Moon Rock EarthCache

This earth cache takes one to a popular tourist attraction, Moon Rock, in the Augrabies National Park. The usual SanParks daily conservation fee applies. Entry is free with a valid WildCard.

An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.

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Q1) Around the posted coordinates you will see some very specific weathering. a) Name the type and how this is formed. b) What other different weathering patterns have you observed on Moon Rock as you walked to the top and describe what causes it.

Q2) Summarise in your own words what ‘granitized’ is.

Q3) Have a look at any of the predominant rocks around S28 35.518 E020 20.372 (or even the rocks along the walkway to the viewing decks) and compare it to the newish exposure you see at S28 35.859 E020 18.947 on Moon Rock and a) Describe any colour variation you might see. b) If any, give your own interpretation as to what would cause these rocks to be different?

Q4)  At S28 35.859 E020 18.947 (or anywhere on Moon Rock) look for a rock that you can view in cross section (from the side) and describe what you see in relation to: a) Compositional bands and b) Orientation c) How does one know this is gneiss and not just granite? d) Given the information what are these black bands composed of?

Q5) At the posted coordinates there is a signpost. What does this indicate?

General overview of topography, geology and soils of the Park

The topography consists of large rock domes scattered in a landscape with an otherwise very low relief. The flat areas in between contain the gently undulating pink gneiss. Drainage lines are sandy and dry. A range of steep rocky hills in the central portion of the Park is formed by dark-weathering quartz-rich granulite. This rock is invariably white on fresh surfaces, but with weathering becomes black. A fairly large flat sandy area occurs north and west of the black hills. Recent river terrace gravel and alluvium consisting of silt and fine sand occurs along the Orange River. The alluvium also forms large islands in the river.

Geological formations are reconstituted sedimentary rocks of the Kheis System and are part of the Archaean Complex. These sediments have undergone large-scale metamorphism, which changed them into crystalline gneiss and granulite (a class of high grade metamorphic rocks of the granulite-facies that have experienced high-temperature and moderate pressure metamorphism). Most of the Park is composed of red biotite granite gneiss, which is one of the three types of granite gneiss called pink gneiss and has a typical brown colour of weathered surfaces. The pink gneiss looks like a typical igneous rock and also resembles granite mineralogically and in chemical composition.

The Augrabies landscape is largely made up of granite and metagabbro shaped by interesting weathering patterns. Metagabbro is a metamorphosed igneous rock which contains no quartz and is made up entirely of dark ferromagnesian minerals and feldspar. This rock is found in the western part of the Park in an area known as the“Swartrante” (Black Ridges).

Granite is a light grey rock consisting of quartz, feldspar, and dark minerals. Due to intense pressure during the metamorphic event, dramatic zigzag folds are visible in the granite throughout the Park.

Different weathering patterns can be seen in the Park, such as hollows in the rock, exfoliation domes, and “popups”. Hollows occur when decomposing feldspar causes hard granite to become crumbly, and wind and rainwater wear away parts of the rock. Exfoliation domes are created by chemical weathering stress along sub horizontal joints, which causes thin slabs of rock to detach from the rock surface. “Popups” appear when thin slabs of rock detach from the rock surface due to extreme changes in the rock, pop up, and lean against another thin slab, forming an “A-tent” shape.

Moon Rock

One of the most popular tourist attractions of the park is Moon Rock, a gneiss dome. The colossal dome originated because of the uplifting of the surface of the earth millions of years ago. The smooth appearance of the dome is the result of large patches of rock scaling off, almost like the layers of an onion that can be peeled off one after the other. This erosion is caused by immense changes in temperature. The temperature of the interior of the rock stays fairly constant throughout, whereas the outer surface is subjected to considerable expansion and contraction. It eventually breaks loose and scales off to form this typically rounded boulder. It is a prominent landmark in the park, a large exfoliation dome measuring around 700 metres by 100 metres and 30 metres high.

Let’s have a closer look at Gneiss, Granitisation, Biotite and Pink Granite Gneiss

Gneiss (pronounced nice) may be simply metamorphosed (to change the form or nature of; transform) granite, or a far more complex rock with possibly four or five different origins, either igneous or sedimentary. It may also include metamorphic rocks which are invaded by igneous materials so that the rock becomes a complex mixture migmatite (a banded, granular metamorphic rock that contains light coloured bands with evidence for partial melting). Schists (a medium-grade metamorphic rock with medium to large, flat, sheet-like grains in a preferred orientation. It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar) are often invaded in this way, producing rocks which contain more feldspar and quartz than ordinary schists. The new minerals are often in small lens-like intrusions. Gneiss is hard to define or describe because it is so varied. In general, it is a coarse-textured rock with the minerals in parallel streaks or bands, but lacking schistosity. It is relatively rich in feldspar and usually contains mica or one of the other dark, rock-forming minerals.

Gneiss is classified by its most conspicuous mineral or according to its origin or structures. Characteristics are usually better seen in the field than in hand specimens.

Muscovite Gneiss is one of the most common kinds, a pale salt-and-pepper appearance, though biotite mica is common in gneiss also. While the name gives no clue as to the origin, muscovite and biotite gneisses may form from highly metamorphosed, shaly sediments.

Granite Gneiss is named to indicate that it is a metamorphosed granite, though this origin is difficult to establish because of the process of granitisation (see below). Granite gneiss is rich in feldspars. The mica or hornblend in it is arranged in parallel bands.

Hornblende Gneiss is a dark rock, much darker than biotite gneiss, in which parallel-oriented hornblende replaces mica. It is probably the end result in the metamorphosis of basic igneous rocks.

Injection Gneiss is gneiss which has been permeated by igneous materials during metamorphism. Like schists which are thus altered, it is also known as migmatite.

Granitisation

Granitisation is a process in which some form of igneous material or another invades sedimentary or metamorphic rock, producing mixtures which eventually alter the rock so that in texture and composition it becomes like granite. This process may be associated with great batholists or it may originate with materials coming from an unknown depth in the crust of the earth. The invasion may not even involve gases or liquids as we ordinarily know them. Gradations of rock from granite to gneiss over wide areas is evidence for a least one form of granitisation.

One example of the process would be the invasion of gneiss (A) by solutions container quartz and feldspar that separate the gneiss along parallel bands. In a later stage (B) some parts of the gneiss are transformed while others retain their original structure. As granitisation continues, the form and structure of the gneiss minerals change (C), though traces of the parallel arrangement still remain. Finally the original rock is completely absorbed, and the resulting rock shows no traces at all of the gneiss. In structure and composition it is granite.

Biotite

It is a dark-coloured mica, brown or black, sometimes green, containing magnesium and iron. It is abundant in some granites and is also common in schists and gneiss. Small barrel-shaped crystals are sometimes found. Biotite may occur with muscovite in metamorphic rocks. Thin cleavage sheets often show light spots, rings or halos. (The mica minerals are distinguished by their perfect basal cleavage, which means that they are easily split into thin, often transparent sheets. Two micas, biotite and muscovite, are so common that they are considered rock-forming minerals.)

Pink granitic gneiss - Light-pink to grey, medium-to coarse-grained, foliated but generally massive or poorly layered granitic gneiss, composed of quartz, microcline, oligoclase, and either biotite or muscovite or both, also locally amphibole or epidote.

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