PRINCIPLES OF VOLCANIC ACTIVITY
ORIGIN OF VOLCANOES
The most basic questions concerning geology are often the most difficult for the geologist to answer. Any answer to the question "What is the ultimate cause and source of volcanoes?" is largely conjectural; however, there are several plausible answers which may be in harmony with what is known from seismic and gravity studies. These studies tell us the earth is solid to depths far below those at which volcanoes must originate. Certain types of seismic waves (the so-called "swaves" or "shear waves") are known to pass through solids, but not liquids. Recording of these waves indicates the earth's crust is solid to a depth of 1,800 miles below the surface. The swaves are not transmitted through the core of the earth, which is assumed to be liquid.
Data from numerous deep oil wells and mines indicate the earth is hotter at its interior than at its surface. This increase in temperature with depth, or geothermal gradient as it is called, carries from place to place on the globe, but generally varies between 30°C and 50°C per mile. The rate of increase must diminish at depths below those for which data are available, or preposterous temperatures must be assumed at the core of the earth.
According to these figures the temperature of the rocks which are 40 miles below the earth's surface would be in excess of 1200° a temperature at which virtually all rocks will melt in the laboratory. However, as seismic data indicate, the rocks at that depth are rigid or solid because of the tremendous pressure of the overlying shell of rock material. It is a fact of physical chemistry that when a solid is put under pressure its melting point is raised.
The existence of the geothermal gradient, among other information, has given rise to a theory held in good esteem by many geologists. The theory assumes the earth formed as a molten ball, and has been cooling since its creation. The earth's surface presumably has cooled and its still-hot interior is cooling still, shrinking the globe as it does so. As the interior shrinks, the solid crust wrinkles and cracks like the skin on a drying apple. Hence the theory provides an explanation for the unrest in the earth's crust as evidenced by earthquakes and volcanic activity.
Volcanoes and earthquake activity generally are confined to narrow belts or mountain ranges, often crudely parallel to the margins of continents. One such belt includes the Andes Mountains of South America, the Cascades, the Aleutians, and volcanic chains in Japan and the East Indies. This long and sinuous chain has been called the "circle of fire", as it rims a good portion of the Pacific Ocean.
If fractures in the earth's crust, however produced, extend to a depth of 40 miles or so they would release the pressure on the rocks at that depth. These rocks, already above their melting point but held rigid by pressure, would fuse or melt and begin creeping up the fracture. A source of volcanism is thus created.
Another possible mechanism whereby rocks in the earth's crust may become molten is by the generation of very high temperatures by radioactive decay. As uranium and thorium decay to produce lead, they give off large quantities of heat. Molten rock produced in this fashion also would tend to rise toward the earth's surface. The molten rock is properly called magma.
Magma is a molten, silicate-melt paste, generated within the earth's crust. Lava, though used synonymously by the layman, is properly reserved for magma which has been erupted at the earth's surface. Lava also refers to rock which has solidified on the earth's surface from a molten silicate melt.
The magma produced beneath the earth's surface tends to make its way upward. This liquid is lighter than the enclosing solid material and tends to rise as it is squeezed by the surrounding solid rocks. Also, the molten rock contains gases which, once released from solution, tend to expand and propel the liquid mass in the direction of least resistanceupward or along a fracture. The magma may rise directly to the earth's surface but commonly it rises to within a few thousand feet of the surface and begins to spread laterally. Here the magma accumulates in a magma chamber. This magma chamber is the stewing pot which furnishes the molten material that will build volcanoes.
The geologic evidence at hand indicates the material which is fused at great depths beneath the earth's surface is basalt. Basaltic magma, therefore, is the fluid from which all volcanic materials ultimately are derived. Basaltic magma consists of a pasty melt, rich in silica, alumina, magnesium, iron, calcium, sodium, potassium, and water vapor. Often this basaltic magma rises directly from the depths where it was created and spills out on the earth's surface in widespreading, fluid sheets. Such material does not halt temporarily in a magma chamber where subsequent changes might produce very different kinds of magma. The material that rises directly from its place of origin, then, most nearly represents the "mother" magma from which all other types of volcanic materials are derived. This material, as previously stated, is basalt. More specifically, such basalts often contain small crystals of the mineral olivine and are termed olivine-basalts.
