Náttúrufræðingurinn - 1970, Blaðsíða 64

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NÁTTÚR U FRÆÐIN G U RIN N
S U M M A R Y
On Lava Balls
by
Jón Jónsson,
National Energy Autliority, Reykjavík.
In the Poldervaart Treatise on Rocks of basaltic Composition (Vol. I, p. 10)
G. A. Macdonald describes features which he calls accretionary lava balls.
Similar features are fairly common in basaltic lava flows in Iceland. They are
of variable size ranging from few centimetres to 2,5—3 m. At several localities
in Landbrot and Medalland, Southern Iceland such lava balls occur. They are
remarkable in being filled with pebbles, sand and evcn silt, which occasion-
ally ntay show regular layering. The balls occur near the top of the lava flow,
at its edge and in pseudocraters. Srnall lava balls filled with diatoms and
other organic matter are also reported. This material originates frorn the layer
immediately below the underlying stratum ancl some of the balls have been
brought front the bottom oí the flow to its top. In the top of the Laki lava
from 1783, rounded rliyolite pebbles have been found, which clearly originate
from the alluvial deposits below, the fragments being completely enclosed by
the molten lavas.
In the top of the lava erupted from Vikraborgir in Askja 1961, the author
saw fragments of acidic pumice originating from the Askja eruption 1875.
The new lava had flown over the pumice layer the day before and pieces of
the acidic pumice had been brought up to the surface of the moving lava.
This shows that „lava advances with a rolling motion resembling the front end
of the endless track on a tractor", as G. A. Macdonald has suggested. From this
it is obvious that xenoliths of older and unrelatecf rock types found in lava
flows don't necessarily have to come up through the volcanic vent itself.