The purpose of this earthcache is to introduce only the basics about granite. The main lessons to be learned will be about the formation of granite and especially why some granite crystals are large and why some granite crystals are small. At the same time, and optional, this earth cache says something about the the Gjesværstappan islands.
What is Granite? Granite is an igneous rock with grains large enough to be visible to the naked eye. It forms from the slow crystallization of magma below Earth's surface. As the magma cools and solidifies, a type of rock called igneous rock forms containing randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma solidified. The more slowly the magma cools, the bigger the crystals. If the magma cools quickly, small crystals form in the rock. This can happen if the magma erupts from a volcano. Obsidian and basalt are examples of this type of rock. If the magma cools slowly, large crystals form in the rock. This can happen if the magma cools deep underground. Granite is an example of this type of rock, but can look slightly different depending on its chemical composition and how quickly it cools.
During the cooling process and whilst magma is still fluid, pieces of older rock may become included in the magma. If the rock was a foreign rock, it is called a xenolith. If it is genetically related to the magma, it is called an autolith, or an "enclosure". With either type, the magma cools around the rock and the xenolith or autolith becomes embedded within it.
Granite can be dark, red, pink, gray, or white in colour, due to differences in mineral composition and texture. Some minerals are millimeters, others can be several centimeters.
Granite is very hard and crackless 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.
And let's log the find...
1. Have a look at a 5cm x 5cm area on the surface of the stone. How many different types of grain crystal do you see? What colour are they?
2. Name at least two minerals from above that you think are in the granite.
3. Is the granite rough-grained or fine-grained?
4. Look carefully at the rock and describe what it looks like. Is it solid or does it contain cracks and joints? Describe.
To log the earthcache, you may log a find as soon as you message the answers to me. If further information is required, I will message you back seeking clarification of your answers. Photos are not required for Earthcaches, but you are welcome to take a photo at the site or from the boat. However, please do not include a photo spoiler. And find the witch at N 71° 08.277 E 025° 21.480.
Close to this EarthCache you'll have an unforgettable nature experience by visiting the busy birdlife on the birdrock Gjesværstappan and see the islands. The three main islands are Storstappen, Kjerkestappen, and Bukkstappen. For conservation purposes it is not permitted to go on shore but from the boat you can experience the fantastic sight and rock formations.
So, for curiosity, a bonus geoscience lesson...
The rocks on all the Stappan islands near Gjesvær have a similar origin. They belong to a high-grade metamorphic complex known as the Gjesvær migmatite complex (GMC), which are found on the western parts of Magerøya from Tunes in the NW to Vannfjordennæringen in the SW. These rocks are mostly paragneisses, that means they are formed mostly from original sedimentary rocks, mainly sandstones, which have been subjected to strong overprint by both heat and pressure. During this metamorphic alteration parts of the rocks started to melt and formed granitic sheets, veins and lenses and most of the rocks became migmatites. Some parts are however more resistant to melting and have retained some of their original characteristics and we can recognise that they were sediments. There are also dark to black lenses and bands that originally were basaltic magmas that were intruded into the metasediments. These metamorphosed basaltic rocks are now amphibolites which some places have quite large garnet crystals in them. Finally there are some larger sheets of granites. One of these was recently dated by us to be 967±4 million years old. This means that the rocks on Stappan as well as the rest of the GMC is older and most likely more than 1000 million years.
Because of the strong overprint by deformation and metamorphism which happened several times in this long history the origin of these rocks is very complicated and therefore difficult to understand. The last major event affecting these rocks in e penetrative manner was the Caledonian orogeny at around 400 to 430 Million years ago. During this event the rocks again were subjected to high temperatures and partial melting.
Kjerkestappen is part of this beautiful story.