Pillar of the Community
Just outside Seattle's Fire Station 30 in Mount Baker, you will find this 14-foot-tall granite sculpture resembling a fire hose. It was built by Brian Goldbloom in 2013, and serves as a symbol of the crucial work our firefighters do for us. This EarthCache will encourage you to take a closer look at the sculpture - perhaps not in the way it was intended - and appreciate all the geology in this relatively small piece of the earth.
🪨 About the Geology
This sculpture is made out of two main types of granite — a lighter pink-ish type and a darker grey type. Using the details provided in this section, please answer the questions at the bottom of this page and submit them to me to claim a “find” on this cache.
🧊 What Is Granite?
Granite is a felsic, intrusive igneous rock, which means it forms from magma that cools slowly underground. This slow cooling allows large crystals to form, giving granite its distinctive coarse-grained (phaneritic) texture. Additionally, it is very strong and mostly resistant to weathering.
🎨 Colors and Minerals in Granite
There are many minerals that make up the different colors of granite. Below are the main ones:
- Quartz – Clear to gray. Adds light gray tones and durability.
- Potassium Feldspar – Pink to salmon. Responsible for pink hues.
- Plagioclase Feldspar – White to gray. Adds dull white tones.
- Biotite Mica – Black or brown. Adds dark specks.
- Amphibole (Hornblende) – Dark green to black. Adds darker speckling.
🧱 What Granite Is Made Of
Granite isn't made of just one mineral — it consists of interlocking crystals of several major minerals. The main “building blocks” are:
🔹 Quartz (gray/white patches)
- Silica-rich; forms late in the crystallization process
- Resistant to weathering — stays glossy and gray
- Adds a “sparkly” or glassy texture
- More quartz = lighter and shinier granite
🔹 Feldspars
- Potassium feldspar (orthoclase) gives granite its pink or salmon color
- Forms earlier during cooling
- Often blocky in shape; may exsolve into stripes with albite
- Plagioclase feldspar is white to gray with visible striations (fine lines)
- Found in more calcium-rich melts (granodiorite, tonalite)
🔹 Mafic Minerals (Biotite, Hornblende)
- Form from iron- and magnesium-rich melt portions
- Show up as black or dark green specks
- More mafic minerals = darker granite (toward granodiorite or diorite)
- But granite is felsic overall — not enough mafic minerals to change classification
⏳ Crystal Size and Cooling Rate
Since granite cools slowly beneath Earth’s surface, minerals have ample time to grow large, resulting in a coarse texture.
- Slower cooling = larger crystals
- Faster cooling (still intrusive) = finer crystals, more uniform texture
- Cooling near magma edges or dikes may cause variation in texture within a single body
🌦️ Weathering and Alteration
After formation, granite may change in appearance due to:
- Hydrothermal alteration: Hot fluids can alter feldspar to clay, dulling the rock
- Oxidation: Iron in biotite or hornblende may rust, adding yellow-orange tints
- Surface polishing/erosion: Smoothed or polished granite becomes more colorful and reflective
🛠️ Why Use Granite?
Granite isn’t just geologically interesting — it’s practical too. Its durability and beauty make it a popular choice for sculptures, monuments, buildings, and countertops. That’s likely why the artist chose it for this piece — it will endure the outdoors and still look beautiful.
🧪 EarthCache Questions
Use the information above to answer the questions below and send your responses to me to claim your find.
- 1a) What minerals are responsible for the color in the lighter section of the granite?
- 1b) What minerals are responsible for the color in the darker section of the granite?
- 2a) What minerals can you identify in the lighter section? What does this suggest about the rock?
- 2b) What minerals can you identify in the darker section? What does this suggest about the rock?
- 3) Do you see any major crystals in the light section? How about in the dark section?
- 4) Based on the texture and crystal size, what does this suggest about how the granite formed?
- 5) Please include yourself or a personal item in a photo at GZ. You may include the sculpture in the background, but avoid focusing on specific details that could give away answers.
Submit your answers by emailing me through my profile or through the Geocaching Message Center.
You may log your find immediately after submitting your answers.