Magma

“ Williams was confusing. Now, two magma areas were showing where only one had shown before.
Esteben Balgos answered Rick's plea for an explanation. Over an excellent dinner of roast suckling pig and bananas steamed with lemon juice, the volcanologist took time to answer their questions.
"There is much we do not know about volcanoes," the Peruvian scientist began. "For example, we do not know exactly what causes magma to form. Magma is, in simplest terms, molten rock. Some event takes place far below, where the earth's crust ends and the mantle begins, and the rock melts." ”

“ A rock is made up of minerals. The igneous rock granite, for example, consists chiefly of quartz and feldspar; other minerals such as mica and hornblende are commonly present.
9 IGNEOUS ROCKS
Igneous rocks result from the cooling of hot, molten rock material or magma. Magma that reaches the surface through volcanoes is called lava. Magma comes from deep within the earth and is made up of a mixture of molten mineral materials. Igneous rocks have been forming throughout the geologic past and are still forming today. ”

“ Whatever the cause, the eventual result is the accumulation of a huge body of molten rock, called magma, enclosed in a deep underground chamber.
Magma, being a mixture of hot liquids and gases that is lighter in weight than the solid rocks surrounding it, tends to rise toward the earth’s surface. ”

“ A molten magma varies greatly in its behaviour according to its position, the solidification of that portion which remained deep-seated in the hidden reservoir beneath the earth’s crust taking place slowly and imperceptibly. A rock such as granite is not formed suddenly by instantaneous cooling, but step by step individual minerals are crystallized from the complex solution, and tend to develop as regular and often most beautiful crystals. ”

“ Some rocks are altered by heat and fluids when they are invaded by hot magma in a process called contact metamorphism.
UNALTERED ROCK METAMORPHIC ROCK MAGMA
14 Dynamic Metamorphism
Still another rock-changing process is one that is associated with mountain building. When mountains are formed, heat and great pressures develop deep within the earth’s crust. The flat layers of rock are then slowly pushed and squeezed so that they bend up into arches, fracture, or slide over each other. These forces cause great changes in the rocks in widespread areas. ”

“ The gases that igneous rocks contain, probably as essential features of the molten magma, and at a temperature above their critical points, escape to a large extent near or at the surface of the earth. The bubbles raised in lava, whereby it is rendered scoriaceous, and the clouds of vapour rising from cooling lava-flows and from the throat of a volcano in eruption, are sufficient evidences of this process. ”

“ Various minerals, the constituents of which exist in the original magma, but which may be hardly or not all recognisable in the superficial lavas, have had leisure to crystallize out in the deep-seated intrusions and appear sometimes among the components of the general body of the rock, or as well-terminated crystals in its drusy cavities. ”

“ Rock masses of igneous origin have no sooner consolidated than they begin to change. The gases with which the magma is charged are slowly dissipated, lava-flows often remain hot and steaming for many years.Post-volcanic Changes. These gases attack the components of the rock and deposit new minerals in cavities and fissures. ”

“ A characteristic of the ultra-basic rocks here, as in other parts of the world, is their gradual transition into one another, showing that [314] the various forms have arisen from consolidation of parts of one and the same magma owing to slight differences in composition or in the physical conditions under which consolidation has taken place. ”

“ Igneous Rocks.—Rocks formed by the cooling and solidification of a hot liquid material, known as magma, that has originated at unknown depths within the earth. Those that have solidified beneath the surface are known as intrusive rocks, or if the cooling has taken place slowly at great depth, as plutonic rocks, e.g. granite. Those that have flowed out over the surface are known as effusive rocks, extrusive rocks, or lavas, e.g., basalt. Volcanic rocks include not only lavas, but bombs, pumice, tuff, volcanic ash, and other fragmental materials or ejecta thrown out from volcanoes. ”

