DO YOU KNOW YOUR ROCKS#3 EarthCache

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Hidden : 10/23/2014
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:

As with the other Earthcaches in this series, we gonna take a look at how we can learn to recognize different rock types inorder to understand more of what is around us.

Shale
Shale is the most abundant sedimentary rock and is in sedimentary basins worldwide.

What is Shale?

Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles that we commonly call "mud". This composition places shale in a category of sedimentary rocks known as "mudstones". Shale is distinguished from other mudstones because it is fissile and laminated. "Laminated" means that the rock is made up of many thin layers. "Fissile" means that the rock readily splits into thin pieces along the laminations.

Black and Gray Shale

A black color in sedimentary rocks almost always indicates the presence of organic materials. Just one or two percent organic materials can impart a dark gray or black color to the rock. In addition, this black color almost always implies that the shale formed from sediment deposited in an oxygen-deficient environment. Any oxygen that entered the environment quickly reacted with the decaying organic debris. If a large amount of oxygen was present the organic debris would all have decayed. An oxygen-poor environment also provides the proper conditions for the formation of sulfide minerals such as pyrite, another important mineral found in most black shales.

Gray shales sometimes contain a small amount of organic matter. However, gray shales can also be rocks that contain calcareous materials or simply clay minerals that result in a gray color.

Red, Brown and Yellow Shale

Shales that are deposited in oxygen-rich environments often contain tiny particles of iron oxide or iron hydroxide minerals such as hematite, goethite or limonite. Just a few percent of these minerals distributed through the rock can produce the red, brown or yellow colors exhibited by many types of shale. The presence of hematite can produce a red shale. The presence of limonite or goethite can produce a yellow or brown shale.

Green Shale

Green shales are occasionally found. This should not be surprising because some of the clay minerals and micas that make up much of the volume of these rocks are typically a greenish color.

Environments of Shale Deposition

An accumulation of mud begins with the chemical weathering of rocks. This weathering breaks the rocks down into clay minerals and other small particles which often become part of the local soil. A rainstorm might wash tiny particles of soil from the land and into streams, giving the streams a "muddy" appearance. When the stream slows down or enters a standing body of water such as a lake, swamp or ocean the mud particles settle to the bottom. If undisturbed and buried this accumulation of mud might be transformed into a sedimentary rock known as "mudstone". This is how most shales are formed.

Above information was obtained from Geology.com

On site questions:
#1. Describe the colour of the Shale found at the site.
#2. What can be seen growing on the Shale that forms a feature at GZ?

Forward answers to our KINGOSRIC handle. Answers not received in 7days will be deleted.

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