While driving this stretch of Main Divide Road, I noticed something interesting. This part of the road was cut into the hill side, which revealed an interesting and common geologic feature that goes mostly unnoticed by motorists. There's a large igneous intrusion known as a dike. Dikes form when cracks or fractures within existing, buried rock (known as the country rock) allow a pathway for magma to rise to towards surface. Dikes, to an extent, can be called conduits although the term is associated more with volcanoes.
Over time, the dike cools and when viewed in cross section (such as here at a road cut) one would see a large straight line of rock cutting across the existing rock. Due to the law of superposition, the layers of rock must have formed before the dike did. This makes sense as in order for the dike to have formed, there must have been rock that existed prior for the dike to be able to cut through it. This means the layers of rock around the dike are older than the dike itself.
Dikes are always igneous in nature, however along the edges one can have localized metamorphism where the dike makes contact with the existing rock. The heat of the liquid rock within the dike (at the time of formation) is enough to not entirely melt the rock along it's edges, but it is enough heat to reorganized its chemistry. When a rock's chemistry changes without melting, the rock is classified as a metamorphic rock. Along the edges of dikes, these "zones of metamorphism" are called "chilled margins" as the margins along the dike were cooled instead of melting and being incorporated into the dikes. These can be seen as dark lines along the dike, typical of a change in chemistry (image above).
If a dike makes it to the surface, it is classified as a sill as lava moves laterally across the landscape. If the dike does not make it to the surface, it is still considered a dike. Above is a diagram of the ideal difference between a dike and a sill. Note, once a sill forms there is nothing stoping it from being buried by sediments or being altered in any way (i.e. tilting due to faulting). Below is a photo of what a dike and sill look like in cross section in the field. Note how the sill is tilted due to tectonic forces.
You may be asking yourself, what if a sill is tilted 90 degrees? How could you tell that it's not a dike? There are three ways to know. One is if the rock that the sill protrudes into is sedimentary. You can figure out its original orientation by looking at fossils, ripple marks, or other sediment structures which can provide clues into the original orientation of the rock. In other words, sed-structures (for short) will help you know the orientation the rock originally formed in. But possibly the easiest way is to look at chilled margins. If chilled margins only exist along one edge of the dike, then you know that what you are looking at is actually a sill. Dikes, if it has them present (note not all do), would have chilled margins along both of its edges. A sill, would only have them (if it is present) along one edge: the edge that marks the bottom of the sill. This makes sense as the top layer of a sill would just be open to the air with no additional layers above it. Lastly, one could look at the mineralogy and crystal sizes of grains along the edges of the dike to determine if it is indeed a dike. Because dikes form underground, they cool much more slowly allowing for larger and more defined crystals to form. On the other hand, a sill, which has one side exposed on the surface, would cool more rapidly and have much smaller crystal sizes.
Works Cited
https://www.tulane.edu/~sanelson/eens212/intro&textures.htm
https://www.quora.com/What-is-the-difference-between-dykes-and-sills
https://www.endeeper.com/blog/magmatism-sedimentary-basin-application-oil-exploration/
https://www.see.leeds.ac.uk/structure/basic_structure/primary/chillmargin.htm
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Santa Ana Mountains - Main Divide Dikes" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with the largest dike you can find. One is abnormally large compared to the others so it shouldn't be hard to see. Be sure not to show other parts of the outcrop in your photos. Upload these images with your found it log. Do not message me any photos.
3. Estimate the (a) number of dikes within this rock outcrop, (b) their lengths and thicknesses, (c) range of colors, (d) angles/orientation, and (e) textures. You don't need to be exact, just give me a range for each.
4. Describe the country rock in which these dikes intrude into. Specifically, tell me its (a) colors, (b) textures, and (c) average grain sizes (if possible)
5. Based on your answers in question 4, is the country rock sedimentary, metamorphic, or igneous? Explain your answer.
6. Do you see any evidence of chilled margins? If so, where do you see it. If not, does this surprise you?
7. Based on your answers above, are these dikes or sills? How can you tell?