Geology Group Diary (06)

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    The group met on Wednesday 10 February at 10.30 am in Merlin's Bridge Welfare Hall.

    The Tertiary period began around 65 Ma and continued up to 2 Ma.  During the break up of the super continent Pangaea, the North American plate and the Eurasian plate rifted apart to form the North Atlantic Ocean. In Palaeocene times (65-56 Ma) Britain was situated above a mantle plume (sometimes referred to as a hot spot) where the earth’s crust was under stress from plate rifting. As a result fissures opened up along a line from Ireland to the Hebrides from which basaltic lavas were extruded and igneous centres developed, surrounded by ring dykes and cone sheets and radiating dyke swarms. A vast lava field (1.8 million km2) called the Thulean Plateau extended from West Scotland  through the Faroes, to Iceland and Greenland.
    The Plateau Basalts
    Up to 2000 metres thickness of basalt lavas were erupted in Antrim, Mull and Skye during the Palaeocene. Even today after extensive erosion, the lavas are 1800 metres thick on Mull. The individual lava flows give rise to stepped topography which is caused by the weathering back of the softer vesicular slag at the top of each flow. Between the lava flows there are often found red lateritic soils. These iron rich deposits contain plant remains, particularly in the leaf beds of SW Mull where a temperate flora of ginkgo, plane, hazel and oak occur. The leaves are well preserved and are thought to have fallen into a shallow lake. In 1819 a magnificent specimen of a tree trunk was discovered embedded in lavas in the cliffs of western Mull; it is known as MacCulloch’s tree!
    The basalt lavas of the Giant’s Causeway and the Isle of Staffa are world famous for their hexagonal columnar jointing. The columns result from the cooling and contraction of the lavas under perfectly stable conditions and they form perpendicular to the base and surface of the flow. The top of the lava flow would be filled with escaping gases which produced a vesicular slaggy texture. On the island of Eigg the lava cooled so quickly that it produced a glassy rock called pitchstone (similar to obsidian) that forms the Sgurr of Eigg.
    The Igneous Complexes
    There are several major plutonic centres extending northwards from the Mourne Mts in Northern Ireland through the Western Isles of Arran, Mull, Ardnamurchan, Rum and Skye. These centres represent the remains of Tertiary volcanoes and their underlying magma chambers.
    On Skye the magma was emplaced in a sequence of Precambrian rocks (Lewisian gneiss and Torridonian sandstone) overlain by basalt lavas. The magma was rich in mafic minerals (plagioclase, pyroxene and olivine). Layered gabbros were produced by the settling out of denser minerals and basaltic volcanoes erupted at the surface. Today we see the Cuillin Hills formed from the deeply eroded gabbronic magma chamber. However, the nearby Red Hills are formed of granitic magma produced within the same igneous complex. The granites were probably formed by the melting of the Precambrian basement rocks. In the Cuillins many minor intrusions occur as cone sheets rising out of the volcanic conduit and sloping outwards in a circle around the volcanic centre.
    The island of Mull is almost entirely formed of an extinct volcano and its eroded magma chamber with three major eruptive centres within the complex. Numerous cone sheets form a distinctive arcuate pattern around central Mull. However, the Mull volcano is also characterised by the presence of ring dykes. These are cylindrical intrusions produced by magma being forced up through ring faults surrounding a central area of subsidence. As magma is removed from the magma chamber the centre of the complex sinks creating a cauldera of subsidence. Think of a full wine bottle as a magma chamber and the cork representing the overlying rocks. Force the cork downwards and wine will be forced upwards around the cork creating a ‘ring dyke’.
    Dyke swarms occur throughout the Tertiary volcanic province and they all demonstrate a NW-SE trend. They are thought to be associated with shearing stresses produced by the opening of the North Atlantic during the early Palaeocene. Some 300 dykes can be seen on the south coast of Arran and these represent a7% crustal extension in a 20 km section. Perhaps the most famous Tertiary dyke is the Cleveland dyke in NE England. This can be traced for 400 kms from Mull to North Yorkshire and is thought to have been emplaced as a single pulse within a few days!

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