TO CLAIM A FIND FOR THIS EARTH CACHE, YOU NEED TO ANSWER THE FOLLOWING QUESTIONS:
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What is the main chemical component of limestone?
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What is the difference between biological and chemical sedimentary rock? Give an example of each.
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a) Describe the limestone used to build the Cape Town City Hall. Examples of this limestone is best observed along the front entrance of the building, as well as along the north-west entrance referenced in the waypoint below.
b) From your observations, would you consider the limestone used oolitic or pisolitic? Explain your answer.
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Facing the building entrance, there are two symmetrical sets of stairs leading up to a common landing, followed by another flight leading up to the entrance level landing. How many steps do you count on the right leading up to the common landing and how many more to reach the entrance level landing? (There is a locked gate across the lower set of stairs, but all relevant steps are still visible from this point, so no need to gain further access passed the gate.)
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You will notice that a different type of rock was used to build these steps, as well as the lower walls and plinths of the City Hall building.
a) Describe this rock in terms of its colour and texture.
b) From your observations, can you identify this type of rock?
c) Why do you think this rock and not limestone was used for the steps? (Hint: think Mohs scale.)
This Earth Cache will take you to the Cape Town City Hall located on the Grand Parade to the west of the Castle of Good Hope. This large Edwardian building was constructed in 1905 and can almost be considered a xenolith amongst the backdrop of Table Mountain Sandstone, the shales of the Malmesbury Formation and the many granitic intrusions in between making up the predominant natural geology of the Cape Peninsula.
But why call it a Xenolith?
To fully understand why the stone making up the City Hall Building is so foreign, we first need to have a look at the natural geology of the Cape Peninsula...
The late-Precambrian age Malmesbury Group is the oldest rock formation in the area, consisting of alternating layers of dark grey, fine-grained greywacke sandstone and slate, seen along the rocky Sea Point and Bloubergstrand shorelines. These sediments were originally deposited on an ancient continental slope by submarine slumping and turbidity currents. The sequence was subsequently metamorphosed by heat and pressure and folded tightly in a NW direction so that the rock layers are now almost vertical.
The Cape Peninsula Granite is a huge batholith that was intruded into the Malmesbury Group about 630 MYA as molten rock (magma) and crystallized deep in the earth, but has since then been exposed by prolonged erosion. The characteristic spheroidal shapes of granite boulders are a result of preferential weathering along intersecting fractures and are well displayed around Llandudno and Simon's Town. Close up, the granite is a coarse-grained rock consisting of large (2-5cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica containing inclusions (xenoliths) of dark Malmesbury hornfels. In some places, intense weathering has altered the granite to kaolin clay soils that cause slope stability problems in road cuttings. High quality kaolin is mined near Fish Hoek and Noordhoek.
The contact zone where the Malmesbury Group was intruded by molten granite can be seen at Sea Point. Here, slivers of dark coloured Malmesbury rocks, altered by intense heat are intermingled and folded with the pale coloured intrusive granite to form a complex mixed rock (migmatite). Large feldspar crystals occur in both the granite and dark hornfel layers
Though initially intruded at great depth, prolonged erosion eventually exposed the granite at surface and it now forms a basement upon which younger sedimentary rocks were deposited.
Table Mountain Group sandstones were deposited on this eroded surface of granite basement, in the stream channels and tidal flats of a coastal plain and delta environment that extended across the region about 450 MYA. The spectacular Chapman's Peak roadway has been constructed along the contact unconformity between granite and the overlying Table Mountain sequence. The sand, silt and mud deposits were lithified by pressure and then folded in the Cape Fold Belt, extending along the southern coast.
The basal Graafwater Formation (300-450m thick) consists of interlayered pale brown sandstone, laminated pink siltstone and dark maroon coloured shale. It is best seen in road cuttings on the slopes of Table Mountain and along Chapman's Peak drive.
The Peninsula Formation (800-1500m thick) consisting of hard, light grey, coarse pebbly quartz sandstone, dominates the steep mountain cliffs.
The Pakhuis Formation tillite (a lithified glacial outwash gravel) occurs on the highest points of Table Mountain, such as Maclear's Beacon. It contains clusters of angular boulders and pebbles and was deposited at a time when the Gondwana continent, of which Africa is a part, was situated close to the south pole.
