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The land and the people The land: The volcanoes A vast amount of lava has poured out of Iceland's volcanoes since the end of the last glacial period — enough to cover about one-tenth of the island — and during historical times there have been more than 125 eruptions from over thirty volcanoes. In Iceland the Earth's crust is only about 10km thick (compared to about 30km in many other landmasses) and is rent with long fractures; below is the mantle and it is here that the molten rock ("magma") gathers before being erupted to the surface, sometimes after accumulating in a higher "magma chamber". As the visitor travels round the island he is confronted by volcanoes of different shapes and sizes and with different types of lavafields around them. The reason for this variation often lies in the state of the magma just before it comes up to the surface. In the case of a very fluid magma the gases can escape easily and the hot material can quietly pour out of the vent and cover a large area, even if the slope down which it travels is not very steep. If such a flow takes place, a "shield volcano" is formed, so–called since its final shape is similar to a shield lying on the ground. A fine example is Skjaldbreiður (north–east of Thingvallavatn) which is 1,060m high and has a slope of only seven degrees. At the summit, these volcanoes may have flat–bottomed craters, with steep sides which often collapse, increasing the size of the crater (300m in diameter in the case of Skjaldbreiður). If the crater is full of magma and regularly overflows then the volcano can be built up by many (perhaps thousands) of sheets of lava with little ash or other airborne material thrown out. Another type of conical volcano is represented by Hverfjall (452 m) to the east of Mývatn. This was built by an explosive eruption, the viscous magma spraying out pieces of hot ash which welded together on cooling and built a steep cone with a slope of some thirty degrees. Many cones, though, are of a composite nature, with alternate layers of lava and airborne material. This combination welds the material together and the volcano can reach a great height. Such a volcano is called a "stratovolcano", the best known example being Snæfellsjökull on the Snæfellsnes peninsula. Iceland's most common type of volcano is built from a "crater row" which is often a number of vents that open up along a fissure. When this erupts it results in a volcano building not a cone, but a ridge; a classic example is the country's most famous volcano, Hekla (1,491m). Crater rows such as Laki (south-west of Vatnajökull) have been amongst the most effusive volcanoes in Iceland`s history. One very different type of crater may be found at Mývatn and Kirkjubæjarklaustur: a "pseudocrater". This is formed when a lava stream flows over wet ground; the water is heated, turns into steam and explosively forces its way upwards through the layer of hot lava. As the cooler crust of the flow collapses back on itself, a crater is formed. Since this type of crater is not fed by magma it is not a real crater — hence the name, "pseudocrater". There are two main types of lava that are produced. In the case of a very fluid magma with little or no gas, the lava will be forced out of the vent easily and at great speed, perhaps throwing fountains of the red–hot liquid many metres into the air. The lava from such an eruption can travel quickly over the ground and may produce channels and tunnels ("lava tubes") through which it runs. When the surface of the lava finally solidifies, it does so in folds and is called "ropey lava" (helluhraun). Great domes of this can be formed (called "tumuli") which may eventually crack under the strain of their own weight. Ropey lava is quite easy to walk across (when cold!). If the magma is more viscous and has a lot of gas dissolved in it, the lava flow often takes the form of big blocks of clinker–like material. While fluid lava runs easily downhill, these large hot jagged blocks move by tumbling clumsily over one another. The progress downhill may be very slow (only a few metres per minute); the front of the advancing pile might be a few metres high. When it has cooled down and set, it forms a great mass of contorted shapes with big spaces between the blocks. The surface of this "block lava" (apalhraun) is sharp and the rock is brittle. It is almost impossible to cross such a lavafield until it has been weathered and moss-covered. The material that is thrown through the air from a volcano may land far from the vent that produced it, so it may be difficult to discover the origin of much loose material that is lying around. Such material varies greatly in size and consistency: the large lumps of lava that are thrown out of a vent are called "bombs", and if they rotate in the air they may become spherical; "pumice" is a very light material formed when sticky, frothy lava which is plugging a vent is suddenly blown out; "ashes" are powdery remains of magma that have been sprayed out and have then solidified during flight. While many of these materials may be produced during the same eruption, it is the lightest ones that will be lifted highest and farthest from the vent. Dust from eruptions may stay in the atmosphere for many years, the biggest eruptions creating a dust cloud large enough to produce beautiful red sunsets not only in Iceland but in many other parts of the world as well. While volcanic eruptions permit solid, liquid and gaseous material to escape from the Earth's interior relatively quickly, there are various secondary volcanic features that release material (and heat) over a long time and at a much less violent rate. The most common examples are the thousands of hot and warm springs found throughout the central (geologically young) region of the country, often at such a powerful rate that they make a roaring sound. Vapour may come from lava cooling underground, or from magma sources that are still losing gases, mixed with water that has been turned into steam. "Fumeroles" may give out steam, hydrochloric acid, carbon dioxide and numerous other gases. When sulphur gases are given out, the fumeroles are called "solfataras"; these can usually be recognised by the sweet, sickly smell they give out and from deposits of the yellow powdered sulphur beside the vent. All these vents have to be approached with care as they often emit superheated steam. In areas where they are hot and powerful, they may exist long enough to turn the surrounding rock to a cream or yellow–coloured clay. Deposits of silica, chlorides and sulphates of various hues can be found at the mouths of the vents, often producing sizeable crystals. Less dramatic are the acid and alkaline springs. Acid springs are usually found at relatively high altitudes or other places where there is little ground water. Carbon dioxide dissolved in hot water produces hot acid springs which are relatively small. In lower areas where there is a lot of ground water and where the reactions between the rocks and the gases have produced alkaline chemicals dissolved in large quantities of water, there are high–volume, warm–water springs. It is these springs that are used to provide warm water for domestic heating (as in Reykjavík and Húsavík) and for glasshouses (for instance at Hveragerði). Sometimes the volume of water is sufficient to produce a warm lake as at Laugarvatn. Since the hot rocks that produce this heat take hundreds or thousands of years to cool (rock is a bad conductor of heat), this useful source of energy can be relied upon not only for heating buildings but also to produce steam for electrical generation. Such a plant, called a geothermal power station, has been built at Krafla to the north–east of Mývatn and large diameter pipes have been bored into Krafla's mountainside to tap the steam and transport it to the power station. The most spectacular secondary volcanic features are geysers, which are deep cylindrical holes in the ground that periodically send columns of warm water high into the air. The best–known examples in Iceland are Geysir (after which all water spouts are named) and Strokkur, both of which were probably formed in a great earthquake in 1294. In a number of areas there are pits of boiling mud. These are formed when steam mixes with volcanic ash in underground cavities; the blue–grey colour of the mud is due to compounds of iron and sulphur. The consistency of the muds vary: the viscous ones let rising bubbles of gas move through them slowly, producing concentric rings on the surface when the bubbles escape; the more fluid ones throw blobs of the mud upwards and to the side of the pool, building "spatter cones" that may reach one metre in height. These should be approached with care as the surrounding clay surface can give way. Altogether there are some 250–300 spring areas, many of them highly coloured due to the clays and other coloured rocks near them and to the deposits around the vents that have slowly built up. In a number of cases, they also allow vegetation to grow, giving life to otherwise barren ground. The most accessible solfatara areas are Námaskarð, Krafla (both near Mývatn), Krísuvík (Reykjanes peninsula) and Hengill (south of Thingvallavatn). The biggest are at Torfajökull (north of Mýrdalsjökull) and Grímsvötn (in Vatnajökull). |
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