Geologic Pictures from Iceland
by Peter Bird, August 2003
Basalts and active spreading
Reykjanes (the Mid-Atlantic Ridge comes on land)
The failing West rift: Þingvellir
The active East rift
1783-1784 AD Eldhraun flow from Laki fissure, East rift
typical surface in Eldhraun (covered with moss, due to daily rain on the south coast)
Neogene layered basalt flows in southeastern Iceland (reflecting smoother pre-Pleistocene topography)
A dike north of Höfn
Breiðamerkurjökull and Jökulsárlón:
Breiðamerkurjökull, showing slope (relative to horizontal cloud ceiling)
Pleistocene (when all Iceland was under an icecap):
Jokulhlaups and sandurs
Silicic and/or explosive volcanism
granodiorite: Iceland has silicic magmatism, too! Presumably, silicic magmas form by (a) partial re-melting of basalt crust at its base (due to deep burial), and/or (b) fractional crystallization of large volumes of gabbro in a magma chamber, leaving a residuum (e.g., Hekla?).
tephra field of the 1104 AD eruption of Hekla This eruption was comparable in volume to the 1980 main eruption of Mount St. Helens in Washington state. It destroyed more than a dozen farms in Þjórsárdalur, and the area has still not been reoccupied.
Isostasy in action
isostatic rebound of a small iceberg (following faster melting of the upper parts, which are exposed to warmer water and/or to weather)
Hvannadalshnúkur: The highest mountain in Iceland is found near the coast, where precipitation, and thus glacial erosion, have been greatest.
All stratified lavas in east Iceland dip to the west: (This is due to loading on top by younger flows to the west.)
a fracture zone scarp(?) in eastern Iceland. The unnamed sinistral transform fault that once connected (and may still connect?) the N end of theWest rift to the S end of the North rift is obscured by surface deposits, but there should still be an imprinted age contrast across a NW-SE line within the Eurasia plate. The NE side should be younger by about 10 Ma, and therefore higher. Even though this fracture zone(?) has not been mapped, and its inactive fault may be buried, one can infer it from the topographic "wall" of mountains just NE of the port town of Höfn (orange dot in this model). Another NW-SE-trending mountain "wall" which might be a fracture zone scarp is found 85 km to the southwest, from Skaftafellsfjöll to Hvannadalshnúkur. Notice the Z-Z shape of the margin of the highlands in this view looking SW across a topographic model.
Erosion of stratified volcanics: waterfalls, columns, & caves
Dyrhólaey (southern tip of Iceland):
stratigraphy: thick basalt flows (with columnar jointing) over sandur deposits
basalt column: basalt flows break easily along columnar joint fractures
Geysir geothermal field
Unusual building materials
Disclaimer: The geologic captions for the pictures on this page include a lot of educated guesses. Anyone who thinks, or knows, that I misinterpreted what I saw is encouraged to e-mail me with a correction. P. Bird, 2003.09.15.