The Project Gutenberg eBook ofVolcanoesThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: VolcanoesAuthor: Robert I. TillingRelease date: September 18, 2014 [eBook #46894]Most recently updated: October 24, 2024Language: EnglishCredits: Produced by Stephen Hutcheson, Dave Morgan and the OnlineDistributed Proofreading Team at http://www.pgdp.net*** START OF THE PROJECT GUTENBERG EBOOK VOLCANOES ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: VolcanoesAuthor: Robert I. TillingRelease date: September 18, 2014 [eBook #46894]Most recently updated: October 24, 2024Language: EnglishCredits: Produced by Stephen Hutcheson, Dave Morgan and the OnlineDistributed Proofreading Team at http://www.pgdp.net
Title: Volcanoes
Author: Robert I. Tilling
Author: Robert I. Tilling
Release date: September 18, 2014 [eBook #46894]Most recently updated: October 24, 2024
Language: English
Credits: Produced by Stephen Hutcheson, Dave Morgan and the OnlineDistributed Proofreading Team at http://www.pgdp.net
*** START OF THE PROJECT GUTENBERG EBOOK VOLCANOES ***
The eruption of Cerro Negro Volcano, near Leon, Nicaragua, during November 1968.The eruption of Cerro Negro Volcano, near Leon, Nicaragua, during November 1968.
The eruption of Cerro Negro Volcano, near Leon, Nicaragua, during November 1968.
U. S. Department of the Interior / U. S. Geological SurveyVolcanoesby Robert I. Tilling
U. S. Department of the Interior / U. S. Geological Survey
by Robert I. Tilling
Cover and Title Page: Lava fountains and flows, Mauna Loa, Hawaii, July 6, 1975.Cover and Title Page: Lava fountains and flows, Mauna Loa, Hawaii, July 6, 1975.
Cover and Title Page: Lava fountains and flows, Mauna Loa, Hawaii, July 6, 1975.
Volcanoes destroy and volcanoes create. The catastrophic eruption of Mount St. Helens on May 18, 1980, made clear the awesome destructive power of a volcano. Yet, over a time span longer than human memory and record, volcanoes have played a key role in forming and modifying the planet upon which we live. More than 80 percent of the Earth’s surface—above and below sea level—is of volcanic origin. Gaseous emissions from volcanic vents over hundreds of millions of years formed the Earth’s earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic eruptions have produced mountains, plateaus, and plains, which subsequent erosion and weathering have sculpted into majestic landscapes and formed fertile soils.
Ironically, these volcanic soils and inviting terranes have attracted, and continue to attract, people to live on the flanks of volcanoes. Thus, as population density increases in regions of active or potentially active volcanoes, mankind must become increasingly aware of the hazards and learn not to “crowd” the volcanoes. People living in the shadow of volcanoes must live in harmony with them and expect, and should plan for, periodic violent unleashings of their pent-up energy.
This booklet presents a generalized summary of the nature, workings, products, and hazards of the common types of volcanoes around the world, along with a brief introduction to the techniques of volcano monitoring and research.
On August 24, A.D. 79, Vesuvius Volcano suddenly exploded and destroyed the Roman cities of Pompeii and Herculaneum. Although Vesuvius had shown stirrings of life when a succession of earthquakes in A.D. 63 caused some damage, it had been literally quiet for hundreds of years and was considered “extinct.” Its surface and crater were green and covered with vegetation, so the eruption was totally unexpected. Yet in a few hours, hot volcanic ash and dust buried the two cities so thoroughly that their ruins were not uncovered for nearly 1,700 years, when the discovery of an outer wall in 1748 started a period of modern archeology. Vesuvius has continued its activity intermittently ever since A.D. 79 with numerous minor eruptions and several major eruptions occurring in 1631, 1794, 1872, 1906 and in 1944 in the midst of the Italian campaign of World War II.
