4.1. Geosites in the Snæfellsnes Peninsula
The two most iconic geosites in the peninsula, which are worthy of mention also for their geotourism-related significance, are Snæfellsjökull Volcano and Kirkjufell. The former (Figure 2
a,b), located at the westernmost edge of Snæfellsnes, is a majestic, snow and ice-capped composite edifice that rises to almost 1500 m a.s.l. This geosite can be evaluated by applying most of the above outlined criteria. In fact, it can be assessed in terms of its aesthetic value, which can be defined as the combination of two factors: visibility and structure [32
]. A geosite’s visibility is greater when it can be clearly spotted from multiple viewpoints and also from a considerable distance. Structure, on the other hand, refers to the fact that features with a vertical development, such as isolated peaks, are generally perceived as the nicest [103
]. This volcano fits both criteria, as it is an isolated peak that, being located at the tip of a peninsula, is perfectly visible from tens of kilometers away. As regards the scientific value, two subcriteria can be applied: firstly, the volcano was the subject of extensive scientific research (e.g., [104
]) and, secondly, it is highly representative of the combination of volcanic and glacial processes. As far as the criterion “artistic or literary importance” is concerned, the volcano has also a cultural value; in fact, Snæfellsjökull was made famous by Jules Verne, who included it in his best-known science-fiction novel, Journey to the Center of the Earth
(1864). Finally, this geosite has a major educational value, being easily accessible and located within Snæfellsjökull National Park (established in 2001), where guided tours to its glacier, as well as recreational/educational activities, are organized on a regular basis. Kirkjufell (“Church Mountain” in Icelandic) is a stunning, 468-m-high peak found on the northern coast of the peninsula, which, along with the beautiful waterfalls, is one of the most photographed natural features in Iceland (Figure 2
a,c). This geosite has a considerable aesthetic value: firstly, it has a peculiar morphologic structure, clearly visible from a considerable distance (although not comparable to the much higher Snaefellsjökull Volcano). It is also very representative of glacial processes that, all over Iceland, carved thick piles of basaltic lava flows, to the point of creating impressive erosion-related landforms, such as Kirkjufell and many others. However, as opposed to its neighboring volcano, this geosite has not been the subject of scientific research in the past. In regard to its educational value, all the subcriteria can be applied: it is perfectly accessible, the processes that led to its formation can be easily understood and there are plenty of possibilities to take part in guided tours.
Along the northern coast of the peninsula, around 20 km east of Kirkjufell, another geosite that is worth highlighting is a scoria cone with a basal diameter of 550 m (Figure 2
d), which is a perfect example of a continuous crater rim with two depressed points. The line ideally connecting the two depressed points is considered [105
] to be parallel to the fracture in the substrate along which magma was rising, leading to the growth of the cone. Another way to define the most probable orientation of near-surface magma paths is to analyze the alignment of pyroclastic cones. In this respect, Tibaldi et al. [47
] documented that 51 pyroclastic cones, clustered in groups of three or more (such as in Figure 3
a), and mostly elliptical in plain view and are aligned in an approximately E–W direction, consistent with the trend of the whole Snaefellsnes Peninsula. As regards the assessment of this geosite, the scientific criterion that can be applied here is representativeness; this is a very clear example of the appearance of such scoria cones all over the country, and in many other volcanic regions of the world. With regard to the educational value, this site is easily accessible, but its didactic relevance is rather low, as it would be difficult to explain to nonexperts the topic of magma paths and their relation to the geometry of scoria cones, as well as to the alignment of multiple monogenetic edifices.
Along the central-southern coast of the peninsula, a very complex system of regional dykes crops out; dykes are major subvolcanic features [108
] that are responsible for feeding fissure eruptions [74
] and flank eruptions at volcanoes [109
]. In some cases, they can also induce the destabilization of volcanic edifices, potentially leading to lateral collapse [110
]. The outcrop in Figure 3
b represents a geosite that is suitable for showing the geometry of dykes in the field.
