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Samson's Ribs

A cache by ScottishHunter Send Message to Owner Message this owner
Hidden : 09/09/2009
1 out of 5
1 out of 5

Size: Size:   not chosen (not chosen)

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Geocache Description:

Samson’s Ribs is a columnar basalt outcrop to the south of Arthur’s Seat. Cache site easily accessible from the road and footpath. Queen's Drive is the main route through the Park, and is closed on Sundays to motor vehicles.

Holyrood Park is a royal park in central Edinburgh, Scotland. It has an array of hills, lochs, glens, ridges and cliffs within its 650 acre area. The park is associated with the royal palace of Holyrood House, and was formerly a 12th-century royal hunting estate, although it is now publicly accessible.

Basalt is an Igneous rock. Igneous rocks (etymology from Latin ignis, fire) are formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth'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 such as an addition of water.
Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. These have diverse properties, depending on their composition and how they were formed.
Igneous rocks make up approximately 95% of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layers of the two other rock types.
Igneous rocks are geologically important because:-

  • their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;

  • their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;

  • in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet.
The most common uses for this rock are as aggregate in highway construction, railroad ballast, and tile. Basalt is also a major component of asphalt.
On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.
During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are often erroneously described as being predominantly hexagonal. In reality, the mean number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observed. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns. (Notable columnar basalts include :-

  • Staffa, Scotland

  • Giant’s Causeway, Northern Ireland

In order to log the cache
1. Estimate the length of the longest column.
2. Estimate the entire width of all of the columns. (I started at the beginning of the fence and ended up just above the large fallen boulder below the road and paced this out).
3. Igneous rock is one of the three main rock types. What are the other two?
4. Take a photo of yourself with GPSr and the outcrop clearly in view and upload with log.

Once you have the answers please e-mail me this info (Do not post the answer in your log!!)

Any log that does not fulfil all four requirements may be deleted.


*****  FTF  Well done to Grim Fun Scenes on first to find  *****



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