In the United States on March 27, 1980, Mount St. Helens Volcano in the Cascade Range, southwestern Washington, reawakened after more than a century of dormancy and provided a dramatic and tragic reminder that there are active volcanoes in the “lower 48” States as well as in Hawaii and Alaska. The catastrophic eruption of Mount St. Helens on May 18, 1980, and related mudflows and flooding caused significant loss of life (57 dead or missing) and property damage (over $1.2 billion). Mount St. Helens is expected to remain intermittently active for months or years, possibly even decades.
The word “volcano” comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan—the blacksmith of the Roman gods. They thought that the hot lava fragments and clouds of dust erupting from Vulcano came from Vulcan’s forge as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful. Today we know that volcanic eruptions are not supernatural but can be studied and interpreted by scientists.
Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products—lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.
Fountaining lava and volcanic debris during the 1959 Kilauea Iki eruption of Kilauea Volcano, Hawaii.Fountaining lava and volcanic debris during the 1959 Kilauea Iki eruption of Kilauea Volcano, Hawaii.
Fountaining lava and volcanic debris during the 1959 Kilauea Iki eruption of Kilauea Volcano, Hawaii.
Driven by buoyancy and gas pressure the molten rock, which is lighter than the surrounding solid rock, forces its way upward and may ultimately break through zones of weaknesses in the Earth’s crust. If so, an eruption begins, and the molten rock may pour from the vent as non-explosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations of fall-back fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materials may be carried by the wind only to fall to the ground many miles away. The finest ash particles may be injectedmiles into the atmosphere and carried many times around the world by stratospheric winds before settling out.
Molten rock below the surface of the Earth that rises in volcanic vents is known asmagma, but after it erupts from a volcano it is calledlava. Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Magmas also contain many other chemical elements in trace quantities. Upon cooling, the liquid magma may precipitate crystals of various minerals until solidification is complete to form anigneousormagmatic rock.
An idealized diagram of a volcano in an oceanic environment (left) and in a continental environment (right).An idealized diagram of a volcano in an oceanic environment (left) and in a continental environment (right).
An idealized diagram of a volcano in an oceanic environment (left) and in a continental environment (right).
The diagram below shows that heat concentrated in the Earth’s uppermantleraises temperatures sufficiently to melt the rock locally by fusing the materials with the lowest melting temperatures, resulting in small, isolated blobs of magma. These blobs then collect, rise through conduits and fractures, and some ultimately may re-collect in larger pockets or reservoirs (“holding tanks”) a few miles beneath the Earth’s surface. Mounting pressure within the reservoir may drive the magma further upward through structurally weak zones to erupt as lava at the surface. In a continental environment, magmas are generated in the Earth’s crust as well as at varying depths in the upper mantle. The variety of molten rocks in the crust, plus the possibility of mixing with molten materials from the underlying mantle, leads to theproduction of magmas with widely different chemical compositions.
If magmas cool rapidly, as might be expected near or on the Earth’s surface, they solidify to form igneous rocks that are finely crystalline or glassy with few crystals. Accordingly, lavas, which of course are very rapidly cooled, form volcanic rocks typically characterized by a small percentage of crystals or fragments set in a matrix ofglass(quenched or super-cooled magma) or finer grained crystalline materials. 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. Subsequent to final crystallization and solidification, such rocks can be exhumed by erosion many thousands or millions of years later and be exposed as large bodies of so-calledgraniticrocks, as, for example, those spectacularly displayed in Yosemite National Park and other parts of the majestic Sierra Nevada mountains of California.
Lava is red hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey in some cases or in others like pasty, blocky masses.
Two Polynesian terms are used to identify the surface character of Hawaiian lava flows. Aa, a basalt with a rough, blocky appearance, much like furnace slag, is shown at the top. Pahoehoe, a more fluid variety with a smooth, satiny and sometimes glassy appearance, is shown at the bottom.Two Polynesian terms are used to identify the surface character of Hawaiian lava flows.Aa, a basalt with a rough, blocky appearance, much like furnace slag, is shown at the top.Pahoehoe, a more fluid variety with a smooth, satiny and sometimes glassy appearance, is shown at the bottom.
Two Polynesian terms are used to identify the surface character of Hawaiian lava flows.Aa, a basalt with a rough, blocky appearance, much like furnace slag, is shown at the top.Pahoehoe, a more fluid variety with a smooth, satiny and sometimes glassy appearance, is shown at the bottom.
