Jökull - 01.01.2013, Page 3

The Stardalur magnetic anomaly, SW-Iceland
Figure 1. Outline map of western Iceland, show-
ing approximate locations of several large mag-
netic anomalies found by Sigurgeirsson (1970a,
1979). Sk: Mt. Skálafell, St: Stardalur, F: Fer-
stikla, Hv: Hvanneyri, Ha: Mt. Hafnarfjall, B:
Breiðakinnarsandur. Prominent negative anoma-
lies also occur between Reykjavík (R) and Kjal-
arnes (K), see Jónsson and Kristjánsson (2002).
The position of the main anomaly peak within the
Stardalur caldera is shown with a cross in the in-
set. The outlines of the Hafnarfjall central vol-
cano and two calderas within it (from Jóhannes-
son and Sæmundsson, 2009) are also shown. –
Kort af Vesturlandi. Nokkrir staðir þar sem óvenju
mikil segulsviðsfrávik komu fram í flugmælingum
Þorbjörns Sigurgeirssonar eru merktir. Einnig er
sýnd afstaða hámarks-segulfráviksins í Stardal til
öskjubrúnar þar, og útlínur Hafnarfjalls-megin-
eldstöðvarinnar með tveim öskjum.
tiary lavas in Iceland where F (the magnitude of F)
is currently close to 50 µT and the average suscep-
tibility is about 0.025 SI units, the average induced
magnetization has an intensity Mi of around 1 Am−1.
The other vector quantity is remanent magnetization
(remanence, Mr) which has generally originated in
part during cooling of the material from its Curie point
(transition temperature to ferromagnetic behavior) to
a much lower temperature. This is called thermal re-
manence, TRM. Another way of acquiring remanence
is through the growth of new magnetic minerals in
the material (chemical remanence, CRM). In a cool-
ing lava, these two processes may well operate more
or less simultaneously, giving rise to thermo-chemical
remanence (TCRM). Additionally, remanent magneti-
zation often builds up gradually in a material when it
is in an external magnetic field of constant direction.
The time constant for this buildup (which generates
a viscous remanence, VRM) can be from seconds to
millions of years. For a rock in situ, all these com-
ponents constitute its natural remanence (NRM). In
exposed relatively unaltered Tertiary lavas in Iceland,
the arithmetic average NRM intensity Mr is about 4
Am−1 (Kristjánsson, 2002). It is most commonly
dominated by TRM which can be either in a direc-
tion close to that of the present geomagnetic field, or
JÖKULL No. 63, 2013 3