Dikes and Migmatites in New Gloucester, Maine EarthCache
Dikes and Migmatites in New Gloucester, Maine
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
INTRODUCTION
This road-cut, along Route 26, between Gray and Poland, exposes some great igneous rocks, metamorphic rocks, and something special in between. It also reveals a significant event in Earth's geologic history.
DESCRIPTION
The main rock type here is light colored granite, primarily from the Sebago pluton. However, you will also notice some darker colored rocks as well. This EarthCache focuses on some of these darker rocks.
You’ll notice that there are two different types of dark-colored rocks here. From a distance, they appear to be similar in color, and possibly composition, but as you get closer, you’ll see they are quite different. Some of these dark rocks appear as wavy, wispy, near-horizontal layers. These dark layers are metamorphic rocks that make up part of a mass of migmatite, a type of rock that represents the boundary between metamorphic and igneous rocks. In a migmatite, a rock is heated to such a high degree that some of its minerals melt and then recrystallize into igneous rock, while other minerals do not melt, but are instead transformed (metamorphosed) into other minerals. The dark/black layers here represent the metamorphosed portion of the migmatite. This EarthCache will not focus on the migmatite, but it is recommended that you peruse some of the resources listed below to learn about migmatites, which are an interesting and somewhat unique rock type.
The dark/black colored rocks that appear as near-vertical strips within the granite are dikes of basalt. Basalt is an igneous rock, like granite, but it has more magnesium and calcium based minerals, and fewer silica based minerals. Because of its different composition, basalt is much darker than granite, and generally has a black, brown, or deep red color. Basalt generally has fine-grained (small) crystals because it cools relatively quickly at or near Earth’s surface, and the crystals do not have time to grow very large. Because basalt forms at (or near) the surface, it is classified as an extrusive igneous rock, whereas granite, which forms below Earth’s surface and has larger mineral crystals, is classified as intrusive. Another difference between basalt and granite is that basalt is believed to derive primarily from magma that originates in the mantle (deeper inside Earth), whereas granite is thought to derive mainly from magma that originates within the crust (shallower). Basalt is often seen in solidified lava flows that cover the surface in places such as Hawaii and Iceland. Basalt is not found extensively on the surface in Maine; here, at this EarthCache, the basalt appears within dikes. A dike is a mass of igneous rocks that cuts through layers of other rocks.
The most interesting thing about the basalt at this EarthCache is how it got to THIS location. First of all, which rock was here first, the granite, or the basalt? It must have been the granite, because clearly the basalt cuts right through it. The granite must have been here first, then something must have happened to create cracks in the granite to allow basalt to work its way inside. But what happened to create these cracks?
The supercontinent we all know and love--Pangaea--formed around 300 million years ago and existed during the late Paleozoic (540-250 million years ago) and the early Mesozoic eras (250-65 million years ago). Maine was situated deep within Pangaea's interior, thousands of miles from the nearest coastlines. Around 200 million years ago, Pangaea started to break apart. Large rifts formed between ancient North America, Europe, and Africa. The present-day Atlantic Ocean began to open up. Today, North America continues to spread apart from Europe and Africa, and that Atlantic Ocean continues to grow at a rate of approximately two inches per year, which is roughly the rate at which your fingernails grow.
It was the initial splitting of Pangaea that created the fractures (cracks) here in the Sebago granite, through which basaltic magma was able to intrude. These fractures extended a long distance through the crust, and probably into the mantle, from whence the basaltic magma came. Some of the basalt probably flowed onto the surface, but was largely eroded away, leaving behind just the dikes as their evidence. So, these striking strips of black rock represent the breaking apart of Pangaea, a major event in Earth's geologic history.
LOGGING TASKS
1. a. Look closely at the mineral crystals in the igneous dikes and describe their texture (size).
1. b. What does the size of the minerals tell you about the rocks that make up these dikes? (The information in this EarthCache description should help you, but remember that real rocks in the field aren't always as described in textbooks!)
2. Look at the mineral crystals in the metamorphic bands. How do they look, and how do they compare to the minerals in the igneous dikes? (It may be helpful to find samples that have broken off from the main outcrops, which will allow you to view the rock more closely and in multiple dimensions.)
3. Observe the color of the igneous dikes, particularly the weathered portions. According to the key in the geologic map of the Gray quadrangle (Source #2), basaltic dikes have "reddish-brown weathering, dark gray" colors, while trachyte dikes have "light greenish-gray" colors. What type of rock do you think is present in the dikes at this EarthCache? Explain your reasoning.
