To log this Earthcache, please email the CO with answers to the following questions:
1. Looking at the front side if the rock (facing away from the building) How many dikes do you see on this specimen?
2. Are there indicators that suggest some which of these dikes might be older or younger than the others? Why?
3. Describe the composition of the dikes. How are they similar or different from each other?
Please do not post answers to the questions within your log entry, nor pictures of the specimen this earthcache focuses on.
How Intrusive!
Magma that cools inside the crust of the planet surrounded by other rocks are called intrusions. Pegmatites are igneous rock “intrusions” that form during the final stage of magma cooling. They are characterized by large crystals, usually 2 cm or larger. Many rare minerals, gemstones, and crystals can be found within pegmatite, including topaz, garnet, tourmaline, beryllium, and lithium, to name a few of the over 300 discovered within these dikes.
Extreme Creativity?
A number of theories exist to explain the creation of these rock bodies. One of the simplest ways to explain pegmatite formation has to do with water.
In most cases of magmatic cooling, water and other compounds evaporate and the magma crystallizes.
Pegmatites represent magmatic cooling in an extreme way. In cases where water molecules and other compounds (e.g., chlorine, fluorine) have nowhere to escape, extreme things happen. The ions present in the water, combined with pressure and heat, allow for larger crystals to develop and rare minerals to form.
The Target Specimen
This rock originated in Canada and was transported to Michigan via glacier. This particular rock is special; it shows pegmatite dikes from different time periods, which allow for additional observations to be made regarding the composition of each intrusion. As with other pegmatites, it also shows larger crystals within the dikes when compared to the granite it has intruded.
References:
http://geology.com/rocks/pegmatite.shtml
http://www.mnh.si.edu/earth/text/2_2_5_0.html
http://en.wikipedia.org/wiki/Pegmatite
http://pegmatopia.ou.edu/
http://www.lsa.umich.edu/ummnh/
Congrats anarcha77, FTF