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Back to School EarthCache

Hidden : 04/27/2009
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Geocache Description:

Back to School

Remeber the old blackboard in your classroom ? That blackboard was actually made of rock; a particular type of rock: SLATE. Well, sit down at your desk and let's learn something more about this old rock.

The Slate Originating from clay or volcanic ash, slate it is one of the hardest rocks known to man. It is composed of several very thin plates that are all parallel and which can be divided easily. This characteristic is called cleavage. Homogeneous in appearance, it is only possible to distinguish the minerals with the aid of the microscope. Slate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, or zircon as well as feldspar. The rock absorbs only 1% of its weight in moisture and does not change on contact with acids.



The Extraction

The Valongo area consists of a large geological structure that is usually designated as the Valongo Anticline. It comprises an asymmetric anticline orientated NW-SE with its axis plunging 5 to 15º to the NW and the axial plane dipping 60º to the NE.The rocks that make up the Anticline range from the Precambrian and/or to the Cambrian up until the Devonian. To the west outcrop the Carboniferous formations. The slate quarries in this region are located in a band along the eastern flank of the Valongo Anticline and date from the Middle Ordovician (ca. 464 M.a).The Ordovician period is one where significant climatic alterations took place. There is reason to suspect that some of the metasediments of this age bracket were transported and deposited by glaciers and icebergs originating in a ice mass that covered the north of Africa.The Ordovician starts out as a discontinuous basal conglomerate that marks the beginning of a transgression. This conglomerate is followed by massive quartzites (Arenigian in age – Lower Ordovician) that form the two crests of the anticlinal structure. These are followed by levels of greywackes and metapelites both dark and light in colour

The industrial extraction of slate at Valongo, near Porto, started in 1865. Currently, the slate is extracted from the quarry (open-pit mine) that, after well inspected, is chosen and sent to get cleaved, sawn and softened. All the work of extraction and processing was done with the aid of steam engines.

The Use

As a good water insulator -slate is widely used in house construction, mainly for roof shingles. Slate has two preferential orientations of break: cleavage and grain. This makes it possible to split slate into thin sheets. When broken, slate maintains fc97b4f7-4261-4af8-81fe-24f435fc364b.jpgd2b58a1a-6a85-45aa-8eaf-e50f469cabdf.jpgits natural appearance while remaining relatively flat and can be easily stacked.
Because it is a good electrical insulator and fireproof, it was used to construct early 20th century electric switchboards and relay controls for large electric motors. Fine slate can also be used as a whetstone to hone knives. Due to its thermal stability and chemical inertness, slate has been used for laboratory bench tops. In the 18th and 19th century schools, slate was extensively used for blackboards and individual writing slates for which slate or chalk pencils were used.
Where slate of fine quality is available it is used for gravestones and commemorative tablets and by artists in various genre.


The EarthCache

To claim this earthcache you must go and see for your self the slates and its quarrying. You can also visit the museum (refer to waypoint below). Send to me the answers for the following questions after visiting the published coordinates:
1. The slate is usually black or dark gray due to the presence of a quality carbon or of a specific mineral. - Which mineral is that?
2. At the given coordinates you can see a quarry.What is the aprox. height of the vertical cuts of the quarry
3. How is Ronnie O'Sullivan is related with slate ?

Take pictures, and after receive my mail validating your answers, you must upload a Picture to your log, a picture that represent you in a school writing in a piece of slate! There are a lots of them at groundzero. use your imagination!

 A special thanks to DanielOliveira who have contributed to this EarthCache. You can visit his notable EarthCache work in: (visit link)

Additional Hints (No hints available.)