4. There are some cracks that cut through the igneous dikes. In what direction do those cracks run? These cracks may have resulted from expansion that occurred as these rocks were uplifted toward the surface and exposed as the rocks above them were eroded away.
5. In what compass direction do the dikes trend (align)? What does this tell you about the forces that enabled their formation? (This may be difficult to determine, but look at the direction the road runs, and compare that to the direction in which the dikes appear to approach the road.)
OPTIONAL TASKS:
6. Please describe one geological observation you made at this location that was not already referred to in the EarthCache description. This is to encourage visitors to “think outside of the box” and go beyond the topic that this cache focuses on.
7. If you’d like, please feel free to post any photos you have taken at this EarthCache, as long as they are appropriate and do not spoil the logging tasks or overall experience for others.
Once you’ve completed your visit, please feel free to log this EarthCache immediately; you do not need to wait until you’ve submitted your logging task responses, and you do not need to wait until you have received a response from the cache owner. However, it would be greatly appreciated if you submit your responses to the logging tasks within 10 days of your visit. Please e-mail your responses to earthcache@geosociety.org and use the GC-code as the subject line.
ACCESS INFORMATION
Route 26 is a very busy road with fast moving traffic, so please use extreme caution when visiting this site. You cannot park your vehicle along Route 26. Instead, you should park at the nearby Maine DOT New Gloucester Park and Ride Lot, and then walk to the cache coordinates. You will have to walk south along Sabbathday Rd., till it hits Route 26, then turn right to walk north along Route 26 to the cache coordinates. you will be approaching Route 26 from the east side, which is where the cache is located. Crossing over to the west side of the road is not recommended, nor is it required in order to complete the logging tasks. Children, strollers, wheelchairs, pets, and visits after dark or in inclement weather are not recommended due to the proximity to the road. A waypoint has been provided to help direct visitors to the parking lot, which is on the west side of Sabbathday Rd., near the intersection of Sabbathday Rd. and Route 26. From Route 26, go east on Sabbathday Rd, then make the first left to turn into the Maine DOT New Gloucester Park and Ride Lot.
Please note that Maine DOT Park & Ride Lots are for commuter parking only, for periods of 24 hours or less. Please do not leave your car there for an extended period of time. Although there are no specific warnings for this lot or this area in general, it is advised that you do not leave valuables in the car while visiting this EarthCache. Here is a map showing the location of this lot: http://www.exploremaine.org/images/maps/parknrides/NewGloucesterPR.jpg
Alternatively, you may park on a side or service road just off of Route 26, north of the intersection with Sabbathday Rd.. Then you can walk to the listed coordinates, or to any other spot that shows the described features in the roadcuts. Parking on/near Pond Rd. works well.
INFORMATION SOURCES
1. Migmatite in New Gloucester Field Locality Web-page, by the Maine Geological Survey (MGS): http://www.maine.gov/doc/nrimc/mgs/explore/bedrock/sites/jul10.htm – Just one of many great geology field sites described in detail by the MGS. Additional geo-resources may be found on the MGS home page here: http://www.maine.gov/doc/nrimc/mgs/index.htm. Clickable maps of the field localities described by MGS may be found here: http://www.maine.gov/doc/nrimc/mgs/frontend/teachers.htm
2. Geologic Map of the Gray Quadrangle, by the Maine Geological Survey: http://www.maine.gov/doc/nrimc/mgs/pubs/online/bedrock/bd-gray.pdf
3. Geologic Description of the Gray Quadrangle, by the Maine Geological Survey: http://www.maine.gov/doc/nrimc/mgs/pubs/online/bedrock/97-3-gray.pdf
4. Roadside Geology of Maine, by D.W. Caldwell – This series, which covers much of the United States, provides accurate geologic information and is a great source for people developing EarthCaches. Information about the entire series may be found here: http://mountain-press.com/series_detail.php?series_key=2
5. Migmatite - American Geophysical Union (AGU) Word of the Day: http://blogs.agu.org/georneys/2011/08/28/geology-word-of-the-week-m-is-for-migmatite/
6. Migmatite - Wikipedia (a very good entry): http://en.wikipedia.org/wiki/Migmatite
7. Maine Park and Ride Information: http://www.exploremaine.org/commuters/parknride/parknride-list.shtml
Additional Hints
(No hints available.)