EarthCaches have no box to find. Instead, they teach you about geology. For more information, see earthcache.org.
Vance Sykes, a member of the class of 1907, came up with the idea to erect a monument to honor NC State University alumni who were killed in World War I. By 1920, Sykes and other NC State alumni formed a planning committee and hired William Henry Deacy, an architect, to design a memorial tower at the entrance to campus on Hillsborough Street.
As constructed, NC State's Memorial Belltower stands 115 feet tall, and it measures 324 square feet at its and tapers to 196 square feet at its top. The tower is made from 1,400 tons of granite quarried from "The Rock" — the largest open-faced granite quarry in the world in Mount Airy, North Carolina — and is set on a 700-ton concrete base. According to a 1920 article in the NC State Alumni News, the Belltower features a "semi-Romanesque style with Gothic treatment of the vertical lines. ... It is military in character and recalls very strongly the beautiful towers at West Point."
You can find the white granite used in this tower all over the country. It was used to construct the World War II Memorial in Washington, D.C., and the Wright Brothers National Memorial at Kitty Hawk, North Carolina.
The history of the granite in this tower goes back about 336 million years. It was then that The Rock formed through a geological process called plate tectonics.
"North Carolina sits on the North American plate." Several times in geological history the North American plate collided with the African Plate. "The last time it happened — approximately 360 million years ago — portions of the African plate sunk beneath the North American plate creating a subduction zone" (a place where two plates come together, one riding over the other) "that set the stage for the formation of Mount Airy’s huge granite quarry."
It was in this subduction "zone, some 12 to 25 miles underneath the American plate, that blobs of molten rock were formed. Due to its relatively lighter weight, the molten rock, also called magma, started to rise toward the surface, intruding the upper portions of the plate."
"The magma slowly solidified over millions of years and formed what’s known as a pluton — basically, a body of igneous rock that develops beneath the earth’s surface. As the plates continued to move and extensive erosion occurred, these plutons — one of which geologists actually refer to as the Mount Airy pluton — appeared at the surface." The Mount Airy pluton is composed of granite, an igneous rock.
The minerals that make up granite are composed of silicon and oxygen. Granite is made out of big crystals. You can easily see them with your naked eye. This is because the granite cooled slowly underground, shielded by the rock surrounding it so that its great heat could escape slowly.
Granite is made out of feldspar and quartz, plus minor amounts of other minerals. The crystals in granite can be a variety of colors: feldspar is typically pink or red, mica is typically dark brown or black, and quartz is typcially clear pink, white, or black.
Igneous Rock Textures
Igneous rock textures are used by geologists to determine the mode of origin of igneous rocks and are used in rock classification. The following are the six main types of textures:
- Aphanitic rocks typically form from lava that crystallizes rapidly on or near the Earth’s surface. Because extrusive rocks make contact with the atmosphere, they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye.
- Glassy textures occur during some volcanic eruptions, when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals.
- Pegmatitic texture occurs during magma cooling when some minerals might grow so large that they become massive (the size ranges from a few centimetres to several meters).
- Phaneritic textures are typical of intrusive igneous rocks. These rocks crystallized slowly below the Earth’s surface. As magma cools slowly, the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye.
- Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. Minerals formed during earlier stages form more slowly, so they remain as large crystals. Sudden cooling causes the remainder of the melt to rapidly crystallize into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization, thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
- Pyroclastic textures occur when explosive eruptions blast the lava into the air, resulting in fragmental, typically glassy material which fall as volcanic ash, lapilli, and volcanic bombs.
Logging Tasks: (Required)
Send the answers to me through in an email or a geocaching message.
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Based on the colors of the granite on the Bell Tower, what rock(s) do you think the granite is composed of?
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Looking at a square foot of the Bell Tower, what percent of the granite consists of dark minerals?
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Which of the 6 igneous rock textures is the granite on the Bell Tower?
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Please post a a picture in your log of you, your GPS, your group, your arm, your geo-pet — just something to show you were here.
References