a. Explain why the oldest rocks are found in the extreme south-east and the extreme north-west of the country.
b. Think of an explanation why there are fissures, and explain their orientation.
Iceland lies on a mid-oceanic ridge, the Mid Atlantic Ridge. This ridge of the seafloor stretches from north to south in the central part of the Atlantic Ocean where the Eurasian and American plates diverge. Here, new oceanic crust is gradually formed from the basaltic magmas that rise from the mantle below the ridge. Iceland is a part of the ridge that lies above sea level. It grows from its centre outward to the east and west.
Why does Iceland lie above sea level? Because a hotspot immediately below the island generates more volcanic activity than elsewhere along the Mid Atlantic Ridge. This explains the extreme rate of magma production on the island.
Usually, a mantle plume beneath a moving oceanic plate will create a row of islands (like in the case of Hawaii, see exercise 4.3). However, the magma producing hotspot below Iceland lies exactly beneath the location where the plates diverge, so that the island grows in both directions. The location of the hotspot itself is stable. It fuels the magma chambers of many volcanic systems on Iceland.
Exercise 4-6*: Islands on a MOR A, I
Iceland is not the only island on a MOR above sea level. a. Look in your atlas for five more islands that lie on a MOR.
Figure 4.8: Iceland with some of its most important volcanoes. The Skjaldbreidur and Ok volcanoes lie northwest of Geysir. Hvannadalshnúkur lies mostly hidden beneath the ice east of the Laki (or Lakigigar) volcano (see Öræfajökull in Figure 4.7)
Now that we know more about Icelandic volcanism we will take a look at one specific eruption that had a devastating effect.
In 1783 and 1784 the Laki volcano (see Figure 4.8) erupted for eight consecutive months. This volcano produced one of the largest lava flows in human history. At least 15 km3 of magma
erupted from a fissure in the crust. When the magma travelled to the surface, gases that were initially dissolved in it were able to escape, because their solubility in molten rock decreases with decreasing pressure. These gases contained highly toxic components, causing severe health problems in the population, poisoning trees and vegetation, and killing more than 60% of the cattle on Iceland. A widespread famine that lasted many years killed about 20% of the population. The effects were felt not only on Iceland. A dry fog hung over the north and west of Europe for months. The summer of 1783 was very hot, while the following winter was extremely cold in Europe and North America. Winters continued to be unusually cold in the following years and many harvests failed. Famine was widespread, and more people died than under normal conditions. A large amount of volatile components were brought into the atmosphere during the eruption. Among these, sulphur dioxide (SO2) and hydrogen chloride (HCl) are potentially harmful. If their
concentrations in the air are high, rainwater will become acid. We will discuss this further in the next sections. First, we will introduce a simplified model of the eruption and estimate the amounts of SO2 and HCl that were released. In §4.6 we will explain how and why volcanic acid rain is
formed, and calculate the pH of rainwater affected by such an eruption. Furthermore, we will discuss examples of possible effects of acid rain.
4.5 The quantity of gas emitted during the Laki eruption
In order to assess the effects of the Laki eruption, we need to know how much of each gas component was emitted. We will focus on SO2 and HCl, as these components were most harmful.
Because the quantities of released volatile compounds were not directly measured when the eruption took place, we will have to reconstruct the data by analyzing the erupted materials that are still there. For this Thordarson and co-workers (1996) used a simplified model, which we will follow here. (see Figure 4.9)
Figure 4.9 shows a hypothetical magma chamber of a certain volume. The magma inside contains the volatile elements sulphur and chlorine (from here called ‘volatiles’ in short) in certain concentrations (vi). Here, the volatiles are completely dissolved in the liquid. When magma reaches
the surface, it cools and solidifies in two different forms: a pile of tephra (ash and rock fragments) and a lava flow. Part of each volatile remains inside these solidified products, but most is expelled into the atmosphere. Some of this released portion stays in the vicinity of the volcano as a local fog. The rest is spewed high into the atmosphere, and is especially relevant in our calculations because this gas is transported away and has a damaging impact at great distances from the volcano.
Schematically, we assume that the sequence of events was as follows:
Magma rises from the magma chamber with all the dissolved volatiles present.
The beginning of the eruption is quite explosive; during this time the tephra is emplaced. It represents part of the original amount of magma. Much gas is released in the eruption plume. Therefore, the concentrations of volatiles in the rest of the magma that remains underground decrease.
This ‘degassed’ magma then rises to the surface as well, but, as it contains less gas, it flows out as a lava in a relatively calm way.
As the lava flows out, it will lose more of these still present volatiles.
Low concentrations of volatiles stay behind as a residue, both in the solidified tephra and in the lava, because ost of the gas escaped to the atmosphere.
Concentrations of a volatile (S or Cl, in ppm):
vi = in melt inclusions (representing original magma in the magma chamber)
vt = in tephra
vl = in lava (during its outflow)
vc = in the solidified lava (after its emplacement)
vs = total concentration in the solidified products
Mass of a volatile, magma, lava or tephra:
mr = original mass of a volatile in the magma chamber
mv =mass of a volatile released into the atmosphere at the crater
ml = mass of a volatile released into the atmosphere during the lava outflow
mc = mass of a volatile released into the atmosphere after the lava emplacement
ms = mass of a volatile in solid eruption products
mtot(r) = total mass of magma degassed at the crater
mtot(l) = total mass of the lava
mtot(t) = total mass of the tephra
All information on the amount of gas release during the eruption can only come from the solid eruption products that are still there: the lava and the tephra. The residual concentrations of volatiles in these rocks can be determined in the lab, by analysing samples taken at the site of the eruption. If we then also know the initial concentrations of volatiles in the magma chamber it is easy to calculate how much gas was emitted in total.
‘Melt inclusions’ are small quantities of the original magma that are locked inside a crystal as a solidified microscopic ‘droplet’. This sometimes happens when crystals grow in a magma chamber. Because the droplets are shut off from the surrounding magma, their composition (including the concentrations of volatile elements) will not change anymore. The crystals reach the surface with the erupting magma, and will be present in the solid eruption products. Geologists use melt inclusions to determine the concentrations of volatiles present in a magma before it is (completely) degassed. Despite the small size of the ‘droplets’ (sometimes no more than a few micrometres)
magma chamber