On the southern side of the Senaefellsnes Peninsula are located the Midhyrna and Lysuskard intrusions, very clear examples of extinct magma chambers that once stored magma feeding eruptions at the surface. Of particular interest are the swarms of inclined sheets surrounding the two intrusions. Sheets are subvolcanic bodies that channel magma from a deep reservoir to the surface. In the field, they are clearly distinguishable from dykes, as the latter are almost always subvertical or vertical, whereas sheets are always dipping at a low angle. Iceland is one of the places on Earth where sheet swarms can be better observed and studied [108
]. At Snaefells, as is the case in other locations in Iceland and at the Isle of Skye [111
], the sheets are inclined towards the subvolcanic bodies from which they were injected. The outcrop in Figure 3
c represents a geosite that may be functional for explaining the geometrical appearance (dipping at a shallow angle) of sheets in the field that are different from those of dykes (which are vertical or subvertical). In the central-southern portion of the peninsula, the 384 sheets that were mapped are arranged in a particular fashion around the Midhyrna and Lysuskard subvolcanic intrusions [47
]. The two aligned ridges in Figure 3
d can be considered a unique geosite (laterally extended for 3.8 km), which is key to pointing out a rather complicated process hardly ever observed in subvolcanic geology: the inclined sheets in the western part of the photograph preferentially dip to the E at a shallow angle towards the location of the Lysuskard composite doleritic–granopyric intrusion, which represents the magma chamber off which they were injected. In the central sector of Figure 3
d, there is the intersection of two different swarms of sheets (highlighted as thin white lines), which dip towards either the W or the E. In the eastern zone (not included in Figure 3
d), the sheets dip preferentially towards the Mydhirna doleritic intrusion. Another feature that is frequently possible to view in the Snaefellsnes Peninsula is represented by sills, subvolcanic intrusions with a subhorizontal attitude [108
], concordant with the underlying and overlying host rocks (geosite in Figure 3
e); especially worthy of attention is the presence of well-developed columnar joints that are formed during the cooling of magma (geosite in Figure 3
f). At the scientific level, all the above geosites are highly representative of the subvolcanic processes that are common in volcanic regions, such as Iceland. All have been the subject of scientific research, leading to publications [47
]. Moreover, they are easily accessible and, with the sole exception of the volcano–tectonic process leading to a sheet swarm intersection (Figure 3
d), all could be suitable for teaching the basics of subvolcanic geology to a public of nonexperts.
4.2. Geosites in the Northern Volcanic Zone
Moving from W to E, the first geological feature in the NVZ is the aforementioned Husavik–Flatey Fault (HFF), an amazing example of an oceanic transform fault that can be observed along its emerged prolongation (Figure 4
a). The HFF, together with the Grimsey and Dalvik lineaments, compose the so-called Tjornes Fracture Zone, which connects the NVZ to the Kolbeinsey Ridge (Figure 1
a). The HFF has an impressive appearance in the field, separating pre-Quaternary from Quaternary volcanites (Figure 4
b) and offsetting structures, such as lava tubes (Figure 4
c), in a dextral sense. The clearest exposure of the fault is the one in Figure 4
d, where the sheer size of the fault plane is visible in its completeness; here, the location of Husavik town a short distance away from this gigantic tectonic element provides an eerie reminder of the seismic hazard the town is prone to. Another view of the dextral displacement along the fault is given in Figure 4
e, where the fault clearly offsets gullies and water divides. Figure 4
f depicts the above-explained triple junction: the field photograph captures one of the few tectonic interactions, visible on Earth’s surface, between a transform fault (the HFF) and a rift system (the ThFS), whose northwesternmost fault (the Gudfinnudgja Fault) can be seen in the distance, with its about 30-m vertical offset [61
The five tectonic geosites that have been introduced above are all of the linear type, as they are related to a strike–slip fault. They can be considered active geosites, as the HFF has produced four historical earthquakes in the last 200 years [61
], and displacements along any sectors of this major fault may take place in the future. Their scientific value is considerable due to the following reasons: Firstly, they are highly representative of the appearance of a major strike–slip structure in the field. Secondly, they belong to a fault that has been documented in a great deal of high-profile publications, as mentioned above. They are also extremely rare at the scale of Iceland, because they belong to an oceanic transform fault that extends onshore, a process that takes place only in this portion of the island. It is worth noting that one of the five geosites in Figure 4
is rare also at the worldwide level—the textbook-example of an emerged triple junction shown in Figure 4
With regard to the educational value of the geosites, all are suitable for explaining the activity of a strike–slip fault. Particularly worthy of notice in this respect are the geosites in Figure 4
c,d. The former is a very good example of the movements along a strike–slip fault, which are easy to visualize and understand thanks to the presence of a displaced lava tube. The gigantic fault plane in Figure 4
d, on the other hand, is key to explaining the existence of a dip–slip component of movement superimposed on the strike–slip one. The geosites in Figure 4
b,d are easily accessible, as opposed to the other three, which would require potential visitors to walk long distances across a harsh volcanic landscape. To our knowledge, none of the above geosites have been the focus of educational activities, probably also on account of the difficulty to access them. The second selected area in the NVZ is the Theystareykir Fissure Swarm (ThFS), marked by the presence of geosites that are representative of a number of active volcano–tectonic processes, such as faulting and fissuring, as well as the development of central volcanoes and associated geothermal areas.