“ In the second place, segregation has taken place in the magma within the terrestrial crust after intrusion, for it is frequently observable in large bosses - 476 - and sometimes in sills, the basic elements having tended to mass themselves towards the margins of the rock, leaving more acid material in the centre. ”

“ Upon this magma floats the solid crust, thirty or forty miles thick, which is composed of various rocks, breaking down at the surface into soil. Three-fourths of the surface of this crust are covered by the water of the oceans, the hydrosphere, the rest being dry land. Outside all comes the atmospheric mantle, chiefly composed of air, which supports life, acts as a blanket to keep the earth warm, and as a shield against the blows of meteorites. ”

“ The final manifestation of volcanic activity in such a region may be the discharge of heated waters, which have ascended from the deep-seated magma far below the surface, and make their appearance as groups of hot springs; these springs persist long after the volcanoes which give rise to them have become quite extinct. ”

“ The planet is supposed to show its
solid crust in cross section, which may be a few miles in
thickness. Below this is such a hot plastic magma as we have
reason to believe underlies much of the solid crust of our own
Earth. Now there is an attraction between the satellite and the
crust of the planet; the same gravitational attraction which
exists between every particle of matter in the universe. Let us
consider how this attraction will affect the planet's crust. ”

“ Precise determination of the location and magnitude of earthquakes by a well-designed seismic network—as the volcano inflates by the rise of magma, the enclosing rocks are deformed to the breaking point to accommodate magma movement. ”

“ The heat, proceeding from the centre outwards, caused local fusion of the rocks, though without forming distinct reservoirs of molten magma, and the fused matter charged with gases rose in liquid threads or tongues, which worked their way upwards, some reaching the superficial part of the earth and escaping through fissures in the zone of fracture, thus giving rise to volcanic phenomena. ”

“ When hot magma moves up through rocks, it not only heats and pushes them, but it also may soak them with liquids and gases, causing the nearby rocks to change into new rocks, by a process called contact metamorphism. ”

“ If we are at a loss to account for the changes in the sequence of lavas during a single volcanic cycle, our difficulties are increased when we find that in some way the magma is restored each time to somewhat the same initial condition. ”

“ Increase of pressure and of temperature, which we know to take place at great depths, would stimulate resorption of sedimentary material, and by retarding cooling would allow time for dissolved foreign substances to diffuse widely through the magma. ”

“ On the other hand, the similarity of volcanic lavas over wide regions is strong evidence that they are drawn from [Pg 226] one continuous magma, consisting of molten matter beneath the solid exterior crust.
(a.) Lines of Volcanic Action.—It has been shown in a previous page that volcanic action of recent or Tertiary times has taken place mainly along certain lines which may be traced on the surface of a map or globe. One of these lines girdles the whole globe, while others lie in certain directions more or less coincident with lines of flexure, plication or faulting. ”

“ While those that have consolidated below the earth’s surface, the intrusive rocks, are notably compact, the magmas which arrive at the surface of the lithosphere before their consolidation reveal special structures dependent either upon the expansion of steam and other gases within them, or upon the conditions of flow over the earth’s surface. ”

“ They come from deep below the surface of the earth. If they cool off while below the surface, they form intrusive rocks, which may later be revealed by erosion. When magmas reach the surface red hot, they form extrusive rocks, such as volcanic rocks. ”

“ Composing that part of the earth which is accessible to us,—or as it is generally called the earth’s crust,—there are rocks of various kinds, of which some have been formed by igneous agency, either as lavas or beds of ashes, or in the form of molten magmas which gradually cooled and became crystalline below a mass of superincumbent strata. With these rocks we need not concern ourselves. ”

“ If magmas never breach the surface to erupt and remain deep underground, they cool much more slowly and thus allow ample time to sustain crystal precipitation and growth, resulting in the formation of coarser grained, nearly completely crystalline, igneous rocks. ”

“ Against this we may place the fact that at great depths whence active magmas ascend the rocks are under so great pressures that every fissure is closed up; in fact in some of the deepest mines the quantity of water found in the workings is often exceedingly small. ”

