Acta naturalia Islandica - 01.02.1946, Blaðsíða 41

ORIGiN OF THE BASIC TUFFS OF ICELAND
35
At the time of the eruption of Mount Katmai, several great bould-
er-flows or landslides occurred, whose only connection with the erup-
tion seems to have been that the accompanying earthquakes served to
start in motion masses of rock that had been in an unstable condi-
tion. One of these flows came down the canyon of Mageik Creek
and covered the flat floor of Katmai valley over an area estimated
at two miles by three-fourths of a mile“.
“The roundness of the boulders in the Mageik Creek Slide gives
the impression that the material set in motion must have been an
old glacial accumulation. Material of that kind may have been in-
volved, but observations on another slide indicate that this is not
necessarily the case.
A few miles away, a section of steep lava-cliffs on the lower
slopes of Mount Katmai fell away, and formed a dam across Katmai
River at the upper entrance to Katmai Canyon. This dam persisted
for several years, and impounded a lake several hundred feet deep
over a wide area. When it gave way, a large part of the dam was
swept by the flood through one-half to two miles of narrow canyon
and spread out over the floor of the Valley at its lower end. The
pounding and grinding undergone by the boulders seem to have been
equivalent in their rounding effects to those produced on the ordi-
nary boúlders of river-channels through hundreds of years of stream-
action.
In making application of these various observations to the condi-
tions of accumulation of the Basic Breccia -series of Yellowstone
Park, and to many occurrences of thick beds of tuffs and breccias
in other regions, the point that it is desired to bring out is that a
vigorously active volcano forms a structure in which conditions of
instability of large masses of rock are likely to occur repeatedly.
Material from the interior of the earth is brought to the surface in
great quantity and has to be disposed of. Violent ejection is one
means of doing this, but evidently this is only one of the forms
that manifestations take. Plugs and domes of viscous material are
frequently protruded, which are likely to undergo shattering explos-
ions. Katmaian eruptions (extrusion of a sand-flow) may occur.
Lavas are poured out, which may be so liquid as to spread for miles
but may approach in viscosity that of the plugs, and congeal on
steep slopes in unstable positions. Whether an inner explosion shat-
ters a protruded mass and starts it in a motion, or whether there is