39. Hvalfjarðareyri Spit - Rhyolitic spherulites, zeolites and dykes

From Hvalfjarðareyri, SW Iceland. Photo Sigurður G. Kristinsson.The bedrock in the Hvalfjörður area belongs to a sequence of late Tertiary to early Quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites. The basalt has been affected by low zeolite facies metamorphism, caused by the burial of the lava succession and high heat flow from the Hvalfjörður and Kjalarnes central volcanoes.

Basaltic and rhyolitic dykes from these volcanoes can be seen cross-cutting the bedrock along the coast. Along the coast, one can also find several types of amygdules. The formation of amygdules depends on temperature, the type of rock and the composition of the circulating fluid. Amygdules in the area are mostly zeolites and silica minerals. The most common zeolites are chabazite, stilbite, analcime, mesolite, thomsonite and heulandite.

Hvalfjarðareyri (Hvalfjörður Spit) is on the south coast of Hvalfjörður east of the Ósmelur moraine. It is a well known locality for finding spherulites, which are small, round, reddish-brown pieces of rock, often found in rhyolite lava. Spherulites form when gas is trapped in the lava, and needles of feldspar and quartz grow radially from a common center. They are commonly 0.5-3 cm in diameter and have often grown together. 

Access

Map of Hvalfjardareyri Spit - Rhyolitic spherulites, zeolites and dykes.Access to Hvalfjarðareyri is convenient. One can drive down from road nr. 47 on to a gravel track. There is ample parking space near the main road and from there one can walk 500 m along the track down to the seashore. Hvalfjarðareyri can also be accessed from Hestaþingshóll, which is slightly further to the east. Travellers should be aware of the tides when walking on the spit and along the coast.

  

Sigurður G. Kristinsson, 2010