Nature Notes

Vol. X December, 1932 No. 12

The Why of a Volcano

Many who have visited "The Mountain" have noted its beauty, observed is glaciers and other natural phenomena and, perhaps, reflected that it was once a raging volcano. There, perhaps, one's knowledge stops. We may wonder why the rocks that we see are what they are and of the type that they are instead of being similar to the rocks that we have seen in other localities. Also some of us may ponder over the question as to why the volcano was constructed where it is instead of, say, fifty miles to the east, west, north, or south of its location.

According to Webster's dictionary a volcano is a "vent in the earth's crust from which hot rock, steam, etc. issue; also a hill or mountain composed wholly or in part of the ejected material". Karl Sapper, a german authority on the subject, has recently defined a volcano as "a place on the earth at which magma or magmatic substances come, or have come, from the interior of the earth".

Now a magma is a molten rock material which, when poured out on the surface of the earth, becomes lava and when cooled becomes lava rock such as that which makes up the cone of Mt. Rainier or that of such an area as the Columbia River Plateau. Rocks formed in this way are fine grained rocks - that is the individual crystals of the minerals can rarely, be ascertained by the naked eye. Had this same magma or molten material cooled below the surface of the earth it would have cooled much more slowly and the result would have been a course grained (plutonic) rock. Cooling slowly the individual crystals would have had time to develop to larger size as in contrast to its having been poured out upon the earth's surface which, because of obvious factors, would have produced more rapid cooling with resulting crystals of a smaller size. Mt. Rainier's cone is composed of rock that cooled rather rapidly but this cone rests upon a base of coarse grained rock - granite or, more properly, granodiorite - which cooled slowly beneath the earth's surface long before "The Mountain" came into being. What was the earth's surface at the time was later eroded away to expose the coarse grained granodiorite - the type of material that we consider as the most prominent rock in this vicinity at about the time the lavas from within the earth began their task of building Mt. Rainier.

In the accompanying diagram Fig. 1 represents an earth surface with a mass of molten magma beneath. Fig. 2 shows this magma having bowed up the overlying strata into a mountain range, after which the magma cooled slowly during a long period of years and so formed granite or granodiorite. Figs 3 and 4 illustrate how this original mountain range was eroded or worn down by streams flowing down its sides until only a rather flat granite platform remained. In the large sketch (Fig. 5) a crack has developed in this platform because of stresses within the earth and in this manner molten magma has poured out to form a volcanic cone such as Rainier.


Why is it though that some lavas form cone-like mountains and others merely form broad, flat surfaces such as the Columbia Rive Plateau? This leads to the explanation of the various types of volcanoes. Some of these fire mountains are of the quiet type, such as those in the Hawaiian Islands, and consist of lava that has flowed out quietly from the vent and spread out to a great radius. Others are formed by intermittent explosions blowing out vast quantities of rock which falls about the vent forming what is known as a cinder cone. Most volcanoes, however, are of the intermediate type in their eruptive activity and in consequence their cones are of a form intermediate between the two already described. They are built up by the alternation of violent explosions and quiet flows examples of this type being Mts. Hood, St. Helens, Adams, and Rainier.

Volcanoes are formed along lines of weakness in the earth's crust and it is interesting to note that they are often situated on or near the backs of mountain chains. Lines of weakness are set up in these regions because of the fracturing of the earth's crust during the time of uplift of the mountain plain. Study any map of the Pacific continents. There is an almost continuous chain of mountains extending from the southern tip of South America northward to the Aleutians, thence southward through Japan and into the southern Pacific Islands. Some of the worlds largest volcanoes are located on the backs of these ranges. The immense amount of molten material below the surface of the earth has had an opportunity to escape at various points along this line of weakness and build up this vast series of volcanoes to such a height that their summits are in many cases continually covered with snow.

Kenneth Bravinder, Ranger-
Naturalist, Season 1932.

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Intermittent eruptions of volcanoes over the years is, in general, caused by the plugging of the volcanic vent due to lessened activity from relief of pressure and then solidification of the lavas which choke the vent. Thus a period of quietude is reached. During the quiet period the molten magma may again seek outlet. Pressure has again been developed. Gases and vapors which have formed serve to reduce, through chemical action, the plug which blocks the exit. Slowly the molten magma is forced upward until it pushes through the surface layers or explodes through them and the mountain is again in action. In general then alternate plugging of the vent, accumulation of pressure and pushing through the point of greatest weakness accounts for succession of volcanic activity. (C.F.B.)

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