While exploring Everett, I came across a unique bench, made from granite (an igneous rock), that contains xenoliths. Granite forms normally as a magmatic pluton cools. This process usually takes tens and sometimes hundreds of millions of years to complete, occuring many miles below our feet. The cooling can occur as the pluton is rising towards the surface or as it is static and sitting in place. As a pluton rises it must break away the surrounding rock. If the pluton is hot enough, this will melt the country rock (the geologic term for the rock surrounding a magmatic pluton) and will become incorporated into the overall pluton. However, if the pluton is much cooler, this will fracture the country rock breaking it into rugged pieces. These smaller rocks will become part of pluton as it cools.
If the pluton is cooling fast enough, these rocks will not have enough time to melt themselves before being suspended in the matrix. The matrix is a term to define the fine grained texture of a rock. Therefore, the overall granitic rock will have the appearance of chocolate chip cookies. Where the country rock fragments are the chocolate chips and the cookie dough is the granite. The geologic term we use for these chocolate chips are xenoliths. It's an odd word to say, so it is pronounced as: Zee-no-liths. Xenoliths are usually irregular in shape as they were fractured prior to being incorporated into the pluton. Below is an image from the Wikipeida article on xenoliths. They are quite distinctive in igneous rocks, and a key characteristic of them.
Xenoliths can be felsic or mafic in composition. In geology, felsic or mafic is a term we use to describe the relative silica content of a rock. Felsic rocks, are lighter in color. Granite, for example, is considered a felsic rock as it contains, quartz, bioite, and k-felds (potasium feldspar or orthoclase) which are silica rich minerals. On the other hand, mafic describes a rock that has a relatively low silica mineral content and contain a high magnesium and iron mineral content. Rocks like basalt are considered mafic because they appear darker in color and contain minerals such as olivine, plagioclase, pyroxene, and amphibole.
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. "Everett Xenoliths" 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 granite bench in the image. This photo MUST be uploaded to your "found it" log. Please do not send any photos through the message center as Groundspeak compresses images. Each log MUST have a unique and different photo. No two logs may contain the same photo.
3. Estimate the number of xenoliths in this rock. You don't need to count all of them. Just give me a range.
4. Describe the xenoliths that are present in this granitic rock. For example, describe thier (a) colors, (b) relative sizes, (c) shapes, and (d) relative abundances.
5. Using the mineral chart in cache page, name at least (a) 2 minerals that make up the xenoliths and (b) 2 minerals that make up the granite. Make sure you keep in mind which minerals are felsic and which ones are mafic!
6. Are these xenoliths mafic or felsic? How can you tell? Explain.
7. Using your answer from question 5, would the country rock be mafic or felsic? How do you know? Explain.
Works Cited
https://slideplayer.com/slide/4062736/
https://en.wikipedia.org/wiki/Xenolith
https://geologycafe.com/class/chapter3.html