It has often been observed that a given volcano will erupt one kind of lava initially, and a very different material at a later time. Also, it is not uncommon for neighboring volcanoes to erupt different kinds of lavas at the same time. The explanation of these phenomena lies in the changes the magma undergoes in the magma chamber.
As the magma begins to cool, crystals begin to grow in the liquid. Those minerals which are stable at higher temperatures form first: olivine, pyroxene, and calcic plagioclase feldspar. These crystals are heavier than the liquid in which they are growing, and slowly settle toward the bottom of the magma chamber. They leave behind a liquid relatively richer in sodium, potassium, and silica. Also solidifying are those portions of the magma adjacent to the cooler walls of the magma chamber. As the magma freezes, the gases it contains are excluded from the crystals, hence are enriched in the remaining liquid. The light gases thus produced, and the remaining liquid portion of the magma, rise to the top of the magma chamber. Geologists call this process differentiation. The result of differentiation is a more-or-less horizontal layering of the gases, liquids, and crystals in the magma chamber.
As the gases accumulate at the top of the magma chamber their pressure increases, and may fracture the rock from the chamber to the surface. The fractures may tap any portion of the chamberthe top, sides, or bottomand may, therefore, release any type of material the chamber contains. The foregoing discussion of differentiation explains why rapidly spaced eruptions usually consist of olivine basalt, the "original" magma. The magma chamber is continually refurnished with fresh olivine basalt from below, which does not have time to differentiate before being expelled at the surface. Conversely, widely spaced eruptions commonly consist of siliceous lavas, the products of differentiation. The outpouring of siliceous lavas often is followed by eruptions of more basaltic, crystalline lavas as the bottom layer of differentiated material follows the more silicic from the chamber.
As Howel Williams has written, crystallization and differentiation are the major causes of diversity in volcanoes. The differences in the liquid products of volcanoes are not wholly dependent upon chemical composition. Differences in temperature and gas content also affect the nature of the fluid issue of volcanoes.
Other things being equal, the higher the temperature, the more fluid the lavas; and the higher the gas content, the more fluid the lavas. Basalts which rise directly from the depths of the earth and which have not been subjected to differentiation retain their gas content and high temperature. Upon reaching the surface, they spread in fluid sheets like thick, muddy water. The temperatures of such lavas are commonly 1000°C to 1200°C Upon solidifying they form smooth, ropy and corrugated forms to which the name "pahoehoe" has been given. Often the crusts of lava flows are jagged, clinkery and splintery. Such lava is called "aa".
The more silicic differentiates of the magma chamber generally have cooled to temperatures between 600°C and 850°C and have been separated from much of their gas content. These lavas are extruded as viscous, pasty masses which flow with extreme sluggishness.
All of the products of volcanic activity, of course, are not liquid. In a magma chamber wherein differentiation has produced a separate gaseous phase near the top of the chamber, fractures tapping that portion of the chamber allow the gas to escape with rapidity and in huge volumes. Measurements of gas exhalation in the Valley of 10,000 Smokes near Mt. Katmai, Alaska, in 1912 revealed 6 million gallons of steam per second, 1-1/4 million tons of hydrochloric acid per year, and 200,000 tons of hydrofluoric acid per year were given off by the cooling pumice flows after extrusion. The relative proportions of these gases are not unique. Of the gases given off by volcanoes most is water vapor (usually 80 to 95 percent of the total gases exhaled) with carbon dioxide the next most abundant, followed quantitatively by sulfur dioxide. Minor amounts of carbon monoxide, hydrochloric acid and hydrofluoric acid gaseshydrogen, hydrocarbons, ammonium chloride, and otherare common.
Some solid material also is ejected by volcanoes. The fragments range in size from blocks weighing hundreds of tons to dust particles fine enough to be carried around the globe by winds. Some of this material consists of pasty, liquid clots which cool in the air as they are blown from the throat of the volcano. Other fragments are solid particles of the volcano which are torn loose, carried upward, and ejected forcefully by the lava and gases rushing up the conduit of the volcano.
If the fragments blown from the volcano are smaller than pea-size they are called volcanic dust and ashes. If they are pea-size to walnut-size they are called lapilli (or cinders, if basaltic in composition). If larger than walnut-size they are called blocks, if solid when blown out, or bombs, if thrown out in a liquid state. Volcanic bombs tend to be elongated or spindle-shaped with slightly twisted ends. They attain this shape when hurling through the air while still a pasty liquid.
