We Irish complain a lot about our climate. This is hardly fair when we compare our weather with the conditions that prevail in many parts of the world.
Apart from the weather, the Earth itself can also be unstable, giving rise to earthquakes and volcanoes. Again, thankfully, Ireland is geologically stable and does not experience these phenomena.
The Earth is roughly spherical and is composed of layers. Its surface is covered with a thin crust representing 1 per cent of the Earth's volume, beneath which are three layers, collectively called the mantle, extending about 2,500km beneath the surface.
Two further inner layers of the Earth, beneath the mantle, are called the outer and inner cores. The distance from the surface to the centre of the Earth is about 6,370 km.
The layers of the Earth differ from each other in composition, density and physical consistency. The 70km deep rocky lithosphere is the upper mantle layer and lies directly below the crust. The layers beneath the lithosphere are under great pressure and are very hot, largely because of the breakdown of radioactive elements. This heat is not easily dissipated and consequently the rocks in the mantle tend to melt and to flow.
The lithosphere has a number of cracks (faults) dividing it into great plates. These plates, bearing the continents and oceans, move relative to each other on the underlying pliable mantle.
In some places plates move away from each other, in other places they slide past each other or one plate dips beneath the neighbouring plate (subduction zone).
Plate movement explains many geological phenomena, including earthquakes and volcanoes. The theory of plate movement is called plate tectonics.
Volcanoes cluster along the boundaries of the Earth's great plates. Rocks in the Earth's mantle, although hot, are usually not molten because of the high pressure. However, molten rock (magma) does occur along the plate boundaries, giving rise to volcanoes.
Molten rock rises from the mantle to form volcanoes at mid-ocean ridges where plates move apart from each other, and at subduction zones, where plates converge. At the mid-ocean ridges, magma flows up from deep in the mantle, melting rocks in shallower parts of the mantle and erupting as lava at the surface.
At a subduction zone, as one plate slides beneath another it slowly warms up. Volatile compounds such as carbon dioxide and water are driven out of the sinking plate and move up through the mantle, melting rocks in the process. The new magma can erupt volcanically.
Subduction-zone volcanoes form a line parallel to the plate boundary, above the descending plate. A ring of volcanoes (the Ring of Fire) is related to subduction zones around the Pacific Ocean. Subduction under the oceans produces chains of volcanic islands called island arcs, e.g. the Aleutian Islands. Subduction beneath a continent produces mountain ranges such as the Andes, with many active volcanoes.
Volcanoes can also arise when a plate moves over a particularly hot part of the fluid mantle (hot spot). A jet of magma (plume) rising up from deep in the mantle is responsible for a hot spot. This magma can break through the lithosphere and erupt as a volcano.
Chains of volcanic islands have been formed in the middle of oceans in this manner. The process is analogous to the scorch marks that form in series on a sheet of paper moved horizontally over a candle flame.
Volcanoes around the world erupt in different ways. Etna in Sicily smokes and steams but it can eject lava flows and showers of pulverised rocks. Volcanoes in Iceland gush fire many metres into the air.
THE likelihood of an explosive eruption depends largely on the viscosity of the magma. Subduction-zone volcanoes, which produce more viscous lava, tend to erupt explosively. Volcanoes associated with mid-ocean ridges or hot spots tend to produce fluid lava.
Gases dissolve easily in hot rocks deep underground where the pressure is high. As the magma rises, some of the gas bubbles off - like a bottle of sparkling water when you loosen the stopper. In low-viscosity magma the bubbles rise to the top easily, or are entombed in the rock as it solidifies. In viscous magma the gas can be held until it builds up enough pressure to blow the molten rock to bits.
A volcanic eruption can produce enough energy to blow a plume of ash 8km into the stratosphere. Ash and debris fall back to Earth. This ash can damage the lungs. The material that reaches the stratosphere spreads around the world bringing exotic colours to sunrises and sunsets for weeks, and sometimes causing strange weather patterns.
But ash can pose an immediate danger. The rising plume of ash can lose energy and collapse - perhaps because the eruption slows down. Ash and rock fragments now fall down and away from the volcano in a pyroclastic flow. The fine ash, mixed with air, makes the pyroclastic flow behave like a fluid and it can rush long distances down a hillside leaving a trail of destruction behind. Some flows reach 400 degrees or more.
(William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.)