A to Z of North Wales Geology
D is for..............DYKE
DYKES are formed when molten rock is forced through cracks in a pre-existing formation and eventually cools to form a cross-cutting, sheet-like feature in 3 dimensions. When exposed to our view some long time later we are treated to a vision of the frozen plumbing of long-dormant volcanoes. If the same molten rock is forced between the beds of a sedimentary formation it would be called a SILL, although every other property would be the same as a dyke of the same composition. Dykes are frequently formed by multiple injections into the same system of cracks to create a composite structure, and it is generally held that repeated intrusion along the mid-ocean ridges to form a SHEETED DYKE complex is responsible for the growth of the oceanic plates.
In these figures I have attempted to draw cross-sections through an outcrop of a system of dykes and sills in a sedimentary sequence. The dykes are shown as connecting through a sill-like feeder which has caused a 'raft' of sedimentary rock to be elevated by the injection of the magma - highlighting the fact that space must be made for an intrusion. Outcrops of sills and dyke are shown as both positive and negative features. I have also tried to indicate that intrusions will try to follow a line of weakness, in this case the junctions between adjacent beds of sedimentary rock. A FAULT is another common line of weakness.
Eruption of lava from a linear surface feature which represents the outcrop of a dyke system is called a FISSURE ERUPTION. An historical example was the Laki Fissure Eruption of 1783 which had lava fountains for a length of over 100km. Dykes may comprise exactly the same range of composition as the erupted lavas that reach the surface, and it may be a matter of some difficulty to distinguish a lava flow from an intrusion. Clues can be gained from the "field relations" - the way in which the contacts between different rock types are displayed at the outcrop. Sometimes outcrops show that a large number of dykes are intruded in a regular pattern which displays a parallel or radiating trend, and this is termed a DYKE SWARM. Many of the dykes on Anglesey are very poorly exposed as a consequence of weathering and erosion, but they can still be found by geophysical methods. If you fly an aeroplane low over the land with a very sensitive compass on board it is possible to make very quickly a map of magnetic anomalies which result from the contrast between quartz-rich sedimentary and metamorphic rocks and the dykes which have abundant magnetic mineral grains. What is revealed is a swarm which can be traced to a focus in Northern Ireland, which is the nearest major centre of Tertiary volcanism. In the opposite direction sporadic outcrops can be followed as far as Llangollen and Shropshire.
Dykes can be recognised at outcrop by the different composition, colour and textures displayed by the contrasting rock types or the way that they have weathered. Sometimes the dyke will break down faster than the country rock and a negative feature will be formed as it erodes. When the dyke is more resistant to weathering it will be left as a prominent or positive feature. Sadly, really bold landforms of this type are seldom seen in North Wales, but on Arran or Mull the dykes often stand like walls 10 metres high on coastal sections. Possible exceptions are the sill-like intrusions which cap several North Walian mountains such as Moel Siabod or Tal y Fan. Many of the dykes on Anglesey are very deeply weathered and are reduced to a crumbly mess within which are embedded rounded boulders with peeling "onion skins" as the minerals containing Iron and Magnesium break down to clays and swell in the process.
When the flow of magma slows down or stops the dyke will start to cool and crystallise. The part which is in contact with the cooler "country rock" will solidify first, and will have a smaller grain-size as it cools quickest. This tendency to fine-grained edges is termed a CHILLED MARGIN, and it might be found that when a solidified dyke has been re-intruded there are chilled margins within the dyke complex itself. In the same way, the centre of a dyke is cooled most slowly and may develop a coarse-grained or GABBROIC texture. Distinguishing a SILL from a lava flow can be difficult, but the key feature of a lava flow is that its upper surface was in contact with air or water and may therefore be weathered or textured in a way that a confined sill would not be. On the other hand, a dyke or sill might be expected to cool and crystallise more slowly than a flow at the surface and to develop a coarser crystal structure or COLUMNAR JOINTING. The columnar jointing is usually at right-angles to the margins of the intrusion, which may give important clues to the real shape of a body whose outcrop is obscure.
DYKES must always be younger than the rocks which they intrude. This is a simple and very useful rule for unravelling the history of rocks found at outcrop. Finding their actual age may be a complex problem since there is no limit to how much younger they may be. The presence of dykes might have profound effects upon the rocks which they intrude, since they will be heated by the hot magma and may develop new minerals or textures common in metamorphic rocks. Such effects are known as CONTACT METAMORPHISM and there are some superb examples in North Wales, particularly on Anglesey.
A long history of geological exploration has shown that dykes in North Wales belong to two distinct populations of different ages. The earliest are contemporary with the volcanism during the Ordovician period and are part of the igneous activity associated with the subduction of the IAPETUS OCEAN. Much later are those which are associated with the opening of the North Atlantic Ocean about 60 million years ago. At that time tension in the crust created pathways for the huge surge of magma that was erupted across the Thulean province now found in Greenland, Scotland, Northern Ireland and the Faroes. Unlike England and Scotland which have numerous igneous intrusions and lavas of Carboniferous age, there are none exposed in North Wales, although they come close at Clee Hill in Shropshire.
Dykes formed from a basic magma are commonly described as being of DOLERITE, or DIABASE according to some authors. Please follow the link for an article about DOLERITES.
©Jonathan Wilkins, 03.2006