FORMATION OF VOLCANOS
Occasionally a frothy or bubbly material, called pumice, is ejected from a volcano. The bubbles often are drawn out into long, delicate, parallel tubes forming a rock light enough to float on water. Pumice is usually silicic in composition.
Volcanic eruptions are as varied as the materials the volcanoes erupt. The eruptions may consist of quiet outpourings of very fluid lava, or they may be explosive ejections of fragmental material.
In the foregoing discussion it was stated that olivine basalt, rising directly from its point of fusion to be spilled out on the earth's surface, does not have the necessary time to differentiate, and therefore retains much of its gases in solution. These gases impart great fluidity to the molten rock, and basalt flows often spread in thin sheets across the earth's surface covering tens or even hundreds of thousands of square miles. It is often difficult to determine the point at which such rocks were extruded because their fluidity will not allow a steep volcano to accumulate around the vent. In many areas the basalt apparently has welled up from numerous, closely spaced fractures and spread evenly over the surface, leaving no trace of the vents from which it was extruded. If such basalts are extruded from a single vent over a long period of time, a gentle dome may be formed.
Eruptions of fluid basalt are termed "quiet" only in a relative sense. Rivers of incandescent rock, flowing down gentle slopes at great speeds, dragging along glassy crusts and pushing ahead of them blocky, clinkery chunks of lava, can hardly be called "quiet" in the absolute sense. The eruptions of the Hawaiian volcanoes are of this type. Occasionally the molten rock will break through the sides of the volcanoes far down the flanks and form a flow atop the older flows making up the base of the volcano. Often the rupture in the flanks of volcanoes is occasioned by the cooling and solidification of material in the vent. A plug thus formed may temporarily seal the vent. As pressure builds up, it may overcome the strength of the flanks of the volcano; molten rock then bursts from the sides below the summit. Or the plug may be disintegrated and blown out, allowing the lava to flow again out the central vent.
If the lava is allowed to differentiate in the magma chamber there is a separation of the gaseous material from the more siliceous and less siliceous magma fractions. As the gas pressure increases, new fractures develop in the roof of the magma chamber and the various types of material can escape. As explained in the previous section of this report, the siliceous material is more viscous than the undifferentiated basaltic magma. It rises along the fractures and flows sluggishly down the flanks of the volcano. It may chill to form a dense, glassy rock called obsidian. Such a flow is Llao Rock at Crater Lake. This flow is greater than 1,000 feet thick, but flowed only a mile down the slopes of Mt. Mazama. Obsidian flows never cover large areasthey tend to be thick and stubby in form.
If the more siliceous magma is charged with gas the eruption may be explosive. As such material is ejected the gas expands rapidly, blowing clots of pasty magma from the vent of the volcano high into the air. Those clots falling outside the crater may solidify while in the air or shortly after striking the ground.
Some eruptions consist almost entirely of gas. As the gas escapes through the fractures it may tear loose fragments of older, solidified lava and hurl them high into the air.
Pumice eruptions tend to be extremely explosive. Eruptions of pumice during recorded history have produced detonations heard thousands of miles away. The coarse material may fall as ash, as the finer particles are wafted around the globe by the prevailing winds. If the quantity of pumice material is great enough, the spewing column of gas and pumice rising from the vent is overcome by gravity, and the boiling mass rushes down the slope of the volcano. This type of incandescent flow has been called a "nuee ardente", or "glowing avalanche". The high content of hot gases gives the mass high mobility. The flows may attain velocities in excess of 100 m.p.h. The pumice lumps discharge gas at extremely high temperatures and pressure, producing a frothing, mushrooming effect. The flow rolls and floats over the surface rather than sliding over it.
Such eruptions are extremely destructive. In 1902 a glowing avalanche flowed down the slopes of Mt. Pelee on the isle of Martinique in the West Indies, and swiftly engulfed the coastal town of St. Pierre. Only two of the city's 28,000 inhabitantsone a condemned murderer locked in a dungeonescaped a searing death.
