Page:Popular Science Monthly Volume 86.djvu/308

304 An examination of the dust from the volcanic eruption of Mt. Lassen shows it to be made up of fine dust and broken fragments of an acid volcanic rock which has been shattered to pieces by a violent explosion. Under the microscope there are to be seen many small angular fragments of quartz, pieces of triclinic feldspar showing twinning structure, perhaps oligoclase in composition frayed sections of brown biotite and grains of magnetite. The original rock could not have been more basic than a dacite and the presence of so much quartz rather suggests a rhyolite. The dust is not an ash in the sense of being a fine residual product of a cinder and there is no evidence of its having come from the cooling of a molten mass. The original rock seems from the appearance of the largest fragments to have been a volcanic tuff formed at some previous activity of the volcano, and the late eruption has simply blown this tuff to dust.

The eruptions of Mt. Lassen while volcanic in their general classification are in the same category as geyser eruptions the difference existing mainly in the fact that the explosions of pent-up steam are so violent as to shatter and throw rock debris in the form of boulders and dust. It is a question whether the explosions are very deep seated.

Some of the mud from the locality is of the same nature as the dust and probably formed from it.

Numerous inquiries have come to the writer as to whether the eruptions of Lassen Peak are to be considered as truly volcanic, and Professor Eakle indirectly raises the same point. This is naturally a question of definition merely. A volcano is primarily an opening in the ground from which the internal forces of the earth project various materials, molten rock being an essential product at some period in the history of the volcano.

Many of the type examples of volcanic eruptions given in standard college text-books are, however, of the explosive type, in which no molten lava is ejected. The noted eruption of Bandai-San in Japan, on July 15, 1888, is an instance. This old volcanic cone, nearly 180 miles from Yokohama, had been without sign of life for a thousand years of recorded history, yet with only a few minutes of warning consisting of rumblings and moderate earthquake shocks the entire top of the mountain was blown away in some fifteen to twenty explosions lasting less than a half hour. There was no fresh lava or pumice thrown out. Ash and steam were projected upward about 4,000 feet, but the main force of the explosion was nearly horizontal, carrying destruction in a northerly direction for about four miles. The quantity of material blown away has been estimated at one third of a cubic mile.

In the case of Lassen Peak the period of quiescence had probably been greater than a thousand years, judging from the effect of erosion on the old cone. The force of the steam explosions to date has been distributed through six months, yet the height of the ash-laden column has several times reached two miles above the mountains. Had the steam been confined more effectively in Lassen and the force, instead