This earthcache will bring you to a natural granite stone located among the brush in Brenton Point State Park. Brenton Point State Park is a public recreation area occupying 89 acres at the southwestern tip of Aquidneck Island in the city of Newport, Rhode Island. The state park offers wide vistas of the Atlantic Ocean where it meets Narragansett Bay.
The granite found in southeastern Rhode Island is typically pink to gray in color with coarse but uniform grains. Composition often includes plagioclase feldspar, quartz, biotite, sphene, zircon, chlorite, and muscovite. Some area rock has been formally mapped as Bulgarmarsh Granite or Metacom Granite Gneiss.
Granite is a very hard, coarse-grained igneous rock that features a wide variety of visible mineral crystals. Igneous rocks form from the cooling and solidification of molten rock. Granite is classified as intrusive, which means it forms from magma that cools and solidifies below the Earth’s surface. This is in contrast to extrusive igneous rocks, which form from lava that cools on the surface.
Granite is formed through a slow and complex geological process deep within the Earth's crust, typically at depths of about 9 to 30 miles. It starts as molten magma, a mixture of melted rock, minerals, and gases, often generated as a result of the movements of the earth’s tectonic plates. When continental plates collide, they create intense pressure and heat that cause the melting of pre-existing rocks. As the magma rises towards the surface, it cools and solidifies, undergoing a process called crystallization. This process occurs over millions of years, allowing the magma's components to rearrange themselves into interlocking mineral crystals. The slower it cools, the larger the crystals.
Granite Mineral Composition:
Quartz: Quartz is a hard, translucent mineral composed of silicon dioxide. It has a glassy appearance that adds a subtle sparkle to granite surfaces. It is significant component of granite, typically making up about 20-60% of the rock.
Feldspar: Feldspar minerals, including both plagioclase and alkali feldspar, are the most abundant minerals in granite, comprising about 65-90% of the rock. Feldspar gives granite its range of colors, from pinks and reds to whites and grays. These minerals are less resistant to weathering than quartz but still contribute to the overall strength of granite.
- Potassium or Orthoclase Feldspar: 20-60%. Typically appears as pink or reddish crystals and is rich in potassium, aluminum, and silicon.
- Plagioclase Feldspar: 10-50%. Ranges from white to gray in color and contains sodium and calcium in addition to silicon and aluminum.
Mica: Mica minerals, such as biotite and muscovite, are also present in granite, though in smaller quantities, usually less than 10%. Mica’s thin, sheet-like structure gives granite its slight shimmer and can add to the rock’s overall visual appeal.
- Muscovite: Light-colored mica, often clear or silvery, giving granite a sparkly appearance.
- Biotite: Darker mica, often black or dark brown, responsible for the speckled look in some granites.
Accessory Minerals: Granite also contains various accessory minerals that make up only a small percentage of the rock, such as amphibole, pyroxene, zircon, apatite, magnetite, ilmenite, and sphene. These minerals are present in smaller quantities but can still influence the overall color, texture, and appearance of the granite.
To log this earthcache, please include a picture of yourself or a personal item near the granite stone, and send me the answers to the following questions:
1. Examine the colors and textures present within the granite. Based on your observations, what do you think some of the minerals are that make up this stone?
2. Examine the different grain sizes and consistencies within the granite. Is it fine, medium, or coarse grained and what does that tell you about the formation of this stone?
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