Geological Exploration of Metro Cleveland (GEM-C)
The Geological Exploration of Metro Cleveland tour is a series of Earthcaches which help to illustrate some geology that is hidden within the urban downtown area of Cleveland. These urban earthcaches help to highlight a wide variety of geological topics that are hidden in plain sight - in fact thousands of people walk by these stones every day without so much as a second thought about their origins!
This tour does not need to be done in any particular order and each earthcache is a standalone lesson. As you wander through the streets of Cleveland take a moment to pause and reflect on the lessons you can learn in just a normal city block!
From the outside the stone is a riddle:
No one knows how to answer it.
-Excerpt from "Stone" by Charles Simic
As with all earthcaches there is no container to find here, rather you must read through the description below and answer a few questions in order to log this cache as found. Please ensure that you send your answers to the CO via email or message in a timely fashion.
Foreign Bodies at the Justice Center
You are standing before the Justice Center. This building is constructed out of an igneous rock, Spanish Pink Granite which was quarried out of the Pedrolonga Quarry in northern Spain near the Portuguese border. Interestingly if you walk around the building you'll find several foreign bodies like the one below.
An example of a foreign body on the Lakeside Ave side of the building.
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 red square on this wall. You'll need to find this to answer the questions below!
Logging Questions
- Examine the wall 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 and 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 personal item) at the posted coordinates. Please ensure that your photo does not show the foreign body in the wall.
References:
- https://dam.assets.ohio.gov/image/upload/ohiodnr.gov/documents/geology/GB5_Hannibal_1992.pdf
- https://education.nationalgeographic.org/resource/xenolith/