more acid as it absorbs more and more CO2. The pH scale measures acidity. Its lowest measurement, 0, represents strong acids. Its highest measurement, 14, represents strong bases. A netural measurement is 7, the pH of distilled water. Seawater near the ocean’s surface should have a pH of 8.2. In the last 200 years, heightened levels of CO2 have reduced surface seawater pH by 0.1, making the water 30% more acidic. With present trends, seawater will be 150% more acidic than in pre-industrial times by 2100. Heightened acidity increases concentrations of hydrogen ions in seawater. It also ties up calcium ions [how? in what molecules?] and thus creates a scarcity of calcium carbonate. Calcium carbonate is required by calcifying marine organisms to build stony skeletons and hard shells. - From Nielson, Frontier Scientists 10/9/2012
