Heart of the Buckeye State EarthCache

This large stone marks the Geographical Center of Ohio (in theory, at least) and is the focus of this EarthCache! 

What Is Granite?
Granite is a coarse-grained igneous rock that forms when magma cools slowly beneath the Earth’s surface. This slow cooling allows large, visible mineral crystals to grow and interlock. Because granite crystallizes deep underground, it is classified as an intrusive igneous rock. Most granite formed hundreds of millions to billions of years ago during periods of continental collision and mountain building. These processes generated large bodies of magma that cooled slowly within the crust, eventually becoming granite plutons that are exposed today through erosion or quarrying.

Mineral Composition of the Granite Marker
The granite used in this marker contains a mix of minerals that give it both strength and visual character. Typical minerals visible in granite include:

• Quartz - Translucent to gray, glassy grains that are very hard and resist scratching. Quartz contributes significantly to granite’s durability.

• Feldspar - Often pink, reddish, or light gray crystals. Potassium feldspar (orthoclase) is common in monument granite and gives many stones their warm coloration.

• Biotite or Hornblende - Dark black or greenish minerals that appear as flakes or elongated grains, adding contrast to the lighter minerals.

• Muscovite - Silvery, reflective flakes that may sparkle in sunlight.

Granite Crystals
The size of crystals in granite tells us how quickly the magma cooled. Just like water freezing slowly forms larger ice crystals, magma that cools slowly underground gives mineral crystals more time to grow.

Coarse-Grained Granite
Crystals larger than 5 mm across (about the width of a pencil eraser) typically mean the magma cooled very slowly — possibly over millions of years. You may see individual quartz or feldspar grains standing out clearly.

Medium-Grained Granite
Crystals between 1-5 mm are typical of granite that cooled at intermediate depths. The cooling was still slow, but not as prolonged as deeper intrusions. This texture is most common in granite.

Fine-Grained or Mixed Texture
If you see small crystals, or a mix of large and tiny grains, this may indicate the granite began cooling slowly underground and then cooled more rapidly; perhaps because the magma migrated toward the surface or was injected into cooler rock. The larger grains are called phenocrysts, and their presence suggests multi-stage cooling.