There is much difference of opinion as to the force which causes molten rock to rise to the surface in the ducts of volcanoes.  Steam is so evidently concerned in explosive eruptions that many believe that lava is driven upward by the expansive force of the steam with which it is charged, much as a viscid liquid rises and boils over in a test tube or kettle.

But in quiet eruptions, and still more in the irruption of intrusive sheets and masses, there is little if any evidence that steam is the driving force.  It is therefore believed by many geologists that it is pressure due to crustal movements and internal stresses which squeezes molten rock from below into fissures and ducts in the crust.  It is held by some that where considerable water is supplied to the rising column of lava, as from the ground water of the surrounding region, and where the lava is viscid so that steam does not readily escape, the eruption is of the explosive type; when these conditions do not obtain, the lava outwells quietly, as in the Hawaiian volcanoes.  It is held by others not only that volcanoes are due to the outflow of the earth’s deep-seated heat, but also that the steam and other emitted gases are for the most part native to the earth’s interior and never have had place in the circulation of atmospheric and ground waters.

Volcanic action and deformation.  Volcanoes do not occur on wide plains or among ancient mountains.  On the other hand, where movements of the earth’s crust are in progress in the uplift of high plateaus, and still more in mountain making, molten rock may reach the surface, or may be driven upward toward it forming great intrusive masses.  Thus extensive lava flows accompanied the upheaval of the block mountains of western North America and the uplift of the Colorado plateau.  A line of recent volcanoes may be traced along the system of rift valleys which extends from the Jordan and Dead Sea through eastern Africa to Lake Nyassa.  The volcanoes of the Andes show how conspicuous volcanic action may be in young rising ranges.  Folded mountains often show a core of igneous rock, which by long erosion has come to form the axis and the highest peaks of the range, as if the molten rock had been squeezed up under the rising upfolds.  As we decipher the records of the rocks in historical geology we shall see more fully how, in all the past, volcanic action has characterized the periods of great crustal movements, and how it has been absent when and where the earth’s crust has remained comparatively at rest.