a is the geological map of a portion of the ThFS about 15 km south of the triple junction; here, the older volcanic and volcanoclastic units pre-date the Last Glacial Maximum (LGM), whereas the younger ones were emplaced after the last deglaciation. In this area, we selected a few geosites, among which a geothermal area (Figure 7
b), home to several pools of hot mud. Another geosite is a 30-m-wide and 300-m-long volcano–tectonic graben, described in detail in a recent paper [39
]; this extensional structure (Figure 8
a) is bounded by two main normal faults, striking NNE–SSW, affecting 2.4-ka-old lavas. Another image (Figure 8
b) documents the offset (12 m) produced along one of the many dip–slip faults that compose this active rift system; a third UAV-captured image, depicting a fracture field (Figure 8
c), enables observing a set of extension fractures, roughly parallel to each other, affecting older, pre-LGM lavas and marked by dilation amounts > 40 cm (in the range of 40 to 120 cm). A close-up field photograph (Figure 8
d) visualizes one of the thousands of extension fractures affecting post-LGM lavas (with dilation from a few centimeters to about 40 cm); here, clear “piercing points” can be spotted, suitable for assessing the vector of fracture opening and the amount of dilation. Finally, Figure 8
e shows the Stórihver recent volcanic cone, with a crater of 60 m in diameter.
Regarding the assessment of the six geosites (Figure 7
b and Figure 8
a–e), all have a major scientific relevance. In fact, they belong to an active rift that has been intensively investigated in the last three decades, as documented by several high-profile scientific publications, some of which are mentioned above. Moreover, all the selected geosites are highly representative of active processes, both geothermal and tectonic ones, the latter leading to the development of dip–slip faults and extension fractures. As opposed to the geothermal and tectonic geosites, which are all representative of presently active processes, the volcanic geosite displayed in Figure 8
e is the result of a localized eruption that resulted in a monogenetic volcanic edifice that is nowadays extinct. With regard to rarity, none of the geosites can be regarded as uncommon, because most of Iceland is pervasively cut by faults and fractures and dotted with monogenetic cones in response to ongoing crustal extension and hot spot-related volcanism.
The above geosites could be undoubtedly used for didactic purposes, as they enable us to explain active extensional processes that can be easily understood thanks to the favorable exposure of the outcrops. However, the educational value of four out of five of the considered geosites is hampered by their limited accessibility; apart from the graben in Figure 8
a, whose location is reachable by car, all the others are found in remote areas; moreover, the floor of the ThFS is riddled with gaping fractures and holes, which make the area relatively unsafe for nongeologists.
In order to overcome these limitations, we produce a series of 3-D models, as illustrated hereunder. The first example (Figure 9
a) is a 3-D view of the 30-m-wide graben (cut by the road), previously shown in Figure 8
a, which represents a typical effect of extensional tectonics across the ThFS.
The 3-D model visualizes, with exceptional detail, the two sets of opposite-dipping normal faults that border the graben, as well as the low-lying floor of the volcano–tectonic structure. The model is also instrumental in highlighting that tectonic subsidence across the graben floor has developed in a differential fashion, as attested by the fault system to the WNW (upper part of the figure), marked by a greater offset than its counterpart to the ESE. The second 3-D model (Figure 9
b) portrays the above-illustrated dip–slip fault (Figure 8
b); here, the geometry of a recent, active dip–slip fault with a steeply-dipping fault plane separating two horizontal surfaces (the top of the footwall block and the hanging wall block, respectively) can be viewed in great detail. The third model (Figure 9
c), aimed at providing a clearer picture of the effects of extensional tectonics, is focused on a segment of the previously shown fracture field (maximum opening 3 m) affecting pre-LGM lava units (Figure 8
The third area in the NVZ is a segment of the northern portion of the Krafla Fissure Swarm (KFS) at a location that is north of the Krafla central volcano. The geological map in Figure 10
a enables us to observe that, as opposed to the ThFS, the KFS is marked by the presence of historical lava fields, as well as both historical and pre-LGM volcanic centers.