All magmas contain dissolved gases, and as they rise to the surface to erupt, the confining pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the lava is a thin fluid (not viscous), the gases may escape easily. But if the lava is thick and pasty (highly viscous), the gases will not move freely but will build up tremendous pressure, and ultimately escape with explosive violence. Gases in lava may be compared with the gas in a bottle of a carbonated soft drink. If you put your thumb over the top of the bottle and shake it vigorously, the gas separates from the drink and forms bubbles. Whenyou remove your thumb abruptly, there is a miniature explosion of gas and liquid. The gases in lava behave in somewhat the same way. Their sudden expansion causes the terrible explosions that throw out great masses of solid rock as well as lava, dust, and ashes.
The violent separation of gas from lava may produce rock froth calledpumice. Some of this froth is so light—because of the many gas bubbles—that it floats on water. In many eruptions, the froth is shattered explosively into small fragments that are hurled high into the air in the form of volcanic cinders (red or black), volcanic ash (commonly tan or gray), and volcanic dust.
During the 1959 eruption of Kilauea Iki, fountaining lava and volcanic debris completely blocked several of the roads in the Hawaii Volcanoes National Park.During the 1959 eruption of Kilauea Iki, fountaining lava and volcanic debris completely blocked several of the roads in the Hawaii Volcanoes National Park.
During the 1959 eruption of Kilauea Iki, fountaining lava and volcanic debris completely blocked several of the roads in the Hawaii Volcanoes National Park.
Geologists generally group volcanoes into four main kinds—cinder cones, composite volcanoes, shield volcanoes, and lava domes.
Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall ascindersaround the vent to form a circular or oval cone. Most cinder cones have a bowl-shapedcraterat the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.
Schematic representation of the internal structure of a typical cinder cone.Schematic representation of the internal structure of a typical cinder cone.
Schematic representation of the internal structure of a typical cinder cone.
In 1943 a cinder cone started growing on a farm near the village of Parícutin in Mexico. Explosive eruptions caused by gas rapidly expanding and escaping from molten lava formed cinders that fell back around the vent, building up the cone to a height of 1,200 feet. The last explosive eruption left a funnel-shaped crater at the top of thecone. After the excess gases had largely dissipated, the molten rock quietly poured out on the surrounding surface of the cone and moved downslope as lava flows. This order of events—eruption, formation of cone and crater, lava flow—is a common sequence in the formation of cinder cones.
Parícutin Volcano, Mexico, is a cinder cone rising approximately 1,200 feet above the surrounding plain.Parícutin Volcano, Mexico, is a cinder cone rising approximately 1,200 feet above the surrounding plain.
Parícutin Volcano, Mexico, is a cinder cone rising approximately 1,200 feet above the surrounding plain.
During 9 years of activity, Parícutin built a prominent cone, covered about 100 square miles with ashes, and destroyed the town of San Juan. Geologists from many parts of the world studied Parícutin during its lifetime and learned a great deal about volcanism, its products, and the modification of a volcanic landform by erosion.
Some of the Earth’s grandest mountains arecompositevolcanoes—sometimes calledstratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.
Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.
Schematic representation of the internal structure of a typical composite volcano.Schematic representation of the internal structure of a typical composite volcano.
Schematic representation of the internal structure of a typical composite volcano.
The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth’s crust rises to the surface. The volcano is built up by the accumulation of materialerupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.
When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface—a telltale remnant of the vanished volcano.
Shishaldin Volcano, an imposing composite cone, towers 9,372 feet above sea level in the Aleutian Islands, Alaska.Shishaldin Volcano, an imposing composite cone, towers 9,372 feet above sea level in the Aleutian Islands, Alaska.
Shishaldin Volcano, an imposing composite cone, towers 9,372 feet above sea level in the Aleutian Islands, Alaska.
An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano—called Mount Mazama—probably similar in appearance to present-day Mount Rainier was once located at this spot. Following a series of tremendous explosions about 6,800 years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large-volume explosionsrapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake. A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known ascalderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long.
Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface.Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface.
Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface.
a. Magma, rising upward through a conduit, erupts at the Earth’s surface to form a volcanic cone. Lava flows spread over the surrounding area.a. Magma, rising upward through a conduit, erupts at the Earth’s surface to form a volcanic cone. Lava flows spread over the surrounding area.
a. Magma, rising upward through a conduit, erupts at the Earth’s surface to form a volcanic cone. Lava flows spread over the surrounding area.
b. As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepen their valleys.b. As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepen their valleys.
b. As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepen their valleys.
c. When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened “volcanic plug” in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas.c. When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened “volcanic plug” in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas.
c. When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened “volcanic plug” in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas.
d. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or “volcanic neck,” a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau.d. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or “volcanic neck,” a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau.
d. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or “volcanic neck,” a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau.
Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a centralsummitvent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior’s shield. They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii—two of the world’s most active volcanoes. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world’s largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.
In some eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.
Mauna Loa Volcano, Hawaii, a giant among the active volcanoes of the world; snow-capped Mauna Kea Volcano in the distance.Mauna Loa Volcano, Hawaii, a giant among the active volcanoes of the world; snow-capped Mauna Kea Volcano in the distance.
Mauna Loa Volcano, Hawaii, a giant among the active volcanoes of the world; snow-capped Mauna Kea Volcano in the distance.
The internal structure of a typical shield volcano.The internal structure of a typical shield volcano.
The internal structure of a typical shield volcano.
A sketch of the havoc wrought in St. Pierre Harbor on Martinique during the eruption of Mont Pelée in 1902.A sketch of the havoc wrought in St. Pierre Harbor on Martinique during the eruption of Mont Pelée in 1902.
A sketch of the havoc wrought in St. Pierre Harbor on Martinique during the eruption of Mont Pelée in 1902.
Volcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as “coulees.” Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes. The nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. The internal structure of this dome—defined by layering of lava fanning upward and outward from the center—indicates that it grew largely by expansion from within. Mont Pelée in Martinique, Lesser Antilles, and Lassen Peak and Mono domes in California are examples of lava domes. An extremely destructive eruptionaccompanied the growth of a dome at Mont Pelée in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed by an incandescent, high-velocity ash flow and associated hot gases and volcanic dust. Only two men survived; one because he was in a poorly ventilated, dungeon-like jail cell and the other who somehow made his way safely through the burning city.
The Novarupta Dome formed during the 1912 eruption of Katmai Volcano, Alaska.The Novarupta Dome formed during the 1912 eruption of Katmai Volcano, Alaska.
The Novarupta Dome formed during the 1912 eruption of Katmai Volcano, Alaska.
Schematic representation of the internal structure of a typical volcanic dome.Schematic representation of the internal structure of a typical volcanic dome.
Schematic representation of the internal structure of a typical volcanic dome.
Congealed magma, along with fragmental volcanic and wallrock materials, can be preserved in the feeding conduits of a volcano upon cessation of activity. These preserved rocks form crudely cylindrical masses, from which project radiating dikes; they may be visualized as the fossil remains of the innards of a volcano (the so-called “volcanic plumbing system”) and are referred to as volcanicplugsornecks. The igneous material in a plug may have a range of composition similar to that of associated lavas or ash, but may also include fragments and blocks of denser, coarser grained rocks—higher in iron and magnesium, lower in silicon—thought to be samples of the Earth’s deep crust or upper mantle plucked and transported by the ascending magma. Many plugs and necks are largely or wholly composed of fragmental volcanic material and of fragments of wallrock, which can be of any type. Plugs that bear a particularly strong imprint of explosive eruption of highly gas-charged magma are calleddiatremesortuff-breccia.
