33. Dyrafjöll - Hyaloclastite ridges and faults

Tindar(s) and tuya(s) are geological terms. They apply to subglacial fissures and central eruptions of pillow lava and hyaloclastite, the latter capped by lava as a rule. A third type is pillow lava mounds or ridges of smooth outline. The tindars form long ridges or a chain of hills, and their hyaloclastite gives rise to sharp and sometimes serrated landforms. Dyrafjöll northeast of Hengill is an example. Three groups occur, two of them crossed by the Nesjavellir road. The older consists of aphyric tholeiite basalt forming a broad, elongated structure of hyaloclastite capped by erosional remnants of lava. The younger group is a complex of tindars and pillow lava ridges, all of which are composed of plagioclase-phyric rocks and small by volume. They run parallel from southwest to northeast and have left valleys between, some closed off when the ridges touch. Dyr (door) is a stream-cut channel between two such valleys. On its edge, a thin tillite layer is seen to separate the two groups. An interglacial shield lava east of Þingvallavatn passes underneath Dyrafjöll. The shield is probably of last interglacial age (c.f. comment 30) and the whole Dyrafjöll complex of last glacial age (i.e. younger than about 100 thousand years). Dyrafjöll are dissected by a number of northeast-southwest trending normal faults. The throw is vertical on both sides towards Háhryggur, which is topographically high but tectonically the lowest part of the rift zone between Mt. Hengill and Hestvík inlet of Lake Þingvallavatn. Nesjavellir is on the eastern flank of the rift zone. It is fault bounded on the east, but repeated eruptions have built up a range on the western side. Two fissure eruptions have occurred on this narrow eruptive strip of the rift zone in Postglacial time, at about 5000 and 2000 years ago. This particular eruptive lineament acts the main upflow zone of the geothermal system, which the Nesjavellir power station harnesses.

Kristján Sæmundsson, 2010