Geology

The Snaefellsnes peninsula has a young volcanic zone that extends along its length, from east to west. This volcanic zone is unusual in Iceland, as it trends in atotally different direction from the main north-south trending volcanic zones of central Iceland. The unusual mountain of Kerlingarfjall (585 meters high) is a volcano, that was built up during eruptions beneath the glacier that covered the Snæfellsnes peninsula during the Ice Age (Figure 4). At the time, the glacier was probably about 500 to 600 meters thick above sea level. The volcanic activity gradually built up the mountain during a series of eruptions. The volcano was erupting basaltic magma, but due to the melting of the glacier during eruptions, violent steam explosions took place in the crater, as the hot magma reacted with the meltwater. We can still see the form of the main crater in the western part of the mountain today. These explosions tore apart the magma into small fragments, turning it to volcanic ash. As the ash fell to the ground around the crater, it accumulated and gradually hardened to the rock we know today as móberg or basaltic tuff. Had these eruptions taken place on ice-free land, then the volcano would have produced a series of thin and wide-spread lava flows. We do not yet have a good idea of the age of Kerlingarfjall volcano, but we know that it is older than ten thousand years, and probably younger than one hundred thousand years. The móberg rock or basaltic tuff that makes up almost all of this mountain often shows crude stratification or layering, with each layer representing an explosion or an eruption.

Aerial photograph of Kerlingarfjall. (Figure 4)

In a few places in the mountain we come across dikes. They are vertical layers of basalt, generally about 20 to 100 cm in width, that extend as sheets of basalt through the mountain. Dikes form when a fissure is created and magma or molten rock invades the fissure from below, soldifying to form basalt. Initially the basalt magma is about 1200oC hot, but cools quickly to form the solid basalt rock. Often we see a black glassy layer ar the outer margin of the dike. The dikes are important, because they represent the veins or arteries of passage of magma through the volcano. A particularly good example of a dike is seen in the narrow gully that we ascend when climbing up to the Kerling or the Old Witch. Sometimes the dikes are only a few cm in thickness, and this gives us evidence of the extreme fluidity or low viscosity of the basalt magma when hot and molten.

Fractured rock:
The main crater of Kerlingarfjall volcano was located in the southwest of the mountain. This is the depression that is the most popular route to climb this volcano. As you ascend, you will enter a broad valley, which is the crater site. The south side of the valley is formed by a sheer vertical wall of basaltic tuff or móberg. When you get closer to this wall, you will notice that it is covered by a dense network of vertical and horizontal lines, as shown in the photograph (figure 5). These are former fractures in the basaltic tuff formation, that were probably created during the frequent explosive eruptions in the crater. Follow this wall to the left (northeast) and you will come to a steep and narrow gully. This leads you upwards to the site of the famous Kerling, the rock pillar that gives the mountain its name. On the way up the gully, notice that you are climbing on the surface of a basaltic dike, that is about one meter wide.

Pillow lava:
The first eruptions of Kerlingarfjall volcano were at the bottom of the Ice Age glacier. Then the crater was at great depth, under a high pressure of ice and meltwater. During such conditions, the eruption produces a rock we call pillow lava. Pillow lava (bólstraberg in Icelandic) is composed of rounded forms of basalt, often 1 to 2 m in diameter, that pile up and form the basal part of the volcano. Because of the great pressure and depth, there are no steam explosions in the crater and only fluid basalt magma is erupted (figure 6). We see such pillow lavas in the lower part of the eastern flank of the Kerlingarfjall mountain.

Kerlingarfjall-Bolstraberg2A schematic view of the formation of pillow lava during a deep subglacial eruption. Here the heat of the volcanic eruption melts the glacier and the erupting magma forms pillows under water. Rocks of this type are found in the lower part of the east flank of Kerlingarfjall. (Figure 6)

 

Cross-fracturing in the móberg cliff on south side of the Kerlingarfjall crater. (Figure 5)

 

Dikes:
Basaltic magma moves within a volcano along fractures in the rock. These fractures are the plumbing system of the volcano. When magma cools and solidifies in the fractures, it forms a hard, dark grey rock that we call a basalt dike (gangur in Icelandic). Dikes are generally vertical structures, and the margins of the dike may contain an outer layer of glassy basalt, one to two cm wide. This is formed as a result of quick cooling of the hot (1200oC) basalt magma in the fissure, as it comes in contact with the surrounding cold rock. The Best place to view a dike in Kerlingarfjall is in the narrow gully you pass up through when climbing up to the Kerling rock pillar. Here the dike acts like a staircase in the narrow gully (figure 7).

Dike in the basaltic tuff of Kerlingarfjall. (Figure 7)

Tuff balls:
The upper part of Kerlingarfjall contains layers that are extremely rich in spherical tuff balls. They can be found lying loose on the surface, but they are mostly encased within the brown basaltic tuff or móberg rock. Most of the tuff balls are about head-size, but they may range up to a meter and in places they are small, or only about 15 cm in diameter (figure 8). These balls are extremely rare in the world. We know of tuff balls of this type in only three localities on Earth: Kerlingarfjall, Valahnúkar (just east of Kerlingarfjall) and the basaltic tuff mountain ofLaki in south Iceland. The most spectacular place to view the tuff balls is in a wall or cliff face that you see when you emerge out of the narrow gully that leads to the top of Kerlingarfjall. Here there are thousands of them, tightly packed together and arranged like sculls in a Roman catacomb. Look closely at the tuff balls, and you will see that the faint layering in the basaltic tuff also passes through the tuff balls. This is important, because it shows us that the tuff balls formed AFTER the tuff was erupted and after it was deposited here. They have grown within the tuff. Thus the balls were NOT thrown out during eruption, but rather formed as a result of differential consolidation or hardening of the tuff rock. For some reason, parts of the tuff harden more rapidly than others and form the tuff balls. The surrounding tuff is softer and weathers out, leaving the balls behind, and gradually they drop out of the rock face. Scientists do not fully understand this process, but there appears to be some radial cristallization process within the tuff, that leads to the formation of the balls. When you see the balls, remember that they are unique and virtually unknown in other parts of the world. Please do not destroy or remove these interesting balls.

Tuff balls in Kerlingarfjall. (Figure 8)

The northern cairn of the shepherds in Kerlingarskarð pass. According to legend, it is the shepherd from the farm Berserkjahraun, who lies buried beneath this cairn. Cairns of this type are known as dys in Icelandic. (Figure 9)

The Cairns of the Shepherds

There are two prominent cairns or rock piles (dys in Icelandic) in the Kerlingarskarð pass, southwest of the Kerlingarfjall mountain. They are located on the side of the old mountain road. Legend has it that these cairns mark the sites of the remains of two shepherds who fought here to the death in the Middle Ages. They were shepherds from the farm of Berserkjahraun to the north and from Hjarðarfell to the south. They argued over grazing lands for their flocks of sheep in this area, and gradually the argument escalated into a bloody battle, that ended with the death of both. It was a custom for travellers passing by the cairns to throw a stone on the pile for good luck, when travelling through this difficult mountain pass.