Turn of the Earth 🌍 EarthCache

Earth Science Lesson: Granite in Motion

Rock Composition: The Spinning Globe and Its Granite Base
The fountain’s centerpiece is a solid sphere made of polished granite, a durable igneous rock prized for its strength and beauty. Both the globe and the basin it sits in are made of granite, which forms deep underground from slowly cooled magma. Though incredibly tough, granite is not immune to the persistent effects of water and time.

Granite consists of:

• Quartz – Clear to milky white, highly resistant to weathering.
• Feldspar – Often pink, white, or gray; vulnerable to chemical changes when exposed to water.
• Mica (biotite or muscovite) – Shiny, flaky minerals that break down more easily and may contain iron, which oxidizes (rusts).

This interlocking mix of crystals gives granite its strength, but in constantly wet environments—like a fountain—it gradually shows signs of weathering.

Weathering Processes at Work: A Fountain That Teaches Geology

Although granite weathers more slowly than many rocks, the conditions at the Lincoln Park Zoo spinning globe make it a perfect natural science lab. Visitors can observe the following types of weathering right on the sculpture:
1.    Chemical Weathering (Mineral Breakdown and Staining)

• Water, especially slightly acidic rain or treated fountain water, reacts with feldspar, slowly transforming it into clay.
• Iron-bearing mica oxidizes, leaving brown or orange rust stains on the stone.
• Water can also leach out minerals, resulting in dark streaks or color patches where the granite is repeatedly exposed to moisture.

2.    Physical Weathering (Surface Abrasion and Texture Changes)

• The globe spins and is constantly in contact with water, which slowly wears down the stone through abrasion.
• Areas where water runs off or drips heavily may begin to feel rougher or less polished over time.

3.    Biological Weathering (Algae and Biofilm Growth)

• Moist, shaded zones encourage the growth of algae, bacteria, or moss. These organisms trap dirt and organic debris, leading to greenish or dark stains, particularly near the waterline.

What to Look For at the Fountain: Signs of Weathering and Erosion

1. Color Differences: Look for areas where the granite is darker or stained compared to drier, untouched stone. Notice if there is a brownish or rusty tint—this may indicate oxidation of iron-bearing minerals.
2. Streaking or Discoloration Patterns: Are there streaks or trails that match where water flows or drips? Check if the discoloration follows the direction of gravity or curves around the basin.
3. Mineral Deposits or Crusts: Look near the base or drain area for white, gray, or off-colored crusty patches—these could be dried mineral deposits from evaporated water.
4. Surface Texture Changes: Run your hand gently along the stone—are some areas smoother, rougher, or pitted? Weathered granite may feel less polished or more textured in wetter zones.
5. Biological Growth: Check for faint green, dark, or slimy patches—these could be algae, moss, or biofilm growing where moisture stays constant.
6. Drip and Splash Zones: Watch where water lands or drips off the globe. Are those areas more stained or weathered? See if there is a clear boundary between frequently wet and mostly dry surfaces.