Volcanic plugs are believed to overlie a body of magma which could be either still largely liquid or completely solid depending on the state of activity of the volcano. Plugs are known, or postulated, to be commonly funnel shaped and to taper downward into bodies increasingly elliptical in plan or elongated to dike-like forms. Typically, volcanic plugs and necks tend to be more resistant to erosion than their enclosing rock formations. Thus, after the volcano becomes inactive and deeply eroded, the exhumed plug may stand up in bold relief asan irregular, columnar structure. One of the best known and most spectacular diatremes in the United States is Ship Rock in New Mexico, which towers some 1,700 feet above the more deeply eroded surrounding plain. Volcanic plugs, including diatremes, are found elsewhere in the western United States and also in Germany, South Africa, Tanzania, and Siberia.
Ship Rock, San Juan County, New Mexico.Ship Rock, San Juan County, New Mexico.
Ship Rock, San Juan County, New Mexico.
Also called “tuff cones,”maarsare shallow, flat-floored craters that scientists interpret have formed above diatremes as a result of a violent expansion of magmatic gas or steam; deep erosion of a maar presumably would expose a diatreme. Maars range in size from 200 to 6,500 feet across and from 30 to 650 feet deep, and most are commonly filled with water to form natural lakes. Most maars have low rims composed of a mixture of loose fragments of volcanic rock and rocks torn from the walls of the diatreme.
Maars occur in the western United States, in the Eifel region of Germany, and in other geologically young volcanic regions of the world. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6,500 feet across and 400 feet deep. Its low rim is composed of loose pieces of basaltic lava and wallrocks (sandstone, shale, limestone) of the underlying diatreme, as well as random chunks of ancient crystalline rocks blasted upward from great depths.
Zuni Salt Lake Maar, Catron County, New Mexico.Zuni Salt Lake Maar, Catron County, New Mexico.
Zuni Salt Lake Maar, Catron County, New Mexico.
Some well-exposed, nearly circular areas of intensely deformed sedimentary rocks, in which a central vent-like feature is surrounded by a ring-shaped depression, resemble volcanic structures in gross form. As no clear evidence of volcanic origin could be found in or near these structures, scientists initially described them as “cryptovolcanic,” a term now rarely used. Recent studies have shown that not all craters are of volcanic origin. Impact craters, formed by collisions with the Earth of large meteorites, asteroids, or comets, share with volcanoes the imprints of violent origin, as evidenced by severe disruption, and even local melting, of rock. Fragments of meteorites or chemically detectable traces of extraterrestrial materials and indications of strong forces acting from above, rather than from below, distinguish impact from volcanic features.
Other possible explanations for these nonvolcanic craters include subsurface salt-dome intrusion (and subsequent dissolution and collapse); collapse caused by subsurface limestone dissolution and/or ground-water withdrawal; and collapse related to melting of glacial ice. An impressive example of an impact structure is Meteor Crater, Ariz., which is visited by thousands of tourists each year. This impact crater, 4,000 feet in diameter and 600 feet deep, was formed in the geologic past (probably 30,000-50,000 years before present) by a meteorite striking the Earth at a speed of many thousands of miles per hour.
In addition to Meteor Crater, very fresh, morphologically distinct, impact craters are found at three sites near Odessa, Tex., as well as 10 or 12 other locations in the world. Of the more deeply eroded, less obvious, postulated impact structures, there are about ten well-established sites in the United States and perhaps 80 or 90 elsewhere in the world.
Meteor Crater, Arizona.Meteor Crater, Arizona.
Meteor Crater, Arizona.
Mount St. Helens, about noon, May 18, 1980.Mount St. Helens, about noon, May 18, 1980.
Mount St. Helens, about noon, May 18, 1980.
During an episode of activity, a volcano commonly displays a distinctive pattern of behavior. Some mild eruptions merely discharge steam and other gases, whereas other eruptions quietly extrude quantities of lava. The most spectacular eruptions consist of violent explosions that blast great clouds of gas-laden debris into the atmosphere.
The type of volcanic eruption is often labeled with the name of a well-known volcano where characteristic behavior is similar—hence the use of such terms as “Strombolian,” “Vulcanian,” “Vesuvian,” “Peléan,” “Hawaiian,” and others. Some volcanoes may exhibit only one characteristic type of eruption during an interval of activity—others may display an entire sequence of types.
In a Strombolian-type eruption observed during the 1965 activity of Irazú Volcano in Costa Rica, huge clots of molten lava burst from the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.