“ We have seen how many of the rocks are made in the sea,—the sandstones and the clays,—but there are two other kinds of rocks, about which we must say a little. The first are the Igneous rocks, which means rocks made by fire. These rocks have solidified, most frequently in crystalline forms, from a molten mass. Lava, which flows hot and fluid, from a volcano, and cooling becomes a sheet of solid rock, is an igneous rock. Some igneous rocks solidify under ground under great pressure, and become crystalline rocks such as granite. ”

“ As the molten lava cools and falls in the form of prodigious clouds of ashes, cinders and dust, for many miles around the volcano, even the light of the sun is obscured, and one cannot see the hand before the face. ”

“ I continually review my own geological muddles, the controversies over steam, flames, volcano swelling, explosion craters, layers in the crust, weighting and underflowing, continental uplift, the globe’s armor plate, contraction wrinkling of basins of sediment, submarine volcanoes, linear chains of volcanoes, siliceous shell, blocks lifted or sunk, planets solar or from the sun’s binary twin, original heat or radioactive heat, thick crust or thin shell, lava reservoirs or lava core, pregeology ancestors of volcanoes, and craters on the moon. ”

“ But the springs called mineral contain an unusual abundance of earthy matter in solution, and the substances with which they are impregnated correspond remarkably with those evolved in a gaseous form by volcanoes. Many of these springs are thermal, i. e., their temperature is above the mean temperature of the place, and they rise up through all kinds of rock; as, for example, through granite, gneiss, limestone, or lava, but are most frequent in volcanic regions, or where violent earthquakes have occurred at eras comparatively modern. ”

“ The top-heavy and the tottering ones are almost within reach; but there are slabs of rock that look like slices out of a mountain—I had almost said like slices out of a red-hot volcano; they stand up against the blue sky and the widespreading background in brilliant and astonishing perspective. ”

“ On the surface of the seething lava in the throat of the volcano there gathers a rock foam, which, when hurled into the air, is cooled and falls as PUMICE,—a spongy gray rock so light that it floats on water. ”

“ Darwin supplied the cross proofs as well as the immediate evidence. You have all heard of volcanoes, those wonderful vents in the surface of the earth out of which pour masses of lava, cinders and ashes, and the like. Now, it is a matter of observation and experience that all volcanoes are placed in areas in which the surface of the earth is undergoing elevation, or at any rate is stationary; they are not placed in parts of the world in which the level of the land is being lowered. ”

“ There can be no doubt that the celestial spaces — at least within the region that the earth traverses — are crowded with angular bits of stony matter, ranging in weight from thousands of pounds down to particles as light as the finest dust. Every night millions of the smaller bits fall swiftly upon our earth’s atmosphere, sparkle for a moment as shooting stars, and are burnt into vapor by the heat engendered from their friction in the atmosphere. ”

“ As soon as one is over the bridge, one finds one's self in a volcanic region, either of real lava, or of the native rock changed by the heat and by fusion. You ascend a mountain, which you might set down at once for gray lava. ”

“ A lower peak, evidently also a dead volcano, stood out from the higher one like a huge flat-topped excrescence, completely surrounded, except where it joined the mountain, by a perfectly bare wall of rock and absolutely inaccessible. ”

“ The person who writes you is mistaken. I nowhere say that the center of the earth is a burning mass. I do say, page 26, ‘The occurrence of volcanoes, and the wide-spread agency of heat or fire in former times, indicate the existence of large areas of melted rock or lava in the earth.’ I mean by this that under volcanic regions are lakes or reservoirs of melted rock. The globe in general is a solid sphere, solid at the center. ”

“ If there were a geologist at present on the moon he would be constrained to admit that his globe was once the seat of volcanic activity of the most widespread character, and of a vehemence and potency the like of which could not be paralleled on any known globe. ”