Welcome to Jerimoth Hill!
As the highest natural point in Rhode Island, Jerimoth Hill offers not only a serene forested trail but also an excellent opportunity to explore one of Earth’s slowest geological processes: the crystallization of granite deep beneath the surface.
What is Granite?
Granite is a coarse-grained igneous rock that forms when molten magma cools slowly deep within the Earth’s crust. This slow cooling gives crystals time to grow, creating a mosaic of interlocking mineral grains.
Granite is made of several key minerals, each of which crystallizes at a specific temperature. By examining these minerals and how they’re arranged, we can reconstruct the cooling history of the granite — a process guided by Bowen’s Reaction Series, a foundational concept in geology.
What is Bowen's Reaction Series?
This key geological concept describes the order in which minerals crystallize as magma cools:
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High-temperature minerals like olivine, pyroxene, and calcium-rich plagioclase form early, around 1200°C.
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As the temperature drops, intermediate minerals like amphibole and biotite form.
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Low-temperature minerals such as potassium feldspar, muscovite, and quartz form last, around 600°C.
This sequence helps geologists interpret the cooling path of the magma. If you see quartz touching feldspar, for example, it suggests the granite fully cooled — all the way to the low end of the crystallization scale.
What else can we learn from Granite?
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.
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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.
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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.
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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.
Granite is famous for its interlocking mosaic of crystals. If you look closely, the mineral grains often fit together like a jigsaw puzzle, with no gaps or cementing material in between. This texture is a direct result of crystals growing tightly against each other as magma cools slowly in a stable environment.
Why is this important?
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Interlocking grains mean the rock formed in place, deep underground. These crystals didn’t break apart and reassemble later — they grew side by side, locking together as they solidified.
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Angular grain boundaries (sharp corners and well-defined edges) suggest slow, uninterrupted crystal growth — like branches of a tree forming over time.
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Lack of porosity (few or no holes or gaps) tells us the granite didn’t cool quickly or explosively. Volcanic rocks like pumice are full of bubbles — granite is just the opposite: solid, dense, and compact.
LOGGING REQUIREMENTS
Please submit your answers within seven days of your online log.
0) What is the name and GC code of the EarthCache you are submitting observations for? GCAZZMB - Granite of Jerimoth Hill
1a) Look closely at the exposed granite. What colors can you see in the granite? What minerals correspond to these colors?
1b) Using the minerals you identified in task 1a, do you think this granite formed at a high or low temperature?
2a) Take a look at the size of the crystals in the granite. Estimate how large the crystals tend to be.
2b) Using your observations in task 2a, do you think this piece of granite cooled slowly or rapidly?
3) Take a look at the grains found in the stone. Where do you think this stone cooled (underground or on the surface)? What observations led you to this conclusion?
4) Take a photo of yourself (or a personal item) and attach this photo to your found log.
Image Sources:
https://www.geologyin.com/2014/09/how-does-bowens-reaction-series-relate.html
https://www.strongtowns.org/journal/2015/10/21/granularity
https://earthsci.org/mineral/rockmin/identification/identification.html