Irazú Volcano, Costa Rica, 1965.Irazú Volcano, Costa Rica, 1965.
Irazú Volcano, Costa Rica, 1965.
In contrast, the eruptive activity of Parícutin Volcano in 1947 demonstrated a “Vulcanian”-type eruption, in which a dense cloud of ash-laden gas explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone.
In a “Vesuvian” eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D. 79, great quantities of ash-laden gas are violently discharged to form a cauliflower-shaped cloud high above the volcano.
Parícutin Volcano, Mexico, 1947.Parícutin Volcano, Mexico, 1947.
Parícutin Volcano, Mexico, 1947.
Mount Vesuvius Volcano, Italy, 1944.Mount Vesuvius Volcano, Italy, 1944.
Mount Vesuvius Volcano, Italy, 1944.
In a “Peléan” or “Nuée Ardente” (glowing cloud) eruption, such as occurred on the Mayon Volcano in the Philippines in 1968, a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move downslope at velocities as great as 100 miles per hour. Such eruptive activity can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mont Pelée on Martinique, Lesser Antilles.
“Hawaiian” eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950; or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano’s rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.
Mauna Loa Volcano, Hawaii, 1950.Mauna Loa Volcano, Hawaii, 1950.
Mauna Loa Volcano, Hawaii, 1950.
“Phreatic” (or steam-blast) eruptions are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted. Phreatic activity is generally weak, but can be quite violent in some cases, such as the 1965 eruption of Taal Volcano, Philippines, and the 1975-76 activity at La Soufrière, Guadeloupe (Lesser Antilles).
The most powerful eruptions are called “plinian” and involve the explosive ejection of relatively viscouslava. Large plinian eruptions—such as during 18 May 1980 at Mount St. Helens or, more recently, during 15 June 1991 at Pinatubo in the Philippines—can send ash and volcanic gas tens of miles into the air. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving deadly pyroclastic flows (“nuées ardentes”) are also commonly associated with plinian eruptions.
Kilauea Volcano, Hawaii, 1959.Kilauea Volcano, Hawaii, 1959.
Kilauea Volcano, Hawaii, 1959.
Taal Volcano, Philippines, 1965.Taal Volcano, Philippines, 1965.
Taal Volcano, Philippines, 1965.
Submarine volcanoes and volcanic vents are common features on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and explosive release of steam and gases. Even very large, deep-water eruptions may not disturb the ocean surface.
Schematic representation of a typical submarine eruption in the open ocean.Schematic representation of a typical submarine eruption in the open ocean.
Schematic representation of a typical submarine eruption in the open ocean.
The unlimited supply of water surrounding submarine volcanoes can cause them to behave differently from volcanoes on land. Violent, steam-blast eruptions take place when sea water pours into active shallow submarine vents. Lava, erupting onto a shallow sea floor or flowing into the sea from land, may cool so rapidly that it shatters into sand and rubble. The result is the production of huge amounts of fragmental volcanic debris. The famous “black sand” beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and sea water. Onthe other hand, recent observations made from deep-diving submersibles have shown that some submarine eruptions produce flows and other volcanic structures remarkably similar to those formed on land. Recent studies have revealed the presence of spectacular, high-temperature hydrothermal plumes and vents (called “smokers”) along some parts of the mid-oceanic volcanic rift systems. However, to date, no direct observation has been made of a deep submarine eruption in progress.
During an explosive submarine eruption in the shallow open ocean, enormous piles of debris are built up around the active volcanic vent. Ocean currents rework the debris in shallow water, while other debris slumps from the upper part of the cone and flows into deep water along the sea floor. Fine debris and ash in the eruptive plume are scattered over a wide area in airborne clouds. Coarse debris in the same eruptive plume rains into the sea and settles on the flanks of the cone. Pumice from the eruption floats on the water and drifts with the ocean currents over a large area.
Submarine eruption of Myojin-sho Volcano, Izu Islands, Japan on September 23, 1952.Submarine eruption of Myojin-sho Volcano, Izu Islands, Japan on September 23, 1952.