In particular, as reported by Hjartardóttir et al. [117
], two major rifting episodes took place within the KFS in the last 1140 years: the 1724–1729 “Mývatn rifting episode” and the instrumentally recorded “Krafla rifting episode” (better known as “the Krafla Fires”), which occurred from 1975 to 1984. During both episodes, there were periods of strong earthquake activity and motions along a number of faults (accommodating the widening and subsidence along tectonic graben). During the “Krafla Fires”, the continuous emplacement of dykes resulted in the opening of eruptive fissures, which, in turn, led to lava fountaining and the outpouring of lava flows.
We individuated six geosites in this area: The first one, displayed in Figure 10
b, is a textbook-example of a recent volcano–tectonic graben affecting post-LGM lava units bordered by two dip–slip faults that diverge from a common point (highlighted by a yellow circle in Figure 10
a). The graben floor, with a maximum width of 90 m, is affected by active stretching, as testified to by the development of an extension fracture field. The other volcanic and tectonic geosites in the KFS are the following: a cluster of recent monogenetic volcanoes (scoria cones), two of which are visible in the background and one at the center of the image (Figure 11
a). The larger cone in the foreground (350 m × 150 m) was formed in 1984, at the end of the “Krafla Fires” eruptive cycle [118
Especially notable is a very recent pahoehoe lava flow, which was outpoured by the crater in the foreground. Another geosite is one of the typical extension fractures (with dilation between 1 and 1.5 m) formed within the lavas older than 7 ka (Figure 11
b). As is the case in the ThFS, in the KFS, the wider extension fractures are found within the older lava units. In Figure 11
c, a field photograph documents a historical lava flow (emitted during the “Krafla Fires”) coming from the left-hand side of the image, which partially infilled a 2-m-wide extension fracture. Figure 11
d is a UAV-captured image, offering a chance to take a look at a geosite composed of the combination of a volcano and an extension fracture field. In the background of the image is the Hituholar volcanic edifice (500 m of basal diameter in E–W direction), made of hyaloclastites at the base and scattered pillow lavas in its upper portion. From its southern base, and extending southward, a fracture field cuts both through 12-ka-old lava units and the volcanic edifice. Finally, in Figure 11
e, another UAV-captured image enables us to observe a geosite made of the two main extension-related structures that characterize active rift zones such as the KFS: These are N–S-trending dip–slip faults (marked by important offsets) and N–S to NNE-trending extension fractures, whose main characteristics are the absence of vertical displacement and the presence of a major dilation compatible to the regional extensional regime. The individual fault and the fractures composing the geosite in Figure 11
e are approximately parallel and spaced about 130 m from each other.
As far as the assessment of the KFS geosites is concerned, all have a considerable scientific value, attested by the several research efforts and publications dedicated to this active rift zone and the recent volcanic episodes that took place there. All are representative of volcanic and tectonic processes within an active rift system. However, none can be considered rare, for the same reasons that were cited above in regard to the ThFS geosites.
Their educational relevance is generally high, though special mention has to be made to the two geosites in Figure 11
a,c. The former is suitable for explaining a number of volcanic features (the geometry of monogenetic cones and the morphology of a recent lava flow) that may be easily understood also by nonexperts. The geosite in Figure 11
c enables documenting of the interaction between volcanic (a lava flow) and tectonic processes (an extension fracture). However, as these geosites are located in remote areas (relatively unsafe as well, due to the presence of a great number of fractures and holes), their accessibility is limited, and their educational value is, therefore, greatly diminished.
As was the case for some of the geosites in the ThFS, in this case, we created some 3-D models: The first one (Figure 12
a) depicts a 100 m × 200 m area marked by the presence of a set of very long and wide (as much as 4 m) extension fractures cutting through 12-ka-old lavas. The 3-D model in Figure 12
b depicts a major N–S-trending dip–slip fault (with a 10-m vertical offset) in a similar way as in Figure 9
b. Finally, the model in Figure 12
c is aimed at capturing a rather common, yet spectacular, occurrence in active rifts: a historical, basaltic, pahoehoe lava flow cascading into a 2.3-m-wide extension fracture. The viewer has a chance to take a look at the so-called ogives, particular structures (common in basaltic, pahoehoe lavas) produced by the bending of the crust during the movement of the underlying, still-molten lava; the convexity of the ogives points in the direction of the lava flow.