This week we will ask you to be an eagle-eyed observer of stones that may decoratively dot your backyard (or, in this case, local elementary school). We’ll share with you an amazing story of a rock….in a rock….and what that means about how these rocks formed! All this geologic history was placed inadvertently in the middle of Sunset Heights Elementary School in Nashua, NH as part of a decorative stone circle memorial.
Let’s look at the picture above. What’s going on? The dark, round rock you see here is what’s called a xenolith and, if you know your Ancient Greek, you’ll know exactly what it is as “xeno” means “foreign” and “lith” means “rock”1. So here we have a foreign rock….but in the middle of another rock?!? We’ll take a cue from Alien and call this other, lighter colored rock the host rock. Because this rock slab was removed from its original location, we can only guess at the history of this host rock and xenolith using some important geologic clues.
Igneous Rocks: intrusive or extrusive?
The first thing we need to do to unravel this mystery is to identify the host rock and xenolith. Both of these rocks are igneous rocks, meaning they formed from molten rock called magma that formed crystals as it cooled. When you hear magma you might think volcanoes (you know I do). However, these rocks are what we call intrusive igneous rocks. Intrusive igneous rocks are cooled slowly beneath the ground. They never get erupted like volcanic, or “extrusive”, rocks.
The cool (literally!) part about intrusive igneous rocks is that the slow way they cool allows minerals in the molten magma plenty of time to form beautiful large crystals. If you’ve ever grown rock candy, you will know that leaving the rock candy to form for a longer amount of time will result in larger sugar crystals. In order for the xenolith to count as a “foreign rock” it needs to have a different composition than the host rock. Here we can tell this based on the very dark color and finer (smaller grained) texture of the xenolith compared to the host. Both of these features give us clues that these two rocks formed under different conditions and from different types of magma.
The second part of our mystery resides in how the xenolith became enveloped by the host rock (and, no, it wasn’t some type of Alien-host situation!) We need to use a tool we have in geology called relative dating to understand what happened. The xenolith is enveloped inside the host rock. So the xenolith must have existed before the host rock magma came poking around. Think of it like chocolate chips in a cookie. The chocolate chips had to exist as solid pieces before you put them in the raw dough! Remember also that the host rock was once molten magma (you can actually think of it as a kind of liquidy mush).
Molten magma is constantly moving around deep below the Earth’s surface. The molten magma that would eventually become our host rock ripped up a piece of another mostly cooled rock (our future xenolith). It’s possible that the xenolith was melted a little by the magma encasing it as you can see the edge of the xenolith seems to mingle a little with the host rock material2.
The molten magma cooled around the xenolith, entombing it as a little chocolate chip in our host cookie to wait around for millions of years until the whole rock slab would be chosen for a stone circle memorial at an elementary school. Xenoliths can give geologists information about rocks that are very deep below the Earth’s surface. Some xenoliths can even originate from the mantle, up to 100’s of kilometers beneath the Earth’s surface3.
Stone circles are found around the globe and often have quite a mysterious history and origin, often associated with astronomical cycles4. Even though the stone slabs we are examining in this post were erected within the past 20 years, they still hold a mystical allure as evidenced by the Ouija board we found abandoned among the slabs. It looks like the small xenolith hitching a ride in its host rock is not the only bit of intrigue found among these stones.
1Mervine, Evelyn. Geology Word of the Week: X is for Xenolith. AGU Blogosphere, 18 May, 2011, https://blogs.agu.org/georneys/2011/05/18/geology-word-of-the-week-x-is-for-xenolith/, accessed 3 May, 2021
2Johnson, Chris, et al., An Introduction to Geology, OpenGeology.org, Salt Lake Community College, 2017.
3Earth Science Australia. Xenoliths/Xenocrysts. http://earthsci.org/mineral/rockmin/petrology/xenoliths/xenoliths.html, accessed 5 May, 2021.
4Burl, Aubrey. A guide to the stone circles of Britain, Ireland and Brittany. Yale University Press, 2005.