DO YOU KNOW YOUR ROCKS #4
-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
As with all the other earth caches in this series, we take a look at how we can learn to recognise different rock types in order to understand more of what is around us.
When reaching GZ you will notice an outcrop of rock. Ussually just passed by but stop and take a look a bit, what this is, is a granite outcrop.
Granite
Igneous Rock Type: Intrusive
Related to: Rhyolite, pegmatite, syenite
Chemistry: Acidic
Color: White, pink, orange, gray, black
Texture: Phaneritic (easy to see crystals)
Origins: Orogenic Plutons
Common Minerals: Quartz, feldspars, hornblende and micas
Accessory Minerals: Tourmaline, phosphates, rare earth oxides, beryl, topaz, zircons, augite, sphene and apatite
Uses: Building material, decorative counter tops, tiles, tombstones, roads, jewellery, curling stones, marbles
Mineral appearance in granite.
Quartz is gray in colour and looks glassy.
Plagioclase Feldspar has a milky white to tan colour and has shiny flat surfaces.
Alkali Feldspar is pink or white in colour and has a more dull shine to it.
Biotite Mica is more a silvery , shiny, flat surface.
Horneblende Has black, shiny, flat pieces.
Granite formation.
As an igneous rock, granite forms from melted or molten rock called magma that never reaches the surface of the Earth in liquid form. When tectonic plates shift or there is a crack the lighter minerals like quartz and feldspar that are at the top get squeezed through these cracks. This magma takes a long time to cool (up to 20 000 years) the minerals in the magma have lots of time to join up and form large crystals that are visible with the naked eye. Erosion and movement of the plates causes these rocks to become visible on the surface of the earth.
WEATHERING
Chemical weathering of granite occurs when dilute carbonic acid, and other acids present in rain and soil waters, readily alter feldspar in a process called hydrolysis. This causes potassium feldspar to form kaolinite, with potassium ions, bicarbonate and silica in solution as byproducts. An endproduct of granite chemical weathering is grus, which is often made up of coarse-grained fragments of disintegrated granite.
Physical weathering occurs on a large scale in the form of exfoliation joints, which are the result of granite expanding and fracturing as pressure is relieved when overlying material is removed by for instance erosion.
Different types of Granite
Granite is a very general term and is applied to a whole host of different rocks. Many different granites have been identified based on their varied chemical and mineralogical compositions. Generally the term granite is used as a suffix to indicate its textural and general composition. If a granite is rich in lithium then the rock might be referred to as lithium granite. Also, petrologists may choose to classify different but similar granitic rock types by using the terms such as granodiorite or syeno-granite. Some granites are named according to their unusual characteristics. In some granites the feldspars had time to form rectangular crystals before quartz and the other minerals crystallized. The resulting rock appears to be a collection of unintelligible letters, numbers or figures and the rock is called "Graphic Granite".
The Stone Industry recognizes granite as one of the hardest of all natural stones.
Above information was obtained from WikiPedia and Geology.com
On-site Questions
Q1: Is the weathering of this rock formation chemical or physical, and give a reason?
Q2: Describe the colour of the granite outcrop?
Q3: using water, wet a piece of the outcrop (please do not attempt to break the Granite)
What minerals is predominent in this Granite outcrop.
Forward answers to our KINGOSRIC handle.
Please forward as soon as possible.
Additional Hints
(No hints available.)