When the world was still young, some 3 500 million years ago, molten rock forced its way through the earth's crust and solidified to form the granite outcrops that can be found in the south west of the Kruger National Park.
Pretoriuskop Rest Camp in the Kruger National Park rests among some of the most spectacular granite outcrops in the whole of Southern Africa. At 3 500 million years old, this is known to be part of the oldest granite bed in the Kruger National Park.
Shabeni Hill is possibly the most impressive of these dome shaped granite outcrops and is found on the S10 Loop road just north of the camp.
Igneous rocks (from the Latin word “ignis” meaning fire) form when hot, molten rock material crystallizes and solidifies.
Igneous rocks cool from magma. The appearance of the rock is created by the composition of the magma and the rate at which it cools:
- If the magma cools deep underground, it will cool slowly.
- If the magma cools at or very near the surface, it cooling happens much more quickly.
The different cooling time results in two different rock types which can also be told apart by the size of their crystals which creates the texture of the rock.
Over time geological processes have brought some igneous rocks to the surface.
Intrusive Igneous Rocks
Intrusive igneous rocks cool underground. Deep in the crust, magma cools more slowly and this gives crystals a chance to grow. Intrusive igneous rocks thus have relatively large crystals that are visible to the naked eye. The formations of intrusive igneous rock can be harder and more lasting than other types of rocks. Granite is the most common intrusive igneous rock.
Other examples of intrusive igneous rocks are:
- diabase
- diorite
- gabbro
- pegmatite
- peridotite
Extrusive Igneous Rocks
Extrusive igneous rocks form above the surface. Lava cools quickly as it pours out onto the surface. Extrusive igneous rocks thus cool more rapidly than intrusive rocks. The rapid cooling time does not allow time for large crystals to form and this means that igneous extrusive rocks have smaller crystals than igneous intrusive rocks. Some extrusive igneous rocks even cool so rapidly the crystals don’t develop at all. These rocks look almost glass like - such as obsidian. Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light that it can actually float in water.
The most common extrusive igneous of volcanic rock is basalt. It is the rock that makes up the ocean floor.
Other examples of extrusive igneous rocks are:
- andesite
- basalt
- dacite
- obsidian
- pumice
- rhyolite
Granite is such an intrusive igneous rock and it is the most common plutonic rock of the earth’s crust. Granite takes its name from the Latin word granum, meaning a grain, in reference to the coarse-grained and generally equi-granular structure of the crystalline rock.
It is relatively light in colour and may appear as red, pink, gray, or white with visible darker mineral grains. Granite comprises some of the oldest known rocks on Earth, and is the most abundant basement rock underlying the relatively thin sedimentary rock cover of the continents. Granite and related marble industries are considered one of the oldest industries in the world, existing as far back as Ancient Egypt
Granite is produced in volcanic arcs, and more commonly in mountain building resulting from the collision of two continental masses. These earliest continental masses were products of the accumulation of volcanic arcs, and this is why granite lies in the cores of all of the continents.
Granite is the plutonic equivalent of rhyolite.
It is mostly composed of minerals namely quartz (a hard crystalline mineral composed of silicon and oxygen atoms) alkali feldspar and plagioclase feldspar along with mica and amphibole minerals.
Feldspars are a group of rock forming tectosilicate minerals and they make up about 41% of the earth’s continental crust. They appear as white mineral grains.
(Tectosilicate is any member of a group of compounds with structures that have silicate tetrahedrons, each of which consists of a central silicon atom surrounded by four oxygen atoms at the corners of the tetrahedron, arranged in a three-dimensional lattice.)
Granitic rocks are classified according to the QAPF diagram for coarse grained plutonic rocks and are named according to their percentage of
- quartz,
- alkali feldspar (orthoclase, sanidine or microcline)and
- plagioclase feldspar on the A-Q-P half of the diagram
(The QAPF diagram is used to classify igneous rocks based on their mineralogic composition. The acronymn stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid")
True granite (according to modern petrologic convention) contains between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of alkali feldspar, also called potassium feldspar.
Potassium/alkali feldspars are differentiated from the plagioclase/sodium feldspar by the lack of fine parallel striations, which are usually present on some cleavage surfaces in the plagioclase minerals.
Thus true granites are classified by the percentage of their total feldspar that is alkali feldspar. Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites and often look pink.
Granites whose feldspar is 65% to 90% alkali feldspar are syenogranites, while the feldspar im monzogranite is 35% to 65% alkali feldspar.
Granitic rocks poorer in quartz are classified as syenites or monzonites, while granitic rocks dominated by plagioclase are classified as granodiorites or tonalites.
PLEASE NOTE THAT YOU DO NOT HAVE TO DRIVE OFF THE ROAD IN ORDER TO DO THIS CACHE AND THERE IS NO CONTAINER AT GZ. AS PER THE NATIONAL PARK RULES PLEASE STAY IN YOUR VEHICLE!
In order to log this cache please answer the following questions in your own words:
- Explain in short the difference between intrusive and extrusive igneous rocks.
- What is the most common plutonic rock? And the most commonly found volcanic rock?
- At GZ you will have a nice view of Shabeni Hill. Do you think the rock contains more alkali or plagioclase feldspar? Give a reason for your answer.
- Send a picture of your GPS device confirming your location at GZ together with your caching name and the date. Be creative or try to get the hill in the background!
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