There's not much information about this sculpture, the name of it, or the artist, neither online nor on a plaque near the sculpture. A helpful man on the street told us it was created by a local artist named Elizabeth Connor. Apparently, it used to reside in a different park a little way up into the city, but was recently moved to this location near the Seattle Ferry Terminal. The sculpture consists of several large chunks of granite, carved into slabs made to look like pieces from the ruins of an old city.
This earthcache is located close to the entrance of the Seattle Ferry Terminal, it is easily accessible in a wheelchair, although parking may vary. I've listed a parking waypoint which is about 100m away, but it is paid parking a majority of the time (times listed in the waypoint description)
Geology Lesson - Xenoliths
An inclusion is a generic term for any foreign rock within a host rock. In the case of igneous rocks, however, a specialised 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 geologists 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 related materials (such as a gabbro xenolith in a basalt host). These foreign bodies are given a specialised 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 crystallised from the same magma.
A true xenolith, or a foreign rock in an igneous host that did not form from the same magma, is always older than its 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 are often brought to the surface by volcanic eruptions. During an eruption, magma ascends from deeper parts of the Earth and passes through the surrounding rock layers. Sometimes, pieces of these rocks break off and get carried along by the rising magma.
As the magma nears the surface and erupts, it forms volcanic rocks like basalt, granite or other types of volcanic deposits. The rock pieces that were carried along in the magma are then embedded within these volcanic rocks - xenoliths are formed. The volcanic rock containing the xenoliths is referred to as the host rock.
Xenoliths are valuable to geologists because they offer insights into the Earth's composition and processes deep beneath the surface, which are otherwise difficult to study directly. They can retain characteristics of their original formation environment, such as mineral compositions and textures that formed under high-pressure and high-temperature conditions.
How can you recognise a xenolith?
- Xenoliths have a different mineral composition and texture compared to the surrounding host rock, making them stand out visually. Their surface might feel different from the host rock.
- Xenoliths can vary in size. They can be small pebbles or large boulders.
- They often have a different colour than the host rock.
- They are commonly found in igneous rocks, such as basalt or granite, where they have been incorporated during the magma's ascent.
- The xenolith may show signs of different metamorphic conditions, such as higher pressure or temperature minerals, compared to the host rock.
Logging Requirements
To log this cache, you will need to answer some questions and take a photo for your online log. You may log your find immediately - we will reply to all messages and emails. If we do not receive your answers and photo within 10-14 days, your log will be deleted. You are welcome to include your photo in your log, but please do not give away any of the answers.
There is a prominent xenolith covered by the red box in the photo below. Use this to answer the questions underneath the image.
1. Compare the foreign rock to the host rock. What are the approximate dimensions of it, and how does it differ in colour and texture?
2. Using the listing above and your observations, is this foreign rock a true xenolith, inclusion, or autolith?
3. Do you think the foreign rock is older, younger, or the same age as the surrounding rock? Why?
4. Take a photo, similar to the example below, showing yourself OR a personal item with a part of the sculpture behind you, and upload this to your online log. Please do not include the xenolith in your photo!
