Volcanic eruptions

Why do volcanoes erupt?

Volcanoes erupt because of density and pressure. The lower density of the magma relative to the surrounding rocks causes it to rise (like air bubbles in syrup). It will rise to the surface or to a depth that is determined by the density of the magma and the weight of the rocks above it. As the magma rises, bubbles start to form from the gas dissolved in the magma. The gas bubbles exert tremendous pressure. This pressure helps to bring the magma to the surface and forces it in the air, sometimes to great heights.

It's sort of like the bubbles of gas in a bottle of soda. Before you open the soda you don't see many bubbles because the pressure in the bottle keeps the gas dissolved in the soda. When you open the bottle the pressure is released and the gas bubbles leave the soda. If you shake up the bottle first, the soda gets pushed out by the bubbles of gas as they rush out.

As the rocks move upward (or have water added to them), they start to melt a little bit. These little blebs of melt migrate upward and coalesce into larger volumes that continue to move upward. They may collect in a magma chamber or they may just come straight up. As they rise, gas molecules in the magma come out of solution and form bubbles and as the bubbles rise they expand. Eventually the pressure from these bubbles is stronger than the surrounding solid rock and this surrounding rock fractures, allowing the magma to get to the surface.

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. When you 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 diagram on the left shows that heat concentrated in the Earth's upper mantle raises 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 the production of magmas with widely different chemical compositions.

Types of Volcanic Eruptions

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, there are use of terms as "Strombolian", "Vulcanian", "Vesuvian", "Pelean", "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.

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.

Parícutin Volcano, Mexico, 1947. 

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 cauliflower-shaped cloud high above the volcano.

 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.

"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).

Taal Volcano, Philippines, 1965.

The most powerful eruptions are called "plinian" and involve the explosive ejection of relatively viscous lava. 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.

Mount St. Helens about noon, May 18, 1980.

There are many different types of volcanic eruptions. Different types of eruptions tend to form different types of volcanoes. Most volcanic eruptions are "builders," adding thin layers of lava or ash to the sides of a volcano, slowly building the easily recognized cone-shaped volcanoes seen around the world. Some eruptions, however, are "destructors" knocking off pieces of a volcano's summit or even destroying the entire mountain. Generally, small-to-medium eruptions build volcanoes, and large explosive eruptions tend to destroy them. Let's look at simple animations of some of the different types of eruptions.

The first animation shows a "building" or constructive eruption of a typical strato-volcano like Mount Rainier or Fujiyama. The volcano itself consists of a pile of successive layers of ash (light brown) and lava (dark brown) resting on a pre-existing surface. Prior to the surface eruption, a new mass of lava has surged into the established vent system deep under the mountain (not shown). Inflation of the mountain is negligible because the amount of fresh lava is small. The eruption begins with the invasion of the fresh lava along the old eruption channel leading to the summit vent. (Successive eruptions do not always occur at the old summit. Often, the lava works its way through weak rock or fractures within the cone and bursts out on the side of the mountain as a "flank eruption"). In this case, the first stage of the eruption begins with a mildly explosive fire fountain of fragmented liquid rock. The hot particles blast out of the vent, but since the gas content is low, the particles almost immediately begin to fall back under the influence of gravity. The particles coat the sides of the cone and build up a new layer of ash. In this case, the layer is thick enough to flow slightly after landing, engulfing some of the neighboring vegetation. Next, there is a short break in activity, during which the ash layer cools.

The second stage of the eruption is effusive: a large mass of gas-free lava percolates to the summit vent, fills it, and flows down the sides of the cone. The resulting lava flows spread out beyond the original mountain and then cool to form a hard cap over the weaker ash layers. Summit vents are typically irregular in shape. For instance, one side is often higher than the other, in which case the lava flows down only one side of the volcano. Usually it takes several effusive eruptions to build a complete new lava layer around a volcano's cone. The illustrated eruption now ends with the cooling of the lava remaining in the vent.

