O Mary Memorial! EarthCache

This EarthCache is located along the pathway leading to the world’s largest statue of Our Lady of Guadalupe. As you make your way toward the world’s largest statue of Our Lady of Guadalupe, you’ll encounter a striking memorial built into the landscape—a quiet place for reflection that also offers a unique geological lesson. This memorial is constructed from two contrasting types of stone: a dark stone forming the top portion and a light stone making up the base. These rocks are not only visually different, but they also tell separate stories about their mineral composition, formation, and long-term behavior in the natural environment.

Granite Memorial at the Our Lady of Guadalupe Statue

The dark stone at the top of the memorial is black granite, an intrusive igneous rock formed from slowly cooled magma deep within the Earth. It is composed mainly of plagioclase feldspar, biotite mica, and hornblende, with very small amounts of quartz. The high content of mafic (magnesium- and iron-rich) minerals gives it a dense black appearance, and when polished—as this stone is—it takes on a glossy, speckled look due to its interlocking crystalline texture.

In contrast, the light-colored stone used on the lower portion of the memorial is white granite, another variety of intrusive igneous rock. This granite consists largely of quartz, potassium feldspar, and plagioclase feldspar, with smaller amounts of biotite. Its salt-and-pepper appearance comes from the mix of light-colored minerals and darker flecks. The greater presence of felsic (silica- and aluminum-rich) minerals gives it a lighter tone and slightly different behavior over time.

Though both rocks are classified as granites and are known for their durability, their weathering patterns differ slightly due to variations in composition. Granite is generally resistant to physical weathering due to its hardness and interlocking structure, but it is susceptible to chemical weathering—especially the feldspars, which can break down into clay minerals when exposed to moisture and slightly acidic conditions. In humid environments, black granite may weather more quickly due to its higher content of iron- and magnesium-bearing minerals, which can oxidize and stain the stone over time. White granite, being more silica-rich, often maintains its color better but may exhibit granular disintegration or surface roughness as the feldspars slowly degrade.

In terms of weathering, both stones are durable, but their mineral content influences how they change over time. The white granite, rich in quartz and feldspar, tends to resist chemical weathering more effectively and may show slow surface pitting or grain loosening. The black granite, with higher levels of iron-bearing minerals, is more prone to oxidation, which can cause surface discoloration or staining. The differing rates and styles of weathering make this memorial an ideal site for observing geology in action.

Common Types of Erosion on Granite

1. Mechanical Weathering:

   • Frost Wedging: Water enters small cracks, freezes, and expands, causing the granite to break apart.
   • Abrasion: Physical scraping by wind, water, or other rocks.

2. Chemical Weathering:

   • Oxidation: Minerals within the granite, like feldspar, react with oxygen, leading to discoloration and weakening.
   • Hydrolysis: Reaction of minerals with water, transforming feldspar into clay and causing surface deterioration.
   • Acid Rain: Sulfuric and nitric acids in rainwater react with the granite’s minerals, accelerating decay.

3. Biological Weathering:

   • Plant Growth: Roots penetrate cracks, exerting pressure and expanding fissures.
   • Lichen and Moss: Organic acids from lichen and moss slowly degrade the rock surface.

4. Thermal Expansion:

   • Repeated heating and cooling cycles cause the granite to expand and contract, leading to cracks and surface flaking.

5. Water and Wind Erosion:

   • Flowing water smooths surfaces, while wind carries abrasive particles that polish or pit the rock.