Welcome to Veterans Memorial Park, and an Earthcache about Granite!!
An Earthcache is a special type of geocache where there is no container to find - instead you are looking for a geological wonder, and will have some questions to answer about your observations before logging that you have found the cache.
This Earthcache will teach you about granite, why it is commonly used for monuments such as these, and the different types and colors of granite that are found worldwide.
EARTHCACHE REQUIREMENTS
To claim a 'find' for this Earthcache you must answer the following questions and send your answers before logging. You should also include a photo at GZ with your log. There is no need to wait for a response before logging, I will reach out if there are any issues. I am not looking for PhD level answers. I ask that you please enjoy the area, and take the time to learn something new.
At GZ, you will be standing in front of a memorial statue at the Veteran's Memorial Park in Clinton, Ohio. This statue is made of granite - a type of igneous stone that is commonly used for monuments, statues, and buildings.
Questions to Answer
1. Observe the various colors of the statue. Using the description of the cache page, describe the approximate percentage of quartz, feldspar, mica, potassium feldspar, and other materials of this particular granite. What color classification would you describe as most closely matching the statue?
2. Take a look at the surrounding area and find one other example of granite (hint: there are LOTS - look at the benches and monuments). Based on the reading and your observations, do you think that the statue is intrusive rock or extrusive rock? Do you think the second sample you found is intrusive or extrusive? Why?
3. From the information on the cache page and your observations, do you think that this is phaneritic (coarse grain) or aphanitic (fine grained)? Describe the size of the crystals in the statue that made you decide on your answer.
4. Include a photograph of yourself, your GPS, a signature item, thumbs up, etc at the fountain. You do not need to show your face in the photo, but your photo must be unique to you.
Geology Lesson:
Granite colors range the spectrum from white to black to pink, based on its composition. Granite is one of the most commonly known types of rocks, used in everything from buildings to sculptures to headstones and fountains. It has been used for thousands of years and is regarded as a symbol of status, strength, and durability.
What is Granite?
Granite is an intrusive igneous rock with large grains (minerals) easily seen by the naked eye. Granite colors are most commonly pink, white, variations of grey and black. Green, Blue and Purple are also less commonly seen. To be considered granite, the rock must contain at least 20% quartz. This quartz is what gives granite its reflective nature.
An intrusive rock is molten rock that cooled within the crust of the Earth and was never expelled as lava. The gradual cooling of molten rock is imperative to create the large crystals of a singular mineral that we see in granites.
Extrusive rock, also known as volcanic rock, is formed when magma cools and solidifies on or near the Earth's surface. This cooling process often occurs rapidly, leading to a fine-grained or a glassy texture. Mineral grains in extrusive rocks are not easily seen with the naked eye, and this type of rock appears as one solid color. Examples of extrusive rocks include basalt and obsidian.
Granite is a type of Porphyritic rock that is made through a process called igneous differentiation, and is generally formed when a column of rising magma is cooled in two stages; Phaneritic Rock stage & Aphanitic Rock stage:
In the first stage, magma cools slowly, deep within the Earth's crust creating large crystal grains, with a diameter of 2mm or more. This phaneritic rock has crystals large enough to be easily seen with the naked eye, without the need for magnification.
In the second stage, the magma is cooled rapidly at a relatively shallow depth, and creates very small grains that are usually invisible to the naked eye requiring microscope magnification to view. Aphanitic rock at this stage is typically referred to as the rock matrix or groundmass.
What Determines Granite Colors?
Granite is a conglomerate (meaning it has many parts, cemented into one rock) of minerals and rocks, primarily quartz, feldspar, mica, amphiboles, and trace amounts of other minerals. Granite typically contains 20-60% quartz, 10-65% feldspar, and 5-15% micas (biotite or muscovite).
The minerals that make up granite give it the unique colors we see in different types of granite.
The relative proportion of different colored minerals in a granite is largely due to the original source of molten rock that cooled to form the granite. If the molten rock was abundant in potassium feldspar, the granite is more likely to take on a salmon pink color. On the other hand, if the molten rock is abundant in quartz and minerals that make up amphibole, you will likely get a black and white speckled granite.
Granite Minerals and Resulting Colors:
- Quartz - transluscent white color
- Feldspar - translucent or opaque off-white color
- Potassium Feldspar - transluscent or opaque salmon pink color
- Biotite - a type of mica, typically black or dark brown color
- Muscovite - a type of mica, typically metallic gold or yellow color
- Amphibole - typically black or dark green color
The combination of the minerals above make up most of the colors we typically see in a granite. Now, let's break down the different types of granite with an overview of what gives them their color classifications.
White granite
White granite is a granite that is composed primarily of quartz (milky white) and feldspar (opaque white) minerals. The small black specks in the granite above are likely small amphibole grains.
If you see a rock that is 100% white, it is not granite but more likely a man-made rock that is created to look like granite or a quartz (quartzite).
Pink Granite
Pink colored granite is a result of an abundance of potassium feldspar within the granite. You can see small specs of milky semi-transparent quartz, dark brown/black amphibole, and opaque white feldspar. However, in a granite like the one above the primary mineral is potassium feldspar.
Black And White Granite
This granite appears to have equal parts quartz, feldspar, and amphibole, making a speckled black and white granite. This is one of the most common types of granite.
Red Granite
Red granite is a variation of pink potassium feldspar abundant granite, where the k-feldspar takes on a redder than pinker color. Also, you can get red coloring from iron oxide in hematite grains or inclusion within feldspar, essentially the same process that makes rusted metal ruby red colored.
Black Granite
Pure "Black granite" is commonly seen in commercial rock, but it is not granite at all. Granite must be at least 20% quartz, which means an all black rock is not a granite unless it also contains at least 20% quartz and have other minerals present. Most commonly, black granite is in fact gabbro, an intrusive igneous rock similar to basalt. Gabbro is primarily composed of the minerals pyroxene, plagioclase, and small amounts of olivine (dark green) and amphibole.
Green Granite
When advertised as green granite, often the stone is actually a green variety of marble, which gains its green coloration from inclusions of serpentine. Granites are not abundant in green colored minerals, but there are a variety of different rock types that do contain green minerals in abundance. One very unusual way to get a green coloration is the inclusion of amazonite, a green variety of feldspar.
Blue Granite
You may see samples claiming to be "blue granite", but this is more likely to be Larvikite, an igneous variety of monzonite and sometimes referred to as "blue granite" despite it not being actual granite. Another common alternative is Anorthosite, a rock that contains abundant blue labradorite and is sometimes sold as blue granite.
Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.