Krantzkloof Nature Reserve
General Geology of Krantzkloof Nature Reserve
The Krantzkloof Nature Reserve’s breathtaking scenery is the result of the deep incisions of the Molweni and Nqutu and Nkonka Rivers – tributaries of the Umgeni – into the Kloof plateau. This plateau comprises erosion-resistant Natal Group sandstone, which forms the reserve’s orange-red, iron oxide stained cliffs which overlie a bed of basement granites.
River Sediments – The sediments which form the Natal Group sandstone were transported by river systems which flowed from an active mountain belt to the north, about 490 million years ago. At this time the earth lacked plant cover and the oceans were dominated by faunas such as trilobites (extinct arthropods), brachiopods (now mostly extinct clam-like creatures), echinoderms (ancestors of starfish and sea urchins) and corals.
Basement granites – Beneath the Natal Group sandstone are even older granite gneisses. These were formed in the roots of ancient mountains about a thousand million years ago at temperatures above 600 degrees centigrade, 10 to 20 kilometres beneath the earth’s surface. These mountain peaks, similar to those of the Alps or Himalayas of today, were gradually eroded until the basement granites were brought to the surface. Further erosion formed a flat surface onto which the Natal Group sandstone was deposited.
These granites are seen as huge dome-shaped boulders along the Molweni River bed and on hillsides above the river. They are coarse-grained crystalline rocks comprising the minerals, feldspar (which is milky pinkish-white), quartz (which looks glassy) and a few dark specs of a brown mica called biotite.
Dolerite intrusions – Between 300 and 180 million years ago a thick sequence of sediments (known as the Karoo Supergroup) was deposited on the Natal Group sandstone. This was followed by a massive upwelling of heat, known as a hot spot, from deep within the earth that melted rocks beneath the Gondwana supercontinent.
Molten rock (magma) forced its way through fissures in the earth’s crust and erupted as basalt lava flows on the surface. These fissures are known as dykes (if vertical) and sills (if horizontal). Magma which crystallised more slowly in the dykes and sills is coarser grained than basalt and is known as dolerite. This is a blackish crystalline rock consisting of feldspar and pyroxene.
The numerous narrow and deep crevices that extend into the sides of the Krantzkloof’s cliffs, often a few metres wide and up to 50 metres deep are dolerite dykes which have weathered and eroded from the more resistant sandstone.
Granite
Granite Basement Rocks upstream from cache site
The word “granite” is used in a variety of ways by different people. A simple definition is used in introductory courses; a more
precise definition is used by petrologists (geologists who specialize in the study of rocks); and, the definition of granite expands wildly when used by people who sell decorative stone such as countertops, tile and building veneer.
These multiple definitions of granite can lead to communication problems. However, if you know who is using the word and who they are communicating with, you can interpret the word in its proper context. Three common usages of the word "granite” are explained below.
Introductory Course Definition
Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of mica and amphibole minerals. This simple definition enables students to easily identify the rock based upon a visual inspection .
Petrologist's Definition
Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components and alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist.
Many rocks identified as “granite” using the introductory course definition will not be called “granite” by the petrologist - they might instead be alkali granites, granodiorites, pegmatites or aplites. A petrologist might call these “granitoid rocks” rather than granites.
There are other definitions of granite based upon mineral composition.
The chart below illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles can each have a range of abundances.
Classification of Granite
Commercial Definition
The word “granite” is used by people who sell and purchase cut stone for structural and decorative use. These "granites" are used to make countertops, floor tiles, curbing, building veneer, monuments and many other products. In the commercial stone industry a “granite” is a rock with visible grains that is harder than a marble. Under this definition gabbro, basalt, pegmatite, schist, gneiss, syenite, monzonite, anorthosite, grannodiorite, diabase, diorite and many other rocks will be called “granite”.
Granite Basement Rocks downstream of cache site
Xenoliths
The Xenolith
Within the Krantzkloof Basement Granites one can find many xenoliths. The published co-ordinates will bring you to a particularly large one close to Splash Rock on the banks of the Molweni River as it winds its way through Kloof Gorge.
A xenolith (Ancient Greek: “foreign rock”) is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing lava on Earth's surface.
A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes.
Although the term xenolith is most commonly associated with igneous inclusions, a broad definition could include rock fragments which have become encased in sedimentary rock. Xenoliths are sometimes found in recovered meteorites.
To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped; an included rock of similar type is called an autolith or a cognate inclusion.
Xenoliths and xenocrysts provide important information about the composition of the otherwise inaccessible mantle. Basalts, kimberlites, lamproites and lamprophyres, which have their source in the upper mantle, often contain fragments and crystals assumed to be a part of the originating mantle mineralogy. Xenoliths of dunite, peridotite and spinel lherzolite in basaltic lava flows are one example. Kimberlites contain, in addition to diamond xenocrysts, fragments of lherzolites of varying composition. The aluminium-bearing minerals of these fragments provide clues to the depth of origin. Calcic plagioclase is stable to 25 km depth.
Between 25 km and about 60 km, spinel is the stable aluminium phase. At depths greater than about 60 km, dense garnet becomes the aluminium-bearing mineral. Some kimberlites contain xenoliths of eclogite, which is considered to be the high-pressure metamorphic product of oceanic basaltic crust, as it descends into the mantle along subduction zones.
The Earthcache
At the published co-ordinates you can find a wonderful example of a xenolith. Examine the xenolith (see pic above) and compare the crystal structure of the xenolith to the surrounding and enclosing basement granite.
Note: Krantzkloof Nature Reserve is run by Ezemvelo KZN Wildlife and a nominal admission fee (R20 in November 2014) is payable to enter the reserve. This can be paid at the main picnic site in Kloof Falls Road.
Note 2: There is an altitude difference of approximately 300m between the parking co-ordinates and the published co-ordinates. Ensure that you are equipped for this hike with sufficient water, snacks etc.
Logging Requirements
In order to claim a find on this earthcache, please complete the logging tasks berlow and email your answers to me via my gc.com profile or direct to technonut[at]stars.co.za .
Task 1
In your own words compare and contrast the xenolith with the surrounding basement granite. Include items such as colour, texture (feel), grain size, porosity (drop some water on each of the rocks) and general appearance in your answer.
Task 2
Measure the approximate circumference of the xenolith and express your answer to the nearest meter.
Task 3 (site specific question)
There is a giant boulder just upstream of the published co-ordinates (see waypoint provided - S29 45.723 E30 51.301). On the downstream side of this boulder, something has been painted. Please describe what it is and note the colour of the paint.
Task 4 (optional)
Please upload a picture of you and your GPS with your online log.
You may log your find before submitting answers, but any logs which are not followed by an email within 5 days may be deleted.
Krantzkloof Nature Reserve from Google Earth
References
Granite
Wikipedia - http://en.wikipedia.org/wiki/Granite
Geology.com - http://geology.com/rocks/granite.shtml
Xenoliths
Wikipedia - http://en.wikipedia.org/wiki/Xenolith
National Geographic - http://education.nationalgeographic.com/education/encyclopedia/xenolith/?ar_a=1
Britannica - http://www.britannica.com/EBchecked/topic/650624/xenolith
Thanks also to Carbon Hunter for some valuable advice in the creation of this earthcache.