To summarize, gas is the propelling force in volcanic eruptions. If the gas content of the lava is high, the resulting flows are fluid and quiet. If the gas content is low, the flows are more viscous. If the gas in a magma chamber has segregated, gaseous eruptions may result. If the gas is associated with silicous magma, pumice eruptions may result.
TYPES OF VOLCANIC ERUPTION
The different shapes volcanic rocks assume are primarily the result of their differences in composition. As already explained, fluid olivine basalts rising directly from the point of their fusion will spread rapidly and evenly. Very fluid lava behaves much like any other fluid and seeks a lower level. If canyons are available near the point of such an eruption the lava flows down them and may be confined by the canyon walls. Such rocks are called intracanyon basalts. They partially fill the canyon to form flat floors between the rugged canyon walls. If such rocks are extruded in relatively flat areas they spread into thin sheets covering vast areas.
Eruptions of basaltic lavas from a central vent produce volcanoes with low, broad profiles, like an inverted dish. This shape is assumed because the fluid lavas flow rapidly from the vent and the height of the volcano grows slowly as its diameter grows rapidly. The Hawaiian volcanoes Kilauea and Mauna Loa are of this type. These volcanoes are called shield volcanoes.
The other extreme in volcanic forms is the cinder cone. This type consists entirely of clinkery ash, cinders, bombs, and blocks of lava. All the material making up a cinder cone is fragmental, and the angle of repose for irregular and jagged fragments is greater than that attained by a congealing fluid lava. The cinder cones, then, have steeper sides than shield volcanoes, and the slope of their sides increases toward the summit. In profile, the very top of the cinder cone may appear flattened due to the conical depression at its top. Examples of well-studied cinder cones are Paricutin in Mexico, and Wizard Island, which protrudes from Crater Lake. The flanks of Newberry Crater in central Oregon are liberally sprinkled with about 200 small cinder cones.
More common than either true shield volcanoes or cinder cones are composite volcanoes. These are composed of alternating layers of cinders and lava flows, and in profile their slopes rise at an angle between those of shield volcanoes and cinder cones. Composite volcanoes are comparatively common because of the workings of the molten rock in the magma chamber. As the material in the chamber differentiates, gas pressure builds up, the volcano erupts, and the chamber emits first the uppermost gas and siliceous lavas, then the more basaltic lavas. As the magma chamber is emptied it is refurnished from below and more basaltic lava is erupted. When the pressure is released by these eruptions, the lava in the vent of the volcano may congeal, plug the vent, and allow the new basaltic magma in the chamber to differentiate again. This cycle may repeat itself many times, with eruptions of more siliceous fragmental ejecta alternating with eruptions of fluid basaltic lava. Numerous examples of composite cones are to be found in the High CascadesMt. Hood, Mt. Shasta, and Mt. Rainier are among them.
As the material in the conduit of a volcano is forced upward and blown out the vent, a certain erosive effect ensues. The gas and lava rushing from the vent are blown out with great energy, and fragments of older lava flows are torn loose and ejected also. The result is a crater, or conical depression in the top of the volcano. These depressions are seldom more than a mile across and usually are littered with volcanic fragments, bombs, blocks, etc., which were not blown beyond the rim of the crater and rolled back down into the depression.
Some volcanic peaks contain circular or conical depressions many miles in diameter. Often the interior rim of the depression is a steep escarpment extending entirely around the depression. Such depressions are called calderas, and result from collapse of the volcanic peak (including the crater) into a void created by the eruption of lavas and evacuation of the magma chamber. At the well-known "Fire Pit" in the summit of Kilauea, the walls often collapse when lava pours from fractures on the slopes of the volcano far beneath the summit. The caldera in which Crater Lake was formed is another example of a depression formed in this manner.
Eruptions of volcanic ash or fine pumice are carried by the wind and dropped to produce a blanket-like deposit. Such material covers the ground evenly, like a layer of fresh snow. Eruptions from Mt. Mazama about 6,600 years ago produced a blanket of pumice which diminishes in thickness away from the volcano, but retains a depth of 6 inches as far as 70 miles away.
Glowing avalanche deposits, because they move across the surface of the ground and are controlled by topography, assume shapes not unlike those of fluid basalts, if the terrain is relatively levelor intracanyon basalts, assuming there are canyons of sufficient depth and width to retain the material.
TYPICAL VOLCANIC FORMS
Last Updated: 01-Jul-2008