Rock of the Month - December 2011

Rock of the Month


Rock of the Month - for December we showcase the Shap granite - a sample from a forthcoming Open University science course SXG288 Practical Science: Earth & Environment. The sample is a constituent of the Igneous and Metamorphic Rocks in the Field topic.

The Shap granite quarry in Cumbria (northern England) is a well-loved field locality, which has been visited by many thousands of OU students over the last 30 years (those taking S23, S260, S276 and more recently SXR276). The quarry is in a geological conservation area and collecting is no longer allowed.

Shap granite is a distinctive coarse-grained granite - an igneous rock formed by the cooling of a large body of molten rock (magma) that was intruded into Cumbrian slates at the end of the Caledonian Orogeny, about 400 million years ago. It has been widely used as a decorative building stone.

The mineralogy of this granite is relatively simple - biotite, quartz and two feldspars (plagioclase and K-feldspar). Fluorapatite, ilmenite and titanite are also present in minor amounts.

The origin of the abundant pink Carlsbad-twinned K-feldspar megacrysts, which are a characteristic of the Shap granite, has been a source of much controversy. The megacrysts occur not only in the granite but also within microgranular mafic enclaves within the granite, leading to the suggestion that the megacrysts are porphyroblasts resulting from late-stage crystallization from potassium-rich metasomatic fluids.

Labelled views

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Stills from 2 rotation pointsRockMonth-2011-12-b.html

View thin section of Rock of the Month

Thin section rotation views

Transmitted light
between crossed polars
Rotation point 1RockMonth-2011-12-c.html
Rotation point 2RockMonth-2011-12-d.html
Transmitted light
plane polarised

Shap granite containing large pink K-feldspar megacrysts.

Width 5 cm across.

Object movie

Hand specimen
Courtesy of Cumbria RIGS Group
map of 
Shap granite





Skiddaw Slates

Eycott Volcanics

Borrowdale Volcanics

Windermere Group

Mell Fell Conglom.