Submarine eruption of Myojin-sho Volcano, Izu Islands, Japan on September 23, 1952.
Geysers, fumaroles (also calledsolfataras), and hot springs are generally found in regions of young volcanic activity. Surface water percolates downward through the rocks below the Earth’s surface to high-temperature regions surrounding a magma reservoir, either active or recently solidified but still hot. There the water is heated, becomes less dense, and rises back to the surface along fissures and cracks. Sometimes these features are called “dying volcanoes” because they seem to represent the last stage of volcanic activity as the magma, at depth, cools and hardens.
Erupting geysers provide spectacular displays of underground energy suddenly unleashed, but their mechanisms are not completely understood. Large amounts of hot water are presumed to fill underground cavities. The water, upon further heating, is violently ejected when a portion of it suddenly flashes into steam. This cycle can be repeated with remarkable regularity, as for example, at Old Faithful Geyser in Yellowstone National Park, which erupts on an average of about once every 65 minutes.
Old Faithful Geyser, Yellowstone National Park, Wyoming.Old Faithful Geyser, Yellowstone National Park, Wyoming.
Old Faithful Geyser, Yellowstone National Park, Wyoming.
Fumaroles, which emit mixtures of steam and other gases, are fed by conduits that pass through the water table before reaching the surface of the ground. Hydrogen sulfide (H2S), one of the typical gases issuing from fumaroles, readily oxidizes to sulfuric acid and native sulfur. This accounts for the intense chemical activity and brightly colored rocks in many thermal areas.
Hot springs occur in many thermal areas where the surface of the Earth intersects the water table. The temperature and rate of discharge of hot springs depend on factors such as the rate at which water circulates through the system of underground channelways, the amount of heat supplied at depth, and the extent of dilution of the heated water by cool ground water near the surface.
Black Growler steam vents (fumaroles), Norris Basin, Yellowstone National Park, Wyoming.Black Growler steam vents (fumaroles), Norris Basin, Yellowstone National Park, Wyoming.
Black Growler steam vents (fumaroles), Norris Basin, Yellowstone National Park, Wyoming.
Mammoth Hot Springs, Yellowstone National Park, Wyoming.Mammoth Hot Springs, Yellowstone National Park, Wyoming.
Mammoth Hot Springs, Yellowstone National Park, Wyoming.
There are more than 500 active volcanoes (those that have erupted at least once within recorded history) in the world—50 of which are in the United States (Hawaii, Alaska, Washington, Oregon, and California)—although many more are hidden under the seas. Most active volcanoes are strung like beads along, or near, the margins of thecontinents, and more than half encircle the Pacific Ocean as a “Ring of Fire.”
The distribution of some of the Earth’s 500 active volcanoes.The distribution of some of the Earth’s 500 active volcanoes.
The distribution of some of the Earth’s 500 active volcanoes.
Many volcanoes are in and around the Mediterranean Sea. Mount Etna in Sicily is the largest and highest of these mountains. Italy’s Vesuvius is the only active volcano on the European mainland. Near the island of Vulcano, the volcano Stromboli has been in astate of nearly continuous, mild eruption since early Roman times. At night, sailors in the Mediterranean can see the glow from the fiery molten material that is hurled into the air. Very appropriately, Stromboli has been called “the lighthouse of the Mediterranean.”
Some volcanoes crown island areas lying near the continents, and others form chains of islands in the deep ocean basins. Volcanoes tend to cluster along narrow mountainous belts where folding and fracturing of the rocks provide channelways to the surface for the escape of magma. Significantly, major earthquakes also occur along these belts, indicating that volcanism and seismic activity are often closely related, responding to the same dynamic Earth forces.
In a typical “island-arc” environment, volcanoes lie along the crest of an arcuate, crustal ridge bounded on its convex side by a deep oceanic trench. The granite or granitelike layer of the continental crust extends beneath the ridge to the vicinity of the trench. Basaltic magmas, generated in the mantle beneath the ridge, rise along fractures through the granitic layer. These magmas commonly will be modified or changed in composition during passage through the granitic layer and erupt on the surface to form volcanoes built largely of non-basaltic rocks.