Brickyard Park's Dark Secrets: Xenoliths and Resistant Granite
A Journey Through Time: Welcome to Brickyard Park, a unique coastal location near Nanoose where you can witness firsthand the power of geological processes and uncover clues about Earth's deep history in this specific region. Unlike many of the sandy, pebbly or sandstone beaches in the surrounding area, Brickyard Park is characterized by significant exposures of granite bedrock. This EarthCache will guide you to explore this fascinating rock and the intriguing dark inclusions known as xenoliths, offering insights into the formation and resilience of this ancient igneous rock.
Logging Tasks: To log this EarthCache, please carefully observe the geological features at Brickyard Park and answer the following questions. Send your answers to the cache owner through Geocaching.com messaging before you log your find.
- Xenolith Observation: At the given coordinates, describe the appearance of at least one xenolith you observed in the granite. Note its color, approximate size (length and width), and texture compared to the surrounding granite. Do you see any sharp or rounded edges on the xenolith? What might the shape suggest about its incorporation into the magma that formed the granite?
- Mineral Identification: Identify and list at least two of the main minerals you can see in the granite. Describe their appearance (color, shape, luster).
- Granite's Resistance: Based on your observations of the granite at Brickyard Park and the surrounding environment, explain in your own words why you think this area is predominantly granite bedrock, while other nearby beaches are mostly composed of sandstone and conglomerate rock, or sand and pebbles (derived from sedimentary rocks). Consider the properties of granite and sedimentary rocks in your explanation.
- Photo: Take a photo of yourself or a uniquely identifying item (your GPS device, a personal trackable,your caching name on a slip of paper, etc) at or very near the given coordinates. Include the granite bedrock in your photo.
This EarthCache aims to highlight the following Earth Science concepts:
- Igneous Rock Formation: Understanding how granite forms from the slow cooling of magma deep beneath the Earth's surface.
- Xenoliths: Identifying and interpreting these "foreign rocks" trapped within the granite, providing a glimpse into the Earth's subsurface.
- Differential Weathering and Erosion: Explaining why the resistant granite forms the prominent shoreline at Brickyard Park, while sedimentary rocks create nearby beaches.
The Story in the Stone: In your visit to Brickyard Park, carefully make your way from the trails to the shore. As you carefully walk along the rocky shoreline, observe the light-colored, speckled rock beneath your feet and in the larger outcrops. This is granite, an intrusive igneous rock. Notice the interlocking crystals of different minerals that forms the granite rock – quartz (glassy), feldspar (pink, white, or grey), and mica (dark, shiny flakes).
Igneous is one of the three primary types of rock, which include sedimentary and metamorphic. Igneous rock is formed by the cooling and slidification of magma (molten rock). The size of the minerals within igneous rock relates directly to the speed at which that rock cools. The faster igneous rock cools, the smaller the minerals will be.
As the magma that forms the granite rises towards the surface and cools, it often travels through "pipes" of older, harder rock, and the magma pulls of bits of those "pipes" which end up incorporated into the cooled granite.
Now, look closely for darker patches or fragments embedded within this lighter granite. These are the xenoliths. They represent pieces of older rock that were incorporated into the molten magma as it rose through the Earth's crust millions of years ago. Their darker color often indicates a different mineral composition, offering a snapshot of the geological layers the magma passed through.
Explore the granite sea side here and locate some xenoliths to observe.
Why Granite Here? Consider the hardness and resistance of the granite you see compared to softer materials like sandstone or conglomerate found on most other nearby beaches. Granite's tightly interlocked crystal structure makes it very durable against the erosive forces of waves, wind, and rain. The surrounding sea-side areas, composed largely of younger, less resistant sedimentary rocks of the Nanaimo Group (formed from accumulated sediments), are more easily eroded away, forming the sandy and pebbly beaches common in the vicinity. The prominent granite exposures at Brickyard Park stand as a testament to its enduring nature.
Important Notes:
- Please be respectful of the environment and do not hammer or remove any rock samples. Observe and appreciate the geology as you find it.
- Be aware of tides and potential hazards along the shoreline. Exercise caution while exploring.
- There is no physical container to find for this EarthCache. Logging is based on answering the questions and submitting the photo.
- Logs without the required logging task answers will be deleted.
Enjoy your geological exploration of Brickyard Park! This location provides a fantastic opportunity to connect with the Earth's deep past and understand the powerful forces that shape our coastline.