Iceland Excursion

Stop 1: Reykanestá and Stampar (Submarine and terrestrial eruptions, feeder dike, base surge)
Start from the international Airport at Keflavik and takr the road no. 44 to the south heading to Hafnir. Head directly south on road no. 425 through Hafnir and to the salt factory at Reykjanes and follow the signs to Reykjanesviti (the lighthouse at Reykjanes).
The Reykjanes ridge comes at shore at the tip of the Reykjanes peninsula. The geology is characterized by recent lava flows and open tensional fissures. At this place we are in the central part of the Reykjanes fissure swarm. At the shore we find remnants of a phreatic crater from the year 1226 which formed the Medieval tephra layer. Part of the crater has been overrun by lava flows from the same eruption. In a small cliff a feeder dike is exposed that can be traced to a spatter cone formed in that year.  
Nice pillow lava can be seen at Bæjarfell, where the present lighthouse stands, and Valahnúkur, where the foundations of the old lighthouse can still be seen.  The pillow lava belongs to a sequence of steep móberg hill composed of pillow lava and hydroclastic tephra and breccia.
Vatnsfell is a rather inconspicuous rise on the coast north Valahnúkur, but is worth a visit for a closer look at the internal make-up of tuff cones. It is made up of lapilli tuff deposits, which are the remnants of two tuff cones formed by submarine explosive eruptions in the early stages of the Reykjanes Fires in about 1210. The internal structures of theses cone are best exposed in otcrops apposite Karl, the sea stack rising, about 300 m off shore. Theolder cone,which makes up the bulk of Vatnsfell proper, was 30 m high and 650 m wide, with the crater located 100 m off shore. The lower part of the older tuff cone consiss of wavy to cross-bedded ash deposits formed by repeated base surges, whereas the upper part consists of coarser-grain lapilli tuff composed of alternating base surges and tephra fall units. On either sides of Vatnsfell, the older cone is capped and flanked by the deposits from the younger one, consists of fine-grain lapilli tuff with repeated pairs of base-surge and tephra-fall layers. Many impact craters formed by ballistic blocks ejected from the crater during the eruptions commonly disrupt bedding in both cones. 

Reykanesviti (lighthouse)

Pillow lava at Valahnúkur

Base surge deposits, Vatnsfell tuff cone

Stampar feeder dike

Stop 2: Krysuvik (High temperature geothermal area)
Continuing to the west on road no. 425, passing the village Grindavik and follow the gravel road no. 427 to Krysuvik.
High temperature geothermal area – High temperature areas occur only in active volcanic zones and probably mainly where there are shallow magma chambers or intrusions cooling underneath inactice volcanic areas. Thus high temperature areas are localised areas of upward ground water flow in active volcanic areas. There are mainly two types of edifice in high temperature areas, fumeroles and solfataras.
Solfataras are common in high temperature areas. They contain blue-grey boiling clay, the colour being due to sulphur compounds of iron which form when sulphuric acid dissolves the rock, the acid forming when hydrogen sulphide (H2S) reacts with atmospheric oxygen. In the solfataras the clay splashes as gas bubbles burst and the clay builds up a rim around them. The rocks of high temperature areas are usually greatly altered.

