Jökull - 01.01.2001, Page 34

Leó Kristjánsson and Ágúst Guðmundsson
Figure 1. Index map of South–Western Iceland. Circles indicate the approximate positions and sizes of five
extinct (Upper Pliocene – Lower Pleistocene) central volcanoes in the vicinity of Skarðsheiði. Stippled region:
Western active volcanic zone. – Lauslegt kort af Suðvesturlandi, þar sem sýndar eru staðsetningar og stærð
fimm fornra megineldstöðva í nágrenni Skarðsheiðar. Gosbeltið er skyggt.
the basalts are zeolite–free. Cores of the central vol-
canoes and their immediate surroundings have suf-
fered high–temperature alteration, resulting in min-
erals such as laumontite, calcite, quartz, epidote and
chlorites.
The lavas which were erupted before the late
Pliocene originally formed a wide plateau but sub-
sequent erosion has left a landscape of fjords, with
mountains reaching up to 1000 m above sea level.
Typical lava flows in hillside exposures have a mean
thickness of 5–10 m and a lateral extent of some km.
In the areas of Iceland where Quaternary volcanic for-
mations outcrop at the surface, material erupted under
subglacial or subaqueous conditions (hyaloclastite)
tends to be dominant. The lateral extent of each unit,
as well as the amount of spatial overlap between units,
is less than in older areas.
Stratigraphic mapping of the pre–Quaternary lava
pile in Iceland has proceeded intermittently, the best–
known effort being that of G.P.L. Walker and his stu-
dents in the fjords of Eastern Iceland in 1955–64 (e.g.
Walker, 1959). Many such stratigraphic studies on
composite sections in different areas have been ac-
companied by detailed paleomagnetic measurements
on core samples collected from selected sections, and
some have included radiometric dating.
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