Xenolith and Foreign Bodies - A Study in Poulsbo
Welcome to Poulsbo! Muriel Iverson Williams Waterfront Park is open to the public from Dawn ‘til Dusk; as responsible Geocachers, please respect all posted signs and operating hours. This park includes several short walking trails, picnic tables and scenic water views. It also contains a large boulder with an interesting foreign body. It’s this foreign rock that we’ll be examining today in our Earth Caching lesson. As a reminder this is an earthcache which means there is not a physical container to find. Rather you must read the lesson, make on site exmainations and then send your answers into the CO via the message center in order to claim your find!
Earth Science Lesson
An inclusion is a generic term for any foreign rock within a host rock. In the case of igneous rocks however, a specialized term “xenolith” is used to differentiate the foreign body. If the host is another rock type (i.e. metamorphic or sedimentary) then the foreign body will only be
classified as an inclusion. Xenoliths are particularly fascinating to geologist because of the manner in which they are formed.
While the term xenolith literally translates to ‘foreign rock,’ some xenoliths are not entirely foreign to the host. They may be a related material (such as a gabbro xenolith in a basalt host). These foreign bodies are given a specialized term and are referred to as “autoliths” or “cognate inclusions.” These terms specifically denote that the crystalline structure of the host and the foreign rock were crystalized from the same magma.
A true xenolith, or a foreign rock in an igneous host which did not form from the same magma, is always older than their host rocks. This is because in order for the foreign body to become surrounded by the igneous rock, it must have already existed as a solid rock fragment when the magma around it solidified. This is not necessarily true with cognate inclusions.
Xenoliths have significant scientific value to geologists. Given that they are older than the igneous rock surrounding them, it indicates that many of them have been carried up from the mantle, which offers scientists a way to examine what sort of rocks exist in the lower regions of the earth’s crust! Depending on the composition of the xenolith (including density and rock type) the foreign rock may have originated from a variety of places within the earth, and this is further invaluable data for geologists.
There is a foreign body below the shaded area on this rock. You'll need to find this to answer the questions below!
Questions
Please do your best to answer all of the questions with as much accuracy as possible, but remember this is a learning experience. Effort matters more than correct answers. If your answers are not correct I will reach out to you and we can discuss. Please ensure you send in your answers within a reasonable amount of time, otherwise your log may be deleted. I will not do this without contacting you first.
- Examine the boulder in front of you. You are looking for the foreign rock that is depicted in the image above. Describe the foreign rock in terms of size, coloration. Do those descriptions match the surrounding rock?
- Based on your reading of the lesson and your answer in question one, characterize this foreign rock as a True Xenolith, an inclusion, or an autolith.
- Do you think the foreign rock is older, younger, or the same age as the surrounding rock. Why?
- Post a photo of yourself or a recognizable object (such as a TB or a perosnal item) at the posted coordinates. Please ensure that your photo does not show the foreign rock.