Igneous Rock: Before discussing the geology seen at the locations highlighted in this Earth Cache, an overview lesson on the formation of igneous rocks is required. Igneous rocks are defined as those formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. A classification of igneous rocks is provided in the figure below. Rocks in the top row are intrusive in nature. Those in the bottom two rows are extrusive.
Many minerals make up magma and lava. Depending on which ones are present, igneous rocks can be classified in various fashions. The very top row of the igneous rock chart shows the range of classifications from Felsic to Ultramafic. Ultramafic rocks are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content) and are low in silicate minerals. Felsic rocks (at the other extreme) are those rocks rich in silicate minerals, and rocks which are enriched in the lighter elements such as silicon (Si), oxygen (O), aluminum (Al), sodium (Na), and potassium (K).
On Iceland more than 90% consist of fine Mafic Basalt which is grayish to black in appearance but on this spot you will find other type of metamorphic rocks.
This Earth Cache will focus on two types of these metamorphic rocks.
Granite: Granite is a magmatic deep rock, that is, it forms when magma deep down into the soil undergoes a slow cooling process. Granite can have a coarse grain structure or fine grain structure depending on the cooling process, the slower the cooling the larger crystals due to the ability of the crystals to merge into larger crystals.
Granite consists of silica-rich minerals such as feldspar and quartz as well as smaller amounts of, for example, mica, hornblende and pyroxene. Granite can have different colors ranging from white to red and black due to differences in mineral composition and texture. Some minerals are millimeters, others can be several centimeters.
Granite is very hard and crackles and has unusually high resistance to pressure, shock and deformation; it is also insensitive to acid rain. Since it is commonplace in many parts of the world, it has become rich in street coverings, such as building blocks, masonry and ornaments and monuments.
The granite is almost always massive because it usually lacks internal structure. Banding does not occur. From a breakdown and machining perspective, granite is commonly called a hardest, unlike marble, limestone and other soft rocks called solstice. Its hardness and hence sustainability have given the granite a reputation as a stone with good qualities.
Gneiss, Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture. Other bands contain platy or elongate minerals with evidence of preferred orientation. It is this banded appearance and texture - rather than composition - that define a gneiss.
Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the size of the mineral grains and segregated them into bands, a transformation which made the rock and its minerals more stable in their metamorphic environment.
Gneiss can form in several different ways. The most common path begins with shale, which is a sedimentary rock. Regional metamorphism can transform shale into slate, then phyllite, then schist and finally into gneiss. During this transformation, clay particles in shale transform into micas and increase in size. Finally, the platy micas begin to recrystallize into granular minerals. The appearance of granular minerals is what marks the transition into gneiss.
Intense heat and pressure can also metamorphose granite into a banded rock known as "granite gneiss." This transformation is usually more of a structural change than a mineralogical transformation. Granite gneiss can also form through the metamorphism of sedimentary rocks. The end product of their metamorphism is a banded rock with a mineralogical composition like granite.
Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartz grains in an interlocking texture. These bands are usually light in color and alternate with bands of darker-colored minerals with platy or elongate habits. The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism.
Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic environment. These minerals might include biotite, cordierite, sillimanite, kyanite, staurolite, andalusite, and garnet. Gneiss is sometimes named for these minerals, examples of which include "garnet gneiss", and "biotite gneiss", not to be confused with augen gneiss.
Augen gneiss is in the same family but is named after, augen (from German "eyes") are large, lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks. In cross section they have the shape of an eye.
Feldspar, quartz and garnet are the common minerals which form augen.
Augen form in rocks which have undergone metamorphism and shearing. The core of the augen is a porphyroblast or porphyroclast of a hard, resilient mineral such as garnet. The augen grows by crystallization of a mantle of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows.
During shearing, the porphyroblast may rotate, to form a characteristic augen texture of asymmetric shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information.
To log this EC you will have to go to GZ and answer the following questions and send them to us via our profile, you do not have to wait for permission to log we will get back to you if we find something wrong.
- On the upper part on the site you will find granite, describe the granite in texture size, crystal sizes and colors.
- Is this granite slowly or fast cooled, justify your answer.
- If you look carefully on the granite you see that it looks like an animal, which?
- Look on the lower part on site, here you will see a type of gneiss, describe this gneiss in texture, crystal size and colors.
- What type of gneiss is this, justify your answer
- Go to WP1 and take a photo of you or your GPS and put it in your log.
At no circumstances write your answers in the log or post pictures of the granit/gneiss.