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.
Send your answers to us in an email via our profile page.
Any logs not accompanied by an email will be deleted.
Logging Tasks:
1. The listed coordinates will bring you to a dolerite split rock. Examine the ‘newly’ exposed rock and describe the colour, grain and texture and whether this rock is best described as aphanitic or phaneritic.
2. Did this rock cool quickly or slowly and give reason for your answer.
3. What type of weathering is taking place on these dolerite rocks? Earth cache Konkoonsies GC4ZH2F has illustrated the various weathering that takes place. (Freeze thaw, Chemical weathering, Onion skin weathering, Chemical weathering).
4. In your own words explain how these dolerite rocks were formed and the reason why we can see them in their present form.
5. At WP1 S32 08.017 E025 40.011 you will find a plague. What is on the plague?
The Karoo Succession
The Karoo Supergroup was formed in a vast inland basin starting 320 million years ago, at a time when the part of Gondwana which would eventually become Africa lay over the South Pole. Icebergs that had carved off the glaciers and ice sheets to the north deposited a kilometre thick layer of mud containing dropstones of varying origins and sizes into this basin. This became the Dwyka Group consisting primarily of tillite, the lowermost layer of the Karoo Supergroup. As Gondwana drifted northwards the basin turned into an inland sea with extensive swampy deltas along its northern shores. The peat in these swamps eventually turned into large deposits of coal which are mined in KwaZulu-Natal and on the Highveld. This 3 km thick layer is known as the Ecca Group, which is overlain by the 5.6 km thick Beaufort Group, laid down on a vast plain with Mississippi-like rivers depositing mud from an immense range of mountains to the South. Ancient reptiles and amphibians prospered in the wet forests, and their remains have made the Karoo famous amongst palaeontologists.
After the Beaufort period, Southern Africa (still part of Gondwana) became an arid sand desert with only ephemeral rivers and pans. These sands consolidated to form the Stormberg Group, the remnants of which are found only in the immediate vicinity of Lesotho. Several dinosaur nests, containing eggs, some with dinosaur fetal skeletons in them, have been found in these rocks, near what had once been a swampy pan.
The Karoo Igneous Event
Finally about 180 million years ago, volcanic activity took place on a titanic scale, which brought an end to a flourishing reptile evolution. Since this massive extrusion of lava, Southern Africa has undergone a prolonged period of erosion exposing the older softer rocks, except where they were protected by a cap of dolerite.
The lava outpourings that ended the Karoo deposition of rocks, not only covered the African surface, and other parts of Gondwana with a 1.6 km thick layer basaltic lava, but it also forced its way, under high pressure, between the horizontal layers of sedimentary rocks belonging to the Ecca and Beaufort groups, to solidify into dolerite, sills and dykes. From about 150 million years ago the South African surface has been subjected to an almost uninterrupted period of erosion, particularly during the past 20 million years, shaving off many kilometers of sediments. This exposed the dolerite, which were more resistant to erosion than the Karoo sediments, forming one of the most characteristic features of the Karoo landscape, namely the flat topped hills, called "Karoo Koppies".
Wikipedia explains dolerite (also known as diabase) as:
Dolerite is a mafic (1), holocrystalline (2), subvolcanic rock (3) equivalent to volcanic basalt or plutonic (4) gabbro (5). Diabase (dolerite) dykes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic (6) chilled margins which may contain tachylite (7).
Redirected from dolerite:
(1) Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron. Most mafic minerals are dark in color and the relative density is greater than 3 (water = 1,an ice cube at 0.91 will float). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, dolerite and gabbro.
(2) Crystallinity refers to the degree of structural order in a solid. Holocrystalline rocks are completely crystalline. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion.
(3) A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that originates at medium to shallow depths within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic and plutonic rocks. Subvolcanic rocks include diabase (also known as dolerite) and porphyry.
(4) A pluton is a body of intrusive igneous rock (called a plutonic rock) that is crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, stocks, dykes, sills, laccoliths (a sheet intrusion,or concordant pluton, that has been injected between two layers of sedimentary rock), lopoliths (a large igneous intrusion which is lenticular in shape with a depressed central region), and other igneous bodies. In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically several kilometers in dimension, without a tabular shape like those of dykes and sills. Batholiths commonly are aggregations of plutons. The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz diorite. Generally light colored, coarse-grained plutons of these compositions are referred to as granitoids.
