209.Subsurface Earth. -Many excavations and borings have been made deep into the earth"s surface, and it has been found that the temperature increaseswith the depth. The rate of increase is not the same in different places, nor is the in- crease always uniform in the same place. The average of a number of deep excavations in different parts of the earthgives a rise of 1° F. for each
70 or 80 feet of descent.
MOUNT SHASTA.
An extinct volcano.
The greater the pressure to which rocks are subjected the more difficult it is to melt them. If it were not for this, the solid part of the earth could not be more than 40 or 50 miles thick, as the interior heat would melt rocks under ordinary pressure. But the earth is too rigid for its interior to be otherwise than solid. So great is the pressure to which it is subjected that probably none of the material deep down in the interior of the earth is in a molten condition.
If the pressure near the surface should be decreased, or if the normal amount of heat at any place should be increased, the material might become fused, and under certain conditions might find its way to the surface. We know that heated material from below does rise toward the surface and intrude itself into the surface rocks and in some places pour forth over the surface.
What causes the uprising and outpouring of this molten material from below the surface of the earth, and how and why it reaches the surface are questions which as yet are unanswerable. But as soon as this igneous material comes within the range of observation, its properties and actions can readily be studied. The following descriptions of some well-known typical volcanoes show some of the results of subsurface activity.
210.Monte Nuovo. -In 1538, on the shore of the Bay of Naplesnear Bai?, that once famous resort of the Roman nobles, after a period of severe earthquake shocks there suddenly occurred a tremendous eruption. From within the earth emerged a mass of molten material blown into fragments by the explosion of the included gases. Within a few days there was formed Monte Nuovo, a hill 440 feet high and half a mile in diameter, having in the top a cup-shaped depression or crater over 400 feet deep.
So great was the explosive force of this eruption that none of the ejected material was poured out in the form of a liquid. The whole hill is made up of dust, small stones and porous blocks of rock which resemble the slag of a blast furnace. The small fragments in such eruptions are called ash or cinders. In a week the eruption was over, and nothing of the kind has since occurred in the region.
When visited by the writer a few years ago, the bottom of the crater was a level field planted to corn. The whole process of formation of this volcanic cone was observed and recorded by residents of the region. Other similar eruptions have been observed,CINDER CONE NEAR MOUNT LASSEN.
but perhaps this is the best
known. We have here what may well be called a young volcano. The cone to-day is almost perfect in form.
In northern California, near Mt. Lassen, which has itself recently become active, another almost perfect cone of this kind is found, which was probably formed much more recently than Monte Nuovo. From this cone both cinders and liquid material or lava were ejected.
211.Vesuvius. -When the Roman nobles were building their mag-nificent villas and baths along the shore of the Bay of Naples, the scenic beauty of the region was greatly increased by a mountain in the shape of a truncated cone, which rose from the plain a few miles back from the shore. Its sides, nearly to the summit, were covered with beautiful fields.
In the top of the mountain was a deep depression some three miles in diameter, partly filled with water and almost entirely surrounded by precipitous rock cliffs. There were no signs of internal disturbance.
VESUVIUS AND NAPLES.
Around the mountain were scattered prosperous cities, the soil was fertile, the vegetation luxuriant. To this natural fortress Spartacus, the gladiator, retreated when he first began to defy the power of Rome.
In 63 A.D. the region about the mountain was shaken by a severe earthquake which did much damage. This was followed by other earthquakes during a period of six teen years. In August, 79, the whole region was frightfully shaken, and the previously quiet mountain began to belch forth volcanic dust, cinders and stones, so that for miles around the sun was obscured, and a pall of utter darkness shrouded the country, lighted at intervals by terrific flashes of lightning.
A large part of the ancient crater, now known as Monte Somma, was blown away, and the villas and towns near the mountain were covered with the ash and cinders ejected. So deep were many of these buried that their sites were utterly forgotten. Pompeii and Herculaneum, after lying buried and almost forgotten for hundreds of years, have been recently partially uncovered.
These fossil cities show the people of to-day how the ancient Romans lived and built. The topography of the country and the coast line were greatly changed by this eruption. Pompeii formerly was a sea coast city at the mouthof a river. It is now a mile
or more from the sea and
MOUNT VESUVIUS.
Showing the famous eruption of 1872.
at a considerable distance from the river.
From the date of its first historic eruption until the present time Vesuvius has had active periods and periods when quiet or dormant. Sometimes the activity is mild, and at other times tremendously violent. At times the material ejected is fragmental and at other times streams of molten lava pour down its sides. Its ever changing cone, unlike that of Monte Nuovo, is composed partly of ash and partly of consolidated lavas. Even as late as 1907 a tremendous outpouring of ash took place which devastated a considerable area.
MOUNT PELEE AND THE RUINS OF ST. PIERRE.