This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The High Temperature has been explained in two principal ways. If we accept the nebular hypothesis of the origin of the Solar System, we must grant that the earth was once a globe of glowing gas, which subsequently condensed, in part at least, to a molten globe, and then solidified on the surface and to unknown depths. One explanation of volcanic heat is that it is due to the originally high temperature of the earth, not yet lost by radiation/ whether in local reservoirs or in a universal, deep-seated layer. By those who accept the latter view, it is generally assumed that the interior of the earth is exceedingly hot, but solidified by pressure, and that when, by folding or fracturing of the overlying rocks, this pressure is partially relieved, the highly heated masses become liquefied along the line of diminished pressure.
In the second class of hypotheses on this subject of temperature, it is assumed that the earth never was in a molten condition, or that it has already so far cooled that its proper heat is no longer sufficient to produce fusion of rock. From this point of view, the great heat is believed to be generated mechanically, by the friction of internal masses under compression and contraction, or, with much less probability, to be due to chemical processes, or even to radio-activity:
Similar divergences of opinion obtain with regard to the nature and origin of the lavas ejected by volcanoes. The view most commonly held is that they are, for the most part, the original, unaltered material of the globe, whether this has always remained fluid, or has been remelted by release of pressure, or otherwise. According to another opinion, volcanic products are formed from the fusion of sedimentary material which was laid down under water, but has been deeply buried within the crust of the earth by subsidence. A third view recognizes both sources of supply. / (3) The problem as to the origin of the steam which plays so important a part in volcanic eruptions is likewise very differently solved by different investigators. One opinion is that the steam, like the lava itself, is primordial and was absorbed from the atmosphere (which then contained all the waters of the sea) when the surface of the globe was still molten. Melted substances will, it is known, absorb many times their own volume of steam and gases, when in contact with them under pressure. From this it is inferred that the lava has contained the steam ever since the first cooling of the surface crust.
A second opinion derives the water from the surface of the earth, supposing that it descends partly through fissures and partly through the pores of the overlying rocks by capillarity. The nearness of most volcanoes to the sea is looked upon as favouring this view. Others, again, employ both methods of explanation, regarding the ordinary steam which impregnates all lavas as primordial, but believing that the violently explosive eruptions are caused by the sudden access of large bodies of water to the lava masses. The evidence of known facts is fit present distinctly in favour of the view that the steam is essentially primordial.