A to Z of North Wales Geology
R is for..............RHYOLITE

RHYOLITE is an igneous rock, and is probably the most common igneous rock in North Wales. There is a good reason for that, which will become clear shortly. Igneous rocks are those which were melted at some time in their life - and have subsequently cooled down and crystallised. Those which crystallised underground are termed Intrusive, and those which crystallised at the surface of the Earth are termed Extrusive - as lava, for example.

Conwy Rhyolite, Scale: 50p coin RHYOLITE is a pale-coloured, extrusive, igneous rock, but its dark secret is that it has never been observed as a flowing lava. Why is that? If you take a lump of a common igneous rock such as lava which flows in picturesque style in television programmes and analyse it, you will find that it contains Silicon, Aluminium, Calcium, Sodium, Potassium, Iron, Magnesium, and a heap of other elements of less importance. If you melt it completely (1100-1200 °C may be required) and allow it to crystallise you will get the original rock back again. BUT, if you START to melt it and do not finish the job (around 700 °C), something strange happens. The liquid part of the rock will contain much more Silicon, Sodium, Potassium and Aluminium than it should (complete analysis here), and the Iron and Magnesium will stay behind in minerals that resist melting until much higher temperatures. All we need to do is to separate our melted part of the rock from the residue and we can create a new rock of very different composition and characteristics. This process is called 'partial melting', and it can happen the other way around by settling Iron and Magnesium-rich crystals out of a cooling magma - in which case it is called 'fractional crystallisation'. The characteristic pale cream or grey colour of rhyolite is due to the fact that it has no minerals containing Iron and Magnesium, which are typically very dark in colour.

RHYOLITES are produced at low temperatures by partial melting of the crust, and most commonly this happens where subduction is causing 'oceanic' crust to descend into the hot interior of the Earth. Because they contain higher than 'normal' amounts of Silicon they are very sticky, and because they are the first products of melting they contain a lot of water (in the form of a dissolved gas) and other volatile matter. The sticky consistency is also a very effective barrier to good crystal formation, so the rock has a translucent or glassy appearance.

Flowage-folding, Yr Arddu. Lens Cap 6cm So why doesn't it flow? At the typically low temperatures and high silica content of Rhyolites, the magma is very viscous. So instead of running up cracks or conduits in a liquid state, it only reaches the surface as a solid which is sufficiently plastic that it can creep slowly, but, like toffee, it can be broken by a sharp blow. The very high strain imposed upon the flowing 'toffee' causes any impurities to be drawn out into streaks and bands, which give the rock a very characteristic appearance. Because the magma is virtually solid when it reaches the surface, it is frequently extruded as a spire. If this collapses it will form a dome of glowing boulders, which slowly grows as new material is intruded into the middle and collapses in turn. Under such conditions the intruding magma may develop contorted flow-bands which are clearly seen on weathered surfaces. A major collapse may allow the confining pressure on the magma to be released suddenly, and then the confined gases can simply blow apart the hot toffee-like magma in a rapidly-deepening chain reaction. The cloud of hot debris is called an IGNIMBRITE, and is a very dangerous phenomenon, which is why Rhyolite volcanoes are viewed as a major hazard.

So, finally we are able to answer the question of why Rhyolites are common in North Wales. They are characteristic of volcanism at the 'destructive' margins of the Earth's tectonic plates. That tells us that 450 Million years ago, North Wales was a part of the subduction zone on the south-eastern margin of the 'Iapetus Ocean' which was vanishing fast. Modern examples of such a situation might be Japan, the North West Rockies (remember Mount St.Helens?) and the Aleutian Islands - all of them situated on the destuctive margins of the Pacific Ocean Plate.


A Chemical and mineral analysis of the CONWY RHYOLITE.

©Jonathan Wilkins, 03.06