The post at the headline coordinates is made from Granite, a very hard rock formed in volcanoes. Very clearly visible within the rock are some large crystals known as Phenocrysts.
All the rocks that we see around us belong in one of three general categories - Sedimentary, Igneous or Metamorphic.
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.
Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses.
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat and pressure causing profound physical and/or chemical change. The protolith may be a sedimentary, an igneous, or even an existing type of metamorphic rock.
A phenocryst is a relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms (meaning that they are well formed, with sharp easily recognised faces) either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than 0.5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts. (Source: Wikipedia)
The granite in this post is likely to have been formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass. Differential cooling permits separation of dissolved metals into distinct zones, creating rich, localised metal ore deposits.
In order to log this Earthcache, please send answers to the following questions via our profile. Feel free to log the find once your answers have been submitted.
1. Approximately how tall is the post?
2. Describe the texture of the rock. Are some sections rougher than others, are the corners angular or smoothed?
3. What colour(s) can you see in the post?
4. Describe the largest phenocryst that you can see - approximately what size is it, what colour is it?