Faults cut across and displace the rock layers. These more easily eroded zones are marked by ravines, for instance, cross-cutting faults separate multiple peaks of the Twelve Apostles.
The present landscape is due to prolonged erosion having carved out deep valleys, removing parts of the once continuous Table Mountain Group sandstone cover from the Cape Flats and leaving high residual mountain ridges.
As you can see, limestone is completely foreign in terms of the natural occurring geology making up the Cape Peninsula - therefore, theoretically a xenolith.
So, where did the City Hall building's limestone come from...?
The building with its distinctive honey-coloured hues was built from Bath Stone, an oolitic limestone imported from Bath in south-west England. This stone was formed during the Jurassic Period (195 to 135 MYA) at a time the region that is now Bath was covered by a shallow sea.
What is Limestone?
Limestone is a sedimentary rock largely composed of calcium carbonate (more than 50%) in the form of the minerals calcite and aragonite.
Most limestone forms in warm, shallow marine waters where organisms such as coral, foraminifers and molluscs occur in abundance. These organisms have shells and skeletal components rich in calcium carbonate. When they die, their calcareous remains accumulate as an organic layer along the seabed; eventually compacting to form limestone rock. In addition, this limestone often contains variable amounts of silica in the form of chert, flint, jasper or siliceous skeletal fragments from sponge spicules, diatoms and radiolarians. Furthermore, quantities of silt, clay and river-carried terrestrial detritus may also be entrained in the limestone. Limestones formed from this type of organic sediment are called biological sedimentary rocks and are often rich in fossils.
Limestone can also be formed when individual carbonate grains called ooids are compacted into what is called oolitic limestone. Ooids are small (<2 mm in diameter), spheroidal, concentricly layered sedimentary grains that form as a series of calcareous mineral layers around a central nucleus. The layers usual comprise calcium carbonate-containing minerals, aragonite or calcite, while the nucleus can be a shell fragment, a quartz grain or any other tiny piece of debris.
When buried under additional sediment, these layers of grains can be cemented together to form an oolite. Strictly, oolites consist of ooids of diameter <2 mm. Rocks composed of grains larger than 2 mm (referred to as pisoids) are called pisolites.
Limestone can also form by way of the direct, chemical precipitation of calcium carbonate from marine or fresh water as seen along hot springs, lake shores and waterfalls.
Stalactites, stalagmites and other cave formations (called speleothems) are examples of limestone formed when groundwater saturated in calcium carbonate evaporates, leaving behind calcite drip formations. These two types of limestone are called chemical sedimentary rocks.
Varieties of limestone
There are many different types of limestone based upon how the rock had formed, its appearance or its composition.
Uses:
Limestone is very widely used in architecture; it is readily available and relatively easy to cut into blocks or more elaborate carvings. It is also long-lasting and stands up well to exposure. However, modern day acid rain can cause damage to limestone structures as these acids react with the calcium compounds in the stone to eventually create gypsum flakes.
CaCO3(s) + H2SO4(aq) → CaSO4(s) + H2O(l) + CO2(g)
Other uses include:
- The manufacturing of quicklime (calcium oxide), slaked lime (calcium hydroxide), cement and mortar.
- Pulverized limestone is used as a soil conditioner to neutralize acidic soils.
- It is crushed for use as aggregate—the solid base for many roads as well as in asphalt concrete.
- Geological formations of limestone are among the best petroleum reservoirs;
- As a reagent in flue-gas desulfurization, it reacts with sulfur dioxide for air pollution control.
- Glass making.
- It is added to toothpaste, paper, plastics, paint, tiles, and other materials as both white pigment and a cheap filler.
- It can suppress methane explosions in underground coal mines.
- Purified, it is added to food and livestock feed as a source of calcium.
- It can be used for remineralising and increasing the alkalinity of purified water to prevent pipe corrosion and to restore essential nutrient levels.
- Used in blast furnaces, limestone binds with silica and other impurities to remove them from the iron.
- It is often found in medicines and cosmetics.
- It is used in sculptures because of its suitability for carving.
References:
- Wikipedia
- http://www.sandatlas.org
- http://geology.com/rocks/limestone.shtml
- http://www.geology.uct.ac.za/cape/town/geology
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