Bucksport Formation Intrusions EarthCache
Bucksport Formation Intrusions
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Often as you drive through a road cut one of the more striking features is an intrusion. This earthcache will look at these intrusions to better help you understand what you are looking at as you drive. Please due this cache south bound only. Watch for traffic....
The fact that in this road cut the layers of rock are almost vertical with intruded vertical beds of granite makes this a unique location. This road cut is part of the Bucksport formation intruded by layers of granite vertically. As you are driving along these different colored rock that cut through a formation give it a striking appearance. These vertical sheets are a type of intrusion.
The subtle features in the intrusions gives the scientist information about how the rocks were formed. The intrusion form in what is called country rock. Country rock is a geological term meaning the rock native to an area. In order for the molten rock to flow into the Bucksport formation, the rock had to pull apart along fractures. The Norumbega Fault that ran along the coast of Maine caused these fractures. Comparing the opposite sides of an intrusion, it is easy to see that they originally fit together and is now separated by the filling material. If the edge of the intrusion is very sharp as it is in this case the dike was formed after the rocks were folded into place.
In geology, an intrusion is a body of igneous rock that has crystallized from molten magma below the surface of the Earth in the country rock. Intrusive rocks also exist in a wide range of forms from mountain range sized batholiths to thin vein-like fracture fillings of aplite. Intrusive structures are often classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel, the body is concordant, while if it cuts across the country rock, it is discordant. Structural types include:
* batholith: large irregular intrusions.
* stock: smaller irregular discordant intrusions.
* dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
* sill: a relatively thin tabular concordant body intruded along bedding planes, often near-horizontal when emplaced, but may be intruded into tilted beds or the entire package may be tilted by later deformation.
* pipe or volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
* laccolith: concordant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.
* lopolith: concordant body with a relatively flat to sagging top and a shallow convex base (spoon-shaped) may have a feeder dike or pipe below.
* phacolith: a concordant lens-shaped pluton that typically occupies the crest of an anticline or the trough of a syncline.
What type of intrusion do you find at the posted coordinates? It is a concordant intrusive dike, which is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters and the lateral dimensions can extend over many kilometers. A dike is an intrusion into an opening-crosscutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it. Dikes are usually high angle to near vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the latter becomes horizontal. Near horizontal or conformable intrusions along bedding planes between strata are called intrusive sills. Looking at these dikes you can see that they are still vertical and have moved little since the time of their formation.
At the coordinates listed above you will be on the edge of a roadside outcrop of the Bucksport Formation. Take some time to look at how the granite formed a number of dikes within the country rock. Look at the edges of the dike and compare the sides to see how they match. If you look carefully you will find sections of country rock that were trapped in the dikes as it formed. To log this Earthcache: You must post a photo of you and your GPS with the outcrop in the background and then send an email to me through my profile with the following information: What is the width of the dike at the posted coordinates? Make sure to use metric measures if possible because this is science at its best. If you don’t have a ruler that’s OK because a meter is just a little bit longer than a yard. Please begin your email with the name of the earthcache and make sure your log includes your photos for others and the number of people in your group.
If you enjoy this earthcache you may want to check the Maine Geological Survey located at (visit link)
They have developed a number of information sheets or field localities giving a great deal of information about geologic features. They also have a number of books and maps about Maine’s natural history/ geology that you might find interesting.
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