Welcome to my Earthcache! An Earthcache is a special type of geocache where there is no container to find - instead you are looking for a unique geological feature of the area and need to answer questions, as well as posting a picture, in order to claim the find. The goal of this Earthcache is to educate visitors about granite, it's development, it's composition and it's uses. All observations can be made from the sidewalk at GZ near Ozona Town Square. Parking is available in the immediate vicinity of the park.
EARTHCACHE REQUIREMENTS
As with all of my ECs, I am not looking for PhD thesis level responses, but I am hoping that you take some time to enjoy the area and learn something new. Please include a list of all cachers with your answer, if answering for more than one caching name. There is no need to send individual answers.
To claim a 'find' for this Earthcache you must answer the following questions and send your answers in a message or email to the owner using the link at the top of the page. You can log your find with a photo at GZ. Send your answers to the tasks. I will be in contact if there is a problem, no need to wait for a response as long as the required photo is included in your log.
Observational Task
At GZ, you will be standing in front of a large granite statue of David Crockett. The David Crockett Monument commemorates the Alamo hero. It is 13ft tall, and was erected in the city park in 1939. The monument is located in Ozona's town square. Your task is to make some observations about the granite that is included in this monument, and answer the questions below.
Questions to Answer
1. Observe the various colours of the monument. There should be at least three different colours of visible within the granite of the monument. Based on the reading and your observations, which color of granite would you classify this as, and what percentage of mica, feldspar, quartz and other materials are present within this granite? (Hint: It might be easiest to present this information in a small chart)
2. Is this monument showing any signs of weathering? Based on your answer, do you think that granite was a good choice for this monument, or do you think another matial would have been better?
3. Based on the information in the cache page and your observations, do you think that this granite used in this monument was originally from Texas? Why or why not?
4. Mandatory: Include a photograph of yourself, your GPS, a signature item, thumbs up, etc at the monument. You do not need to show your face in the photo, but your photo must be unique to you. If you are caching with a group you can use the same photo, but each log must upload a photo.
Geology Lesson:
Granite colours 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 (apparently) monuments. 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 colours are most commonly pink, white, variations of grey and black. Green, Blue and Purple are also possible. To be considered granite, the rock must contain at least 20% quartz. This quartz is often what gives granite its reflective nature.
An intrusive rock means that molten rock cooled within the crust and was never expelled as molten rock. The gradual cooling of molten rock is imperative to create the large crystals of a singular mineral that we see in granites. In contract, extrusive rock is any rock derived from magma that was poured out or ejected at Earth's surface.
What Determines Granite Colours?
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 colours we see in different types of granite. This minerals contained within granite will vary, even between rocks from the game general location, based on the minerals and impurities that are present when the rock is formed.
The relative proportion of different coloured 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 colour. 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.
Local Granite - From the Texas Mountains
Granite in the Texas mountains is primarily pink and red, but can also be gray. The pink granite that is predominent in the Texas mountains, comes from ancient volcanic processes approximately 1.1 Billion years ago, and appears to be predominantly pink because it contains higher-than-average quantities of potassium feldspar. Examples of pink granite that can be found within Texas include:
- Granite Mountain in Marble Falls, Texas is a solid dome of pink granite that's also known as Sunset Red.
- Enchanted Rock in the Llano Uplift is a pink granite monadnock that's the largest in the United States.
Texas is famous for red and pink granites, widely employed for architectural purposes. This finished specimen on the Capitol grounds in Austin displays the polish granite takes. Granite Mountain quarry is the source of most Texas pink granite that has been used extensively for building, especially in Austin. The “mountain” is really only a moderate hill, but even a century of quarrying has barely reduced the visible bulk, and the granite extends thousands of feet under the surface!
Granite Colours:
What makes up the various parts of granite? What mades it so shiny? Granite is considered a conglomerate, which means that it is made up on many different parts that have been compressed into a solid rock. Some of the minerals that are frequently included in granite include:
- Quartz - transluscent white colour
- Feldspar - translucent or opaque off-white colour
- Potassium Feldspar - transluscent or opaque salmon pink colour
- Biotite - a type of mica, typically black or dark brown colour
- Muscovite - a type of mica, typically metallic gold or yellow colour
- Amphibole - typically black or dark green colour
- Topaz - unique to granite formed in the southern US, topaz may resemble a transparent piece of glass, somewhat heavier than quartz, which it resembles.
The combination of the minerals above make up most of the colours we typically see in a granite. Now, let's break down the different types of granite with an overview of what gives them their colour.
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 coloured 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.
Specific to Texas: Llanite is a type of Texas pink granite found nowhere else in the world. Llanite’s uniqueness results from crystals of sky-blue quartz mingled with the granite’s rusty-pink feldspar and other minerals.
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 colour. Also, you can get red colouring from iron oxide in hematite grains or inclusion within feldspar, essentially the same process that makes rusted metal ruby red coloured.
Is it granite or not?
Blue Granite
You may 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 granite. Another common alternative is Anorthosite, a rock that contains abundant blue labradorite and is sometimes sold as blue granite.
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 other minerals present. Most commonly, black granite is in fact gabbro, an intrusive igneous rock similar to basalt. Gabbro is primarily composed of minerals pyroxene, plagioclase, and small amounts of olivine (dark green) and amphibole.
Green Granite
When advertised as green granite, often times the stone is actually a green variety of marble, which gains its green coloration due to inclusions of serpentine. Granites are not abundant in green coloured 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.
How old is Granite?
Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age (4 Billion to 541 Million Years Ago). Based on boring samples from western texas, it is estimated that a majority of the granite in the Western part of the state is approximately 1.1 Billion years old. 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.
This EarthCache has been prepared by a proud:
