Things you will need to log this Earthcache:
- GPS
- Camera (optional)
To log your find, please email me the answers to the following questions. You can send them by my profile or click here to open a new email. Please note that this email link does not work with all email clients.
- Estimate the width and height of the slope area which is only covered with small plants (no trees).
- The areas of the slope that are not cover with plant is the Tufa rock. What color is this Tufa?
- At the time of your visit, was water flowing from the slope?
- What kind of plants are growing on the slope?
- Not required but always appreciated, post to your on-line log a photo of your GPS. Your face can be included if you so wish.
Definition of Terms
Tufa
[too-fuh]
Tufa is highly porous type of limestone which forms near bodies of water with a high content of dissolved minerals, especially calcium carbonate. It forms by two basic methods, biological and chemical precipitation of Calcium Carbonate, CaCO3. It's color can vary depending on the other dissolved minerals contained within the water.
Tuff
A porous volcanic rock also called 'Tufa'. Sometimes confused with Tufa.
pH
The pH scale measures how acidic or basic (alkaline) a substance is. It ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic. Each whole pH value below 7 is ten times more acidic than the next higher value. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, a pH of 10 is ten times more alkaline than a pH of 9.
Pure water is neutral, with a pH of 7.0. When chemicals are mixed with water, the mixture can become either acidic or basic. Vinegar and lemon juice are acidic substances, while laundry detergents and ammonia are basic.
Carbonate Mineral
Any member of a family of minerals that contain the carbonate ion, CO32-, as the basic structural and compositional unit. The carbonates are among the most widely distributed minerals in the Earth’s crust. Generally the carbonate minerals are found at or near the surface. They represent the Earth's largest storehouse of carbon.
There are approximately 80 known carbonate minerals, but most of them are rare. The commonest varieties, calcite, dolomite, and aragonite, are prominent constituents of certain rocks: calcite is the principal mineral of limestones and marbles; dolomite occurs as a replacement for calcite in limestones, and when this is extensive the rock is termed dolomite; and aragonite occurs in some recent sediments and in the shells of organisms that have calcareous skeletons.
The Geology
Geochemistry
Tufa is formed when alkaline waters of an ambient temperature that are supersaturated with carbonate minerals reaches the surface. Due to the lower atmospheric pressure, CO2 degases, resulting in an increase in pH. Carbonate minerals solubility decreases with increased pH which enables the precipitation of the supersaturated carbonate minerals.
Geo-thermally heated hot-springs sometimes produce similar, but less porous carbonate minerals deposits known as travertine.
Tufa at Mono Lake near Rush Creek
Biological
Tufa is also formed when alkaline waters that are supersaturated with carbonate minerals reaches the surface. The metabolism of algae, bacteria and mosses is important for tufa formation due to their consumption of CO2. Again the Carbonate mineral solubility decreases with increased pH (more alkaline) which increases the precipitation of the supersaturated carbonate minerals . With this method the Tufa forms on the surface life forms. Some geologist feel that all Tufa is formed from a biological base. Fresh Tufa formed this way often contains remains of plants or their molds.
Local Geology
At the posted waypoint you will be looking at a steep slope (no climbing please), where ground water is seeping to the surface. This water has traveled through the limestones of the Late Silurian and Early Devonian periods (400 to 420 million years ago) that Mackinac Island is made of. Along the way Calcium Carbonate is dissolved. Depending on the amount of the water flowing and the ambient noise, you may be able to hear what sounds like rain as the water trickles down the slope. You should also be able to see water flowing at points on the slope as well at the base of the slope in the small road side ditch. You may find a piece of Tufa that you can examine. Be gentle with it as it can crumble easily depending on how much Tufa is inside.
Mackinac Island Tufa
References used:
- http://chc.sbg.ac.at/sri/thesaurus/node.php?id=58
- http://www.wisegeek.com/what-is-tufa-stone.htm
- http://www.epa.gov/acidrain/measure/ph.html
- http://en.wikipedia.org/wiki/Tufa
- http://www.epa.gov/acidrain/measure/ph.html