Náttúrufræðingurinn - 2009, Blaðsíða 36

Náttúrufræðingurinn
100
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Um höfundinn
Árni Hjartarson (f. 1949) lauk B.Sc-prófi í jarðfræði frá
Háskóla Íslands 1974, M.Sc.-prófi í vatnajarðfræði frá
sama skóla 1994 og Ph.D.-prófi frá Kaupmannahafnar-
háskóla 2004. Hann hefur starfað sem sérfræðingur
hjá Orkustofnun og starfar nú hjá Íslenskum orkurann-
sóknum.
Póst- og netfang höfundar/Author’s address
Árni Hjartarson
Íslenskar orkurannsóknir/Iceland Geosurvey
Grensásvegi 9
IS-108 Reykjavík
Arni.Hjartarson@isor.is
Summary
The Búrfellshraun lava and
the caves of St. Mary
Búrfellshraun lava near Hafnarfjörður
SW-Iceland was erupted 8000 years ago
(~6000 BC) from the cinder cone Búrfell
(Table 1). The crater is located near the
eastern border of a shallow rift valley
belonging to the Trölladyngja volcanic
rift system. It is only 7 km from the cen-
tre of Hafnarfjörður town. The lava is
olivine tholeiitic with prominent phen-
ocrysts of feldspar and olivine. The
eruption can be divided into four phases
(fig. 2).
I. The lavas from the initial phase are
entirely covered by younger lavas
but can be found in boreholes and
small exposures near Straumsvík.
From the crater it flowed along the
rift valley towards SW. Then it left
the valley and entered the shore near
Straumsvík.
II. In the second phase the lava found its
way between the hillocks SW of
Hafnarfjörður and entered the shore
were the harbour is now. Part of this
lava tongue is covered by lavas from
phase 4.
III. The lava from this phase is mostly
visible at the surface. When the lava
had filled up the rift valley it found a
new course towards the shore just
NW of the crater and covered the
coastal area around Hafnarfjörður
harbour. During this phase the lava
channel of Búrfellsgjá and related
lava tubes were developed.
IV. In the final phase small lava fields
around the crater were formed along
with the lava channel Kringlóttagjá.
Maríuhellar, the caves of St. Mary, are
the best known of many caves in
Búrfellshraun (Table 2). They have been
developed in the same lava tube that
has collapsed in two places to form
three individual caves (fig. 5). In earlier
times they were used as shelter for
sheep. In the last decades the fourth
member has been added to the group, a
single lava shelter in the neighbourhood
that has got the name Jósefshellir, St.
Joseph’s Cave. The reason for the name
of Maríuhellar is probably that in the
middle ages the caves were owned by
the church and monastery of Viðey,
which were dedicated to the holy moth-
er of God.