Granite and typical Intrusions – Amazing Geology on Guernsey

To answer the question at Waypoint 2 you hav to take a torch cause there are no lights in the Dolmen but as there is a gap in the stone ceiling by the entrance, it is not completely dark .

Granite

The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a completely crystalline rock. Granite is a intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. The grains large enough to be visible with the unaided eye.It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock.These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals.

Granite is nearly always massive (lacking any internal structures), hard, and tough and is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that they encounter in daily life. These include countertops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments. Granite is used all around us - especially if you live in a large modern city.

Introductory geology textbooks report that granite is the most abundant rock in the continental crust. At the surface, granite is exposed in the cores of many mountain ranges, within large areas known as "batholiths," and in the core areas of continents known as "shields."

The large mineral crystals in granite are evidence that it cooled slowly from molten rock material. That slow cooling had to have occurred beneath Earth's surface and required a long period of time to occur. If these granites are exposed at the surface today, the only way that could have happened is if the granite rocks were uplifted and the overlying rocks were eroded.

Here is a summary of what you might observe on a surface of granite:
 

Feldspar

Feldspar minerals are abundant in granite. They are usually white, gray, pink or reddish in color. Many grains will exhibit two directions of cleavage that intersect at right angles. You should be able to observe this cleavage pattern in granite with a hand lens.

Quartz

Quartz will usually be a transparent mineral that is colorless or gray in color. Many grains will exhibit a conchoidal fracture - with a vitreous luster on the conchoidal fracture surfaces

Mica Minerals

The mica minerals expected in granites include muscovite or biotite. Micas occur in very thin sheets. They will often be in "books" of numerous sheets stacked upon one another. The surfaces of these sheets will have a highly reflective vitreous luster. The edges of a "stack of sheets" will look similar to the edge of a stack of playing cards.

Amphibole Minerals

Amphibole minerals such as hornblende are dark in color and will often have a prismatic habit.
 

Example of a granite stone

Dark grey-brown to medium gray grains: quartz; light brown, yellowish and whitish: feldspars; black: biotite.

Intrusions in ingenious rocks

Xenoliths

The word comes from the ancient Greek word xenos (foreign) and lithos (stone). Xenoliths are fragments of foreign rock within e.g. a plutinite, therefore they differ from the rock in which they are found. When similar in nature, these stones are called autoliths or related inclusions. Because they had to already exist as a solid rock fragment when the magma or lava solidified around them, true xenoliths are always older than their surrounding host rocks.

Xenoliths often consist of different types of rock that became embedded in magma as it cooled. They are usually easy to recognize because they often differ significantly in composition, density, and color from the surrounding rock. Xenoliths can range in size from single crystals to rock fragments of several meters.

Xenoliths are of particular importance for science. On the basis of the entrained foreign rocks in the magmatic rocks, conclusions can be drawn about the composition of the Earth's mantle and crust as well as their rock and mineral content. In the further analysis of the minerals and rocks age, chemism and formation temperature can be determined on the basis of isotope determinations, elemental composition and melting point of minerals.

Quartz veins

Quartz is often found in veins that cut through rocks. Although the term "vein" suggests this, the veins of quartz and other minerals are usually not thin tubes, but rather thin sheets. The veins can form under various conditions, and depending on these conditions, may or may not contain quartz crystals.

Even though certain types of quartz veins do never bear any crystals, it sometimes makes sense to follow large quartz veins to look for crystal-bearing fissures: Should a rock that contains old large quartz veins have been folded later due to tectonic forces, the quartz veins represented a disturbance (a discontinuity in the otherwise homogeneous mechanical properties), and alpine-type fissures are likely to open up between the quartz and the host rock.

The dimensions of the veins vary in width and length, from centimeters to decimeters thick and from decimeters up to several meters long. Quartz veins are mostly fracture related and have clear contacts with their host rocks.

The simplest type of a quartz vein is the filling of an already present crack in rocks. The crack might form during folding of the rock in mountain-building processes, by shattering during tectonic events, by a decrease in pressure during the uplift of a rock, or because a rock cools down and shrinks. Hot brines that percolate the rocks and originate at greater depths with higher temperatures will precipitate the minerals they carry with them in cracks at lower temperatures and pressures. This process may continue until the crack is completely filled or may stop before, leaving "pockets" in the vein that are sometimes outlined by crystals. Hot brines that enter a crack in the rock from some distant hot source like a granite pluton first cool and precipitate most of their load rather quickly. The result is milky quartz, either massive or made of interlocked milky quartz crystals. Later, when the crystal growth slows down, the crystals may get less milky or even clear. In the majority quartz veins, most of the quartz is precipitated as massive, milky quartz. Well-formed crystals, if found at all, are only a small portion of the vein filling.

Images of quartz veins
 

So lets take a closer look of the two geologic formation at the coordinates.

To log this Eartcache, go to the given coordinates and answer the following questions. This via email or the message center to me:
 

1. At Waypoint 1 you found a Millenium Stone(*)
   a) Describe the granite. What is the dominant Colour. How does it feel (soft or rough)?
   b) How do you estimate the percentage distribution of quartz, feldspar and mica?
   c) You can see a quartz vein. What is the average width?
   d) What do you think, is the vein more a tube or more a sheet? How would you justify your observation?
    

2. At Waypoint 2 you see the neolithic grave „La Varde“(**). To answer the question please step inside.
   a) In the middle of the grave is a granite pillar stone? Describe the granite. What is the dominant colour. How does it feel (soft or rough)? Compare this Pilar stone brought in 1898 with one of the other granite stone surrounding you?
   b) How do you estimate the percentage distribution of quartz, feldspar and mica?
   c) Do you think that all stone, even the Millennium stone have the same origin? How would you justify your observation?
   d) You will find some Xenoliths as described above. How many hand-sized Xenoliths can you see, standing at the pillar looking north-west Find the Xenolith from the picture above.
   What is the height and width?
   
   Additionally

3. Take a photo of yourself and/or a personal item (e.g. your GPS) and attach it to your log!

* The Millennium Stone was dragged with considerable difficulty from the beach at L'Ancresse where it had lain for who knows how long and erected as part of the 2000 celebrations
** La varde is Guernsey’s oldest passage grave which is bottle shaped in plan, with a narrow entrance leading to a wider chamber. It was built during the Neolithic period between c 4000-2500 BC and was in use until the late Bronze Age C1000 BC.

Many thanks to the Heritage Services, States of Guernsey for the permission to integrate La Varde as an ancient scheduled monument into this eartcache

Source
https://geology.com/rocks/granite.shtmlwikipedia.de
www.sciencedirect.com/topics/earth-and-planetary-sciences/quartz-vein
quartzpage.de
www.alexstrekeisen.it/english/meta/veins.php