The Location
Sterkspruit Quarry
The Rock Cycle
One of the bases of geology is the Rock Cycle. All rocks follow this cycle, and it can take many millions of years to complete the cycle.
Like most Earth materials, rocks are created and destroyed in cycles. The rock cycle is a model that describes the formation, breakdown, and reformation of a rock as a result of sedimentary, igneous, and metamorphic processes.
The Rock Cycle
All rocks are made up of minerals. A mineral is defined as a naturally occurring, crystalline solid of definite chemical composition and a characteristic crystal structure.
A rock is any naturally formed, non-living, firm, and coherent aggregate mass of solid matter that constitutes part of our planet.
Igneous rocks
Igneous rocks form in two very different environments. All igneous rocks start out as melted rock, (magma) and then crystallize, or freeze.
Volcanic processes form extrusive igneous rocks. Extrusive rocks cool quickly on or very near the surface of the earth. Fast cooling makes crystals too small to see without some kind of magnifier. Basalt is dark rock, gray or black on a freshly broken surface, and weathers brown or red, because it contains lots of dark-colored minerals.
Intrusive igneous rocks cool in plutons (Pluto was the Roman god of the Underworld.) deep below the surface of the Earth. Slow cooling allows the growth of large crystals. Crystals in intrusive rocks are visible without magnification. Granite has the same minerals as rhyolite, but in much larger crystals.
Sedimentary Rock
Sedimentary Rocks are those rocks made up of pieces of other rocks. We call the pieces of rock "clasts" (Clast means "broken piece"). A clast is a piece of rock broken off from another rock. Clasts of rock are eroded from larger rocks, transported (moved) by wind or water and deposited in a basin.After some period of time, the clasts are lithified (lithos is the Greek word for stone). The sedimentary rocks we see today were once gravel, sand, silt, mud, or living things. We decide what to name sedimentary rocks based on the size of the clasts that make up the rock.
For most sedimentary rocks, this is easy. Sandstone is made of sand, siltstone is made of silt, mudstone is made of mud and so on. Even volcanic ash can become sedimentary rock!
The only difficult sedimentary rocks to remember are conglomerate and breccia. Conglomerates are made up of rounded, gravel-size particles (To a geologist, gravel is anything from 2mm to 4 meters in diameter), and breccia is made up of angular, sharp-edged, gravel-sized clasts.
Limestone and chert are classified as sedimentary rocks, but most limestone and chert are grown by living organisms rather than broken from other rocks. Some limestones have fossils, but most limestones and cherts have recrystallized, and the remains of the creatures that made them are no longer visible.
Metamorphic Rock (The focus of thie earthcache)
Metamorphic Rocks form when sedimentary, igneous, or other metamorphic rocks are subjected to heat and pressure from burial or contact with intrusive or extrusive igneous rocks. ("Meta" means change, and "morph" means form.)
Heat and pressure from burial cause molecules of flat minerals like mica to line up perpendicular to the direction of greatest compression. Deep burial means higher pressure and hotter temperatures, and very high temperature and pressures cause the formation of new minerals, and mineral grains.
Low-grade metamorphic rocks like slate and phyllite break in flat pieces, and have a sheen on the surface.
Schist is shiny, and many schists contain garnets, staurolites or other mineral crystals that have grown within the rock.
Gneiss is a foliated metamorphic rock. Layers of dark and light minerals stripe the rock, and sometimes it is possible to see how the direction of pressure deep in the Earth changed as the minerals formed. The change in direction forms eye-shaped pods of minerals, called augens ("augen" is German for "eye.")
Marble forms when limestone is intruded by a pluton which heats the limestone.
Quartzite is an important metamorphic rock. Quartzite is metamorphosed sandstone. Some quartzite is so pure that it can be used to make computer chips. The quarry you are looking at here is a good example of the pink variety of quartzite which was once sandstone and which was subjected to high pressure and elevated temperatures many millions of years ago.
Quartzite
More about QuartzIte
Quartzite has a grainy, sandpaper-like surface which becomes glassy in appearance
Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colors, such as yellow, green, blue and orange, are due to other mineral impurities.
Pink Quartzite
When sandstone is cemented to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism. The grainy, sandpaper-like surface becomes glassy in appearance. Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite.
Orthoquartzite is a very pure quartz sandstone composed of usually well rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite. Although few fossils are normally present, the original texture and sedimentary structures are preserved.
Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little for soil, therefore, the quartzite ridges are often bare or covered only with a very thin layer of soil and (if any) little vegetation.
The Earthcache - Logging Requirements
In order to claim a find on this earthcache, please complete the tasks below and send the answers to me via my gc.com profile.
Task #1
Have a good look at the quarry from the posted co-ordinates and estimate the height of the quarry face. Use the picture on the cache listing, and also take note of any machinery and trucks that are working in the quarry to make your estimate.
Task #2
At the published co-ordinates there are always samples of the quarry stone which have fallen from lorries transporting the stone up the hill. Find a ~ 25mm stone, clean and / or wash the stone. Then describe the stone with regards to texture, sheen, colour & hardness.
Task #3 (Optional)
Compare and contrast the stone you have found with sandstone that you have seen in other parts of the Durban area, using the same parameters as in Task #2 above. (e.g. in Krantzkloof Nature Reserve)
Task #4 (Site visit verification)
On the opposite side of the road is a sign relating to the asphalt plant. Please supply the last four digits of the fax number.
Task #5 (Optional)
Please post pics of the quarry, your stone, and yourself with GPS at the published coordinates.
You may log your find prior to emailing your answers, but found logs where an email has not been received within 72 hours may be deleted.
