Metaline Formation
First, let's discuss what you are looking at. 200 Million years ago the rock across the street (and scattered at your feet if you are at the posted coordinates) was a broad coastal plain of sediment, which had eroded off our continent. The coast at that time was not far west of here.
As our continent began to move west at a rate of 2" to 3" per year, the coastal layers of sedimentary rock were buckled. In time, this buckled rock was further contorted by colliding with the offshore islands that now form Western Washington.
You are looking at the top of these contorted coastal sediments that were once at the edge of the North American continent, the Ledbetter Formation.
Beneath the Ledbetter lies the Metaline Formation, containing mineralized zones of zinc and lead ore. In the early days the Pend Oreille Mine, just north of here, extracted zinc and lead ore from the Josephine Zone. Today, ore is mined from the Yellowhead zone.
Weathering Process
Now, let's discuss weather. Weathering is the chemical and physical processes that change the characteristics of rocks on the Earth’s surface. In order for weathering to occur, the environment of a rock sample must change and the rock needs to be exposed to some form of water and the air. Human processes such as pollution, which can be a large factor in acid rain, along with the acts of other living organisms, can cause chemical weathering to occur at faster rates.
The weathering process occurs when rocks are exposed to the hydrosphere (water) and atmosphere (air). These weathering agents can change the physical and chemical characteristics of rocks. As rocks are broken down (weathered) they can be classified as different types of sediments based on their diameters:
Boulders: greater than 25.6cm
Cobbles: between 6.4cm and 25.6cm
Pebbles: between 0.2cm and 6.4cm
Sand: between 0.006cm and 0.2cm
Silt: between 0.0004cm and 0.006cm
Clay: less than 0.0004cm
Physical Weathering
Physical weathering occurs when rocks are broken in to smaller pieces without changing the chemical composition of the rock. Think of a physical change (e.g., ripping a piece of paper) where the sample will change in size but all its other characteristics will remain the same. There are a few types of physical weathering such as:
Frost action/ice wedging is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water. Water can seep into the cracks of a rock and as the climate cools the water freezes and expands breaking the rock apart. A very similar process occurs on roads, which causes potholes.
Abrasion is the physical wearing down of rocks as they rub or bounce against each other. This process is most common in windy areas, under glaciers, or in stream channels.
Exfoliation is the peeling away of large sheets of loosened materials at the surface of a rock. Common in shale, slate, and mica.
Chemical Weathering
Chemical weathering occurs when a rock is broken down by chemical action resulting in a change in the composition of a rock. Such as the change a piece of paper would go through after being burned. The main agents of chemical weathering are oxygen, rainwater, carbon dioxide, and acids produced by decaying plants and animals that leads to the formation of soil. There are a few types of chemical weathering such as:
Oxidation occurs when oxygen interacts chemically with minerals. For example, when a nail rusts oxygen combines with the iron in the nail to form iron oxide.
Hydration occurs when water interacts chemically with minerals. For example, when hornblende and feldspar unite with water they eventually form into clay.
Carbonation occurs when carbon dioxide interacts chemically with minerals. When carbon dioxide is dissolved in water, it forms weak carbonic acid. Carbonic acid when it comes in contact with the surface of the earth dissolves large masses of limestone, creating caves and caverns. Other common terms associated with carbonation are sink holes, karst topography, stalactites and stalagmites.
Weathering rates overall depend on 3 different factors:
- Particle size/surface area exposed to the surface
- Mineral composition
- Climate
Verfication
In order to verify what you've learned from visiting this earthcache, please e-mail via the geocaching site to the cache owner the answers to the following questions:
1. Do not cross the road, stay where it is safe. Look around you and find some of the remnants of the giant rock that is across the highway. Would these rocks be classified as Boulders, Cobbles, Pebbles, Sand, Silt or Clay?
2. Is the rock across the way influenced by Physical or Chemical process? What type of weathering do you hypothesize is at play here?
3. What evidence leads you to this conclusion?
4. Give your best guess of what type(s) of "rock" you are looking at (for example: Granite, Dolostone, Limestone, Quartzite, Other)?
5. Photos are encourage, but not required.