Acta naturalia Islandica - 01.02.1946, Blaðsíða 73

ORIGIN OF THE BASIC TUFFS OF ICELAND
67
2. The temperature is lower and the melt is very viscous, at least
after having reached the surface of the earth, in spite of the gas
it still retains. Yet, it has still fluidity enough to flow in a thick
stream. The decrease of pressure from bottom to surface in the flow,
local differences of gas content, and the continued loss of gases from
the lava mass may have conspicuous effects on the mode of con-
solidation. A composite lava is formed, consisting of three more or
less distinct, roughly parallel zones, viz. columnar, very fine-grained
lava, a lava of block jointing, and pure, mostly fragmental glass. The
predominance of one zone or another varies from case to case, and
all degrees of variations from an entirely crystalline lava to pure glass
will be possible. Most often the columnar zone is at the top and the
glass at the bottom, but the order may be the reverse.
3. The melt, probably after losing some of its gases and while still
losing gases flows on the surface in a thick, very viscous flow and be-
comes entirely glassy. Somtimes a compact, although more or less
jointed mass of glass is formed, more often, however, the glass, on
cooling, crumbles into a mass of porous fragments.
4. At a still lower temperature the magma consolidates to glass
already on its way up to the surface on account of the loss of gases,
and a mixture of glass fragments and gases flows out of the volcanic
vent. A typical exemple of this is the flow on Skriða (p. 55). and it
seems to have a modern representative in the sandflow of the Valley
of Ten Thousand Smokes.25) The young heaps of fragmental material
(Jarlhettur, Hítardalur etc.) are also of this type.
This phase might also be explosive and was probably often so. The
main volume of the Younger tuffs in Iceland is probably of type 4,
especially sandflows and breccia flows.
5. Deep explosions and a tumultuous whirling of glass fragments
and pieces from the wall rock produce a mixture of worn glass frag-
ments and more or less rounded blocks and pebbles. This mixture
seems to flow rather quietly to the surface of the earth, the blocks
then getting scratched and even faceted as was the case in Swa-
ben.18) On extrusion this porridge may have considerable mobility
thus giving rise to layers of grey conglomerates which are of
markedly morainic appearance.
In some cases, as in the dyke in the north slope of Ljósavatns-
skarð (p. 48) and in the veins in the southern escarpment of that
pass, foreign blocks are absent, and we get mainly rounded grains