Krysuvik geothermal field

Krysuvik geothermal field

Stop 3: Hvalfjörður (Late glacial and intrabasaltic sediments, zeolite facies)
Take road no. 1 from Reykjavik to the north. Just a few hundret meters before the Havlfjörður tunnel turn right and take the old ring road (road no. 47).
The route to Hvalfjörður from Reykjavík passes below the slopes of Mt. Esja (914 m), which dominates the horizon north of Reykjavík. Mt. Esja is the result of Pliocene-Pleistocene volcanism, and contains basaltic lavas as well as gabbroic intrusions and rhyolite, numerous basaltic dikes, clastic (volcanic and glacial) sediments. The Esja succession covers about 1.3 million years, between 3.1-1.8 million years ago. The mountain got its present shape as a result of repeated episodes of glacial erosion during the Pleistocene. The western part of Mt. Esja is an eroded flank of the Kollafjörður caldera volcano, which was active 3-2 million years ago. At the base of the mountain there are basalts and dolorite dikes, but higher in the strata horizons of palagonite (formed by supglacial eruptions) and tills appear. The eastern part of Mt. Esja and Móskarðshnjúkar (rhyolite) are the remains of a younger volcanic centre, the Stardalur central volcano, active after 2 million years ago.
For getting a glimpse of the Pliocene strata can be visit at a section at the base of Mt. Esja at a site called Kiðafelssá, where clastic sediments of glacial nature outcrop close to sea level, and have a discussion on the origin of the sediments. Approximately 120 m of tholeiitic and olivine tholeiitic lavas separate the two earliest diamictite units in Hvalfjörður. The younger unit forms an impressive cliff along the coastline of the fjord Hvalfjörður, and is interpreted as a localized succession of flow units based on its overall stratigraphic framework and composition. The diamictite which in places rests on stratified fine-grained sandstone and conglomerate, displays a facies association consisting of massive, matrix supported diamictite overlain by clast supported diamictite intercalated with finer grained waterlain sediments and sediments of fluvial origin. The overall appearance of this section suggests continuing sedimentation with occasional interruption of single lava flows. Pebble fabric from this exposure is inconsistent and in general shows a unimodal pattern. Post-depositional tectonic and igneous activity is indicated by basaltic dykes and mud dykes that cut through the diamictite (Geirsdóttir, 1991).
Burial of the lava pile and the higher heat flow through the hydrothermal activity in relation to central volcanoes have formed various zeolites, which can easily be collected on the shore line of the Hvalfjör
ður. By careful study  of the zeolite variation, zeolite facies zones can be detected like in Eastern Iceland. Additional dolerites and rhyolites associated with the central volcanoes can be found around the fjord.
The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjör
ður area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (Stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after burial and is marked by zeolite mineralization caused by higher heat flow from the Laxárvogur and Hvalfjörður central volcanoes. Altogether eleven different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled “zeolite zones” occur in the Hvalfjörður area.

Geological map Hvalfjörður

Basalt steps, dipping in SE direction



Thin section showing alteration




Zeolite facies

Stop 4: Þingvellir (Plate boundary, volcanism and history)
Take road no. 448 from Hvalfjörður until you reach the crossroad with road no. 36. Turn to the east and head the þingvelir.
Þingvellir area is a part of the North Atlantic rift system. It can be described as an area of sea-floor spreading, displaying the close association of crustal rifting and volcanism. The rift zone constitutes a graben bounded by major faults. The rift valley narrows from about 20-25 km in the NE to about 10 km in the SW. The extension appears as nearly parallel fissures and down-dropped fault blocks running along the length of the valley. On the west (North American) side of the valley, the blocks step down toward the east, while the situation is reversed on the east (European) side. Thus the valley floor is a linear block that has subsided as the valley walls were pulled apart by plate motions. The valley walls are moving apart at a rate of about 7 mm per year, and during the past 9000 years the estimated horizontal extension is of the order of 70 m. The floor is subsiding at about 1 mm per year, with a total subsidence of 40 m for the past 9000 years. Rifting within the graben is episodical, with the last major earthquake activity occurring in 1789. During the 1789 earthquakes the graben floor subsidised 1-2 m.
Almannagjá is 7.7 km long. Its greatest width is 64 m, and its maximum throw is 30-40 m. It marks the eastern boundary of the North American plate. Its equivalent across the graben, marking the western boundary of the Eurasian plate is Hrafnagjá. It is 11 km long, 68 m wide and has a maximum throw of 30 m. The Þingvellir faults are believed to be the surface expressions of deeply rooted normal faults. The numerous fissures encountered on the valley floor are of similar origin.
The bedrock of the Þingvallavatn catchment consists mostly of postglacial lavas that are most extensive in the central part of the graben and Late Pleistocene pillow lavas and hyaloclastites of subglacial origin and sub-aerial lavas. The Þingvellir graben is surrounded by volcanoes, that illustrate the connection between rifting and volcanism. 
The Þingvellir lava (Þingvallahraun) floors the northern part of Lake Þingvallavatn and the graben floor north and east of the lake. It originated in a major fissure eruption to the southeast of Hrafnabjörg, around 9100 years ago. The many single flows of this lava are best exposed in the fault scarp of Almannagjá, where numerous sheets of individual lava lobes have been successfully stacked as the eruption progressed.
Mt. Skjaldbreiður (1060 m) is a huge shield volcano that dominates the horizon to the north of the Þingvellir graben. It was formed during a prolonged eruption about 9000 years ago.





Stop 5: ? (????)