Protecting Port Aransas
This is an Earthcache, which does not have a physical container, but provides a learning exercise and a chance to show that knowledge in this setting. Please send me the answers to the questions below to validate your log.
Aransas Pass is the water passage between Mustang and St. Joseph islands, located at 27°50' north latitude and 97°03' west longitude. A deepwater entrance has been dredged into channels leading to Corpus Christi, Aransas, and Red Fish bays. This natural pass over a sandy bar was known to exist as early as 1528 when it was clearly indicated on the Bratton map. It was called Aránzazu by Governor Prudencio de Orobio y Basterra on his map of 1739; the name was altered to Aransas on the map of a Captain Monroe of the ship Amos Wright (1833). Powers and Hewetson colonists came into Copano Bay across the Aransas bar in 1830–34, when the water depth was variously reported to be seven to eighteen feet.
By 1885 jetties, a breakwater, and a mattress revetment along the channel face of Mustang Island had greatly retarded the erosion. A south jetty, known as the Mansfield Jetty, was also constructed of brush mattresses and stone. By 1889 an eighteen-inch-thick riprap cover, which effectively curtailed the erosion, had been installed.
RIPRAP
Riprap is rock or other material used to armor shorelines against erosion. It is also called rip rap, shot rock, rock armor, and rubble. Granite and limestone are common rock used or sometimes concrete rubble from building or pavement demolition is spread.
TEXAS GEOLOGY
Geoscientists have documented the state's changing landscape was formed by a process that started billions of years ago in the Precambrian Era and continues today. Uplifting of mountains, inundation of vast inland seas, river transport of large volumes of eroded sediment, volcanic eruptions, and earthquake activity have all happened through the geologic development of Texas. The state is characterized by Sedimentary Limestone Deposits that are several thousand feet thick and more than 570 million years old. The limestone was formed during geologic periods when much of Texas was covered with shallow, warm inland seas. Since Texas is covered by so much limestone, it is unusual to find an area of pink granite.
TYPES OF ROCKS
The world has three types of rocks: Igneous, Sedimentary, and Metamorphic.
Igneous rocks are rocks that have solidified from magma or lava upon cooling. They can be intrusive (solidified from magma underground) or extrusive (solidified from lava @ or near the surface. Most of the earth's crust is formed from igneous rock. One rock type in this category is granite.
Sedimentary rocks are formed by the accumulation of small to large particles derived from all 3 types of rocks and in some cases, organic material and then undergoes compaction, cementation, or evaporation. These particles are deposited at the earth's surface and within bodies of water while being transported by the wind, water, ice, mass movement, and glaciers. There are three classifications of sedimentary rocks: organic, clastic, and non-clastic. Examples of organic sedimentary rocks are coal and limestone, while examples of clastic include shale and conglomerate. Non-clastic includes rock gypsum and rock salt.
Metamorphic rock is an igneous, sedimentary, or other metamorphic rock that has been squeezed by incredible pressure deep underground or exposed to very high temperatures altering its structure, mineral alignment, or chemical composition. Examples include slate, quartzite, and marble.
GRANITE
Granite is an igneous rock formed from magma that consists mainly of quartz, mica, and feldspar with colors ranging from black to gray. It forms from the slow crystallization of magma below the earth's surface. The grains in granite are large enough to be seen with the unaided eye (more later). It is widely used in construction because of its characteristic massive, tough, and hard properties. Some popular uses are in sculptures, memorials, buildings, and curling stones necessary for the sport of Curling. Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction. A well-known venue for granite climbing includes Yosemite Park in the Western United States.
The Granite here came from the city of Marble Falls, Texas where operations began in the 19th century. The granite is also known as Sunset Red Granite which has a distinctive color. It is an enormous granite pluton rock formation located in the Llano Uplift, which is a circular geologic dome of Precambrian Rock, primarily granite in Central Texas. The area is rimmed by limestone ridges. It is termed an uplift due to the raised status of the rocks in comparison to adjacent, completely buried rocks. The ancient faulting that produced the uplift may have happened around the time of the Ouachita Orogeny. It's like an island of granite in a sea of limestone. Most granite is primarily white, grey, and black. The color here is what makes it uncommon, the rarest granite color. Also, since Texas is covered by so much limestone, it is very unusual to find much of this color of granite. The rarer feldspar gives it a distinctive color.
Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse-grained igneous rocks are "intrusive," in that they cooled at depth in the crust where they were insulated by layers of rock and sediment. Fine-grained rocks are called "extrusive" and are generally produced through volcanic eruptions.
Grain size can vary greatly, from extremely coarse-grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Fine-grained rocks, where the individual grains are too small to see, are called aphanitic. Basalt is an example. The most common glassy rock is obsidian. Obviously, there are innumerable intermediate stages to confuse the issue.
Fine Grain
Coarse Grain
Logging Requirements:
To demonstrate the educational value of this cache, please answer the following questions. Submit the answers to these questions by messaging us through our profile page. Do not post your answers with your log. Photos are optional but appreciated and encouraged
1. Explain the depth of red color in the rock. Is it dark red, light pink, or in between? What is the reason for that?
2. What is the size of the grains primarily in the stone? Are they closer to the coarse large grains >5 mm, medium, between 1 and 5 mm, or fine <1mm?
3. Using your observations ...... please explain why Riprap is important? What is the function of the Jetty?
4. What shapes and approximate sizes have the granite been cut to? Considering how long these blocks have been in place, do you see evidence of any erosion?
5. Take your best estimate, how long do you think the Jetty is?
References:
Texas State Historical Association: Aransas Pass https://tshaonline.org/handbook/online/articles/rra03
ScienceDirect: Riprap https://www.sciencedirect.com/topics/earth-and-planetary-sciences/riprap
Texas Monthly: Taken for Granite https://www.texasmonthly.com/travel/taken-for-granite/
Geology.com: Granite https://geology.com/rocks/granite.shtml