Cornwall Virtual Fieldtrip Taster

Overview

Cornwall and Devon in the South-West of England show classic geological features associated with the Variscan mountain building episode which occurred around 290 million years ago. The rocks of Devon show evidence of extreme crustal shortening in their carboniferous sediments, including thrust faults and impressive chevron folding as seen at Millook Haven and other sites. The Cornish granites are younger, at around 250-270 Ma and show processes occurring in the mid-crust during later stages of mountain building. They are particularly important for the exposure of their contacts with the surrounding country rock and the economic mineralisation associated with late-stage hydrothermal fluids which emanated from the granites. This is a taster of some of the locations and samples we consider during the VFT course.  

Cligga Head

Hydrothermal mineralisation associated with the later stages of granite emplacement and cooling produced important ore deposits. The Virtual hand specimen here shows a typical vein of greisen (quartz-muscovite-topaz rock) replacing granite seen at Cligga Head. Click on this to move the viewing angle, zoom in and out with the mouse wheel, and right-click drag to change the centre view.

A greisen and ore-forming reaction reaction can be written as:

But at Cligga Head the main economic mineral was wolframite (a tungsten-iron oxide) and this site was for many years the main source of tungsten in the UK. The video discusses the processes of Greisen formation.

Rinsey Cove

Cornwall shows world-class exposures of roof-complexes which occur between the top of granite magma chambers and the surrounding country rocks. The granite here forms the headlands either side of Rinsey Cove and Mylor Slate formation dips down into the top of the granite forming a roof pendant. The softer slate has been eroded away to form the cove and the contacts with the granite are exposed on either side of the sandy beach. This is a great example to study the difference between vertical and very shallow-dipping intrusive contacts in granite, with lithium-rich, comb-textured pegmatites forming below the shallow roof and a much simpler vertical contact. The dykes and sills emanating from the granite into the slate show how granite is emplaced into the country rock by the process of stoping. The different angles of contact exposed on the wave-cut platforms provide a great introduction to the principles of geological mapping.

Use the 3D models below to explore the igneous contacts at Rinsey Cove for yourself. They are divided into three separate segments covering the Western contact, the Eastern Contact and a major structural fault which is not marked on the geological sketch map.