(5) Gabbro refers to a large group of dark, often phaneritic (coarse-grained), mafic intrusive igneous rocks chemically equivalent to plutonic basalt. It forms when molten magma is trapped beneath the Earth’s surface and slowly cools into a holocrystalline mass.The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basaltic magmatism at mid-ocean ridges that produce ophiolites. It is also present in association with continental volcanism. Due to its variant nature, the term "gabbro" may loosely be applied to a wide range of rocks, many of which are merely "gabbroic", technically speaking. In comparison, their extrusive counterparts, basalt and andesite cover most of the gabbroic designations.
(6) Aphanite, or aphanitic as an adjective, is a name given to certain igneous rocks which are so fine-grained that their component crystals are not detectable by the unaided eye (as opposed to phaneritic igneous rocks, where the matrix grains are visible to the unaided eye). This geological texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. As a rule, the texture of these rocks is not quite the same as that of volcanic glass (e.g. obsidian), with volcanic glass being even finer grained (or more accurately, non-crystalline) than aphanitic rocks, and having a glass-like appearance. Aphanites are commonly porphyritic, having large crystals embedded in the fine groundmass or matrix. The large inclusions are called phenocrysts. They consist essentially of very fine-grained minerals, such as plagioclase feldspar, with hornblended or augite, and may contain also biotite, quartz, and orthoclase.
(7) Tachylite (also spelled tachylyte) is a vitreous form of basaltic volcanic glass. This "glass" is formed naturally by the rapid cooling of molten basalt. It is a basic type of igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is very brittle and occurs in dykes, veins and intrusive masses. The word originates from the Greek takhus meaning swift.Tachylites have the appearance of pitch and are often more or less vesicular and sometimes spherulitic. They are very brittle and break down readily under a hammer. Small crystals of feldspar or olivine are sometimes visible in them with the unaided eye. All tachylites weather rather easily and by oxidation of their iron become dark brown or red. Three modes of occurrence characterize this rock. In all cases they are found under conditions which imply rapid cooling, but they are much less common than acid volcanic glasses (or obsidians), the reason being apparently that the basic rocks have a stronger tendency to crystallize, partly because they are more liquid and the molecules have more freedom to arrange themselves in crystalline order.
The dolerite gives itself away in a number of ways, notably by its:
Massiveness, meaning it shows no internal structure like bedding;
Colour – dark grey to almost black, on a fresh surface, sometime with the crystalline texture visible;
Rounded shape (may also weather spheroidally) – boulders on surface;
Very conspicuously red soil that forms over it;
Positive topographic expression, forming ridges and cappings
If you’ve ever tried to break off a piece from a fresh outcrop of dolerite, you will know it’s a hard rock, and you will better understand why it builds high ranges and plateaus.
Dolerite has been of importance to human evolution and South Africa’s culture.
Artefacts and Tools
Firstly, all across the Karoo you can find Stone Age artefacts and tools. They were made out of hornfels, a hard, fine-grained surface shale that was heated (metamorphosed) by dolerite. Hornfels (also known as lidianite) is very hard and like glass or obsidian, it breaks with a ‘conchoidal’ fracture. With it, Stone Age people created tools for cutting, scraping and stabbing. There are some places along dolerite dykes intruding shales, where the ground is littered with hundreds of worked hornfels fragments. One can imagine such sites to literally have been stone tool ‘factories’. Without dolerite to make hornfels, one can wonder whether this Stone Age culture could have thrived as it did across the Karoo Basin.
Dolerite Rock Gongs
Dolerite has qualities linked to early human creativity. Many large dolerite boulders near the top of flat-topped hills give off a bell-like sound when struck. Karoo rock gongs are almost without exception located among clusters of engravings and are recognisable as sets of hammered or pecked patches usually on dolerite boulders, nearly always on the lips of rocky ridges.
Etched in Ironstone
Dolerite boulders were also used as ‘canvases’. As ironstone (dolerite) ages, the exposed area darkens to reddish brown and then pitch black, as if the rock were becoming sunburnt. This is called a rock varnish, patina or oxidation. Some of the planet’s earliest human artistic expression is etched on dolerite, not just here but at sites all over the Karoo.
References
Nick Norman 2013 book Geology Off the Beaten Track
Julian ‘Goonie’ Marsh, Professor Emeritus of Geology at Rhodes University in the Eastern Cape
http://en.wikipedia.org/wiki/Diabase