Acta naturalia Islandica - 01.02.1946, Blaðsíða 40

34
TRAUSTI EINARSSON
may be considered equaily typical depart widely from these in the
manner of distribution of their ejecta.
Probably volcanic processes of several kinds were responsible for
the building up of the thick beds of basic breccia in Yellowstone
Park.. We should not suppose that the ordinary explosive ejection
of blocks and lapilli was lacking here. The deposits in Mount Wash-
burne and Sepulcher Mountain, for instance, are close to vents, and
a portion,at least, was explosively ejected. Probably in general this
process was an important factor. This, however, would tend to heap
up material close to the vents, and if these accumulations remained
undisturbed, we should hardly have the miles of horizontal strata
or the rounded boulders that we see.
The fonnation of the spine of Montagne Pelée, and the accompany-
ing avalanches of clastic material that occasionally burst forth from
it in the form of nuée ardentes, have called attention to a type of
eruption that has since been recognized as of common occurrence.
It is now generally accepted that in such cases very viscous lava
is pushed up from a vent in the form of spines, domes, or plugs. At
intervals, shattering explosions occur, which disrupt portions of the
rnass and cause it to rush down the slopes. Continued evolution of
gas from the shattered blocks is believed to be an important factor
in maintaining mobility, and these streams of fragmental material,
carrying large boulders, have been known to continue their course
for miles after reaehing gentle grades.
By a variation of this process a new element is introduced. In the
volcanoes of Java especially, but also in other regions, the waters
of the crater lakes have been involved in eruptions, and wide-
spreading mud-flows have resulted.
Though the evolution of magmatic gases or the presence of water
from crater lakes has been definitely recognized as involved in these
flows, and must have greatly increased mobility, in other instances
boulder-flows of great magnitude have occurred in which only a
modicum of water or mud seems to have been present as a lubricating
agent, and yet an astonishing facility of movement has been ex-
hibited. A well-known instance is that of the eruptions of Bandai
San, in Japan, in 1888. This has been attributed to a subterranean
explosion, by which a part of the Mountain, of enormous magnitude,
was upheaved and set in motion. It flowed down from the Mountain
into the lowland, and covered an area of more than 25 square miles.