The illustrated sequence of explosive activity followed by effusive is fairly typical. Rising lava usually has some dissolved gas. If the fresh lava slows or temporarily stops in the subsurface vent system, the dissolved gas tends to migrate to the top of the mass of lava. Thus the first lava that comes out is usually more gas-charged and explosive than the lava expelled later during the same eruption. Of course, lava movements can be complicated, and lava compositions and gas contents do vary, so no two eruptions are alike.

The next animation illustrates destructive eruptions of different types. Although the general impression is that volcanoes "blow up," the forces in explosive volcanic eruptions are generally too small to literally blast large pieces of the existing volcanic cone upward. During violent eruptions some of the material lining the erupting vent is ripped free and carried into the explosion cloud with the new lava, but most of the material lost from the pre-existing cone during a destructive eruption simply breaks free and rumbles down the volcano's side as a giant rock avalanche or sinks downward into the earth. Sometimes only small sections of the original cone are lost this way, but sometimes--depending on the amount and location of the lava inside the mountain before the eruption--the entire summit of the volcano can be destroyed.

The 1980 eruption of Mount St. Helens is an excellent example of a destructive eruption. The material that was blasted upward so spectacularly and blown across eastern Washington and Idaho (about 1/4 cubic mile of material) was mostly made from the new gas-charged lava that had invaded the mountain's interior during the few months preceding the eruption. Nearly all of the rock missing from the pre-eruption cone went sideways down the hill instead of up into the air.

As destructive as a Mount St. Helens-type eruption is to the original volcano, there are some eruptions in which the entire volcano is annihilated! This happened at Krakatoa (and several earlier eruptions like Tambora and Santorini). Before the 1883 eruption, Krakatoa was a majestic cone at least 7000 feet high on an island 3 by 5 miles in size; after the eruption, only a shallow lagoon surrounded by a ring of small islands remained. Where had the main island gone? Conventional wisdom said that it had been blown up into the sky. But Krakatoa was the first of this type of eruption to be studied in detail, and no old volcanic rocks were found in the ash blasted all over Indonesia. The old rocks were found underneath the new lagoon--the old volcano had not blown up, it had sunk! Geologic studies at similar eruption sites have shown the same thing: the center of the mountain in this type of eruption sinks straight down, leaving behind a large crater called a "caldera." 

Let's look at another animation that illustrates how this type of volcano forms. Initially, we find the dormant (resting) volcano sitting on the earth's surface, but underlain by a huge underground lake of liquid rock called a magma chamber (magma is lava that is still underground). A "pipe" or conduit (not shown) connects the bottom of the magma chamber to the region where the liquid rock is formed. Occasionally, new masses of magma come up the conduit into the chamber. The new magma has dissolved gases that migrate to the top of the chamber. Just like putting air into a tire, adding new magma to the chamber increases the pressure inside and causes the old volcano on the surface to bulge upward and crack. Gas-charged magma creeps up the new cracks, causing the volcano to bulge even more. Eventually, the new magma opens a crack all the way to the surface, which becomes the exit vent for a new eruption. When that happens, the pressure in the magma at the top of the vent drops rapidly, causing the dissolved gas to form bubbles at an explosive rate. The magma in which the bubbles are forming is fragmented to ash and blown out the top of the vent. As that magma is blown out, the magma just below it in the vent suddenly has no rock on top of it, so it forms explosive bubbles and rockets out the vent as well, followed by the magma below it, and so on down the vent. The zone of explosive bubble formation and rocketing ash moves downward into the top of the magma chamber, allowing that magma to fragment and escape up the vent as well. As the upper part of the magma chamber empties, the overlying rock finds itself without visible (or any other) means of support, and does what comes naturally--it sinks. At the end of the eruption, most of the rock of the original cone has sunk down and left a big hole. Sometimes the hole fills with volcanic rocks from later, smaller volcanic eruptions (see the Valles Caldera); sometimes it fills with water forming a volcanic lake, like Crater Lake in Oregon. If at sea, the caldera fills with sea water, becoming a lagoon, like Krakatoa.

Magma chambers can get very large, as much as a hundred miles across. Consequently, caldera-type eruptions can be very large. The largest known explosive volcanic eruptions are of this kind.

Sizes of Eruptions

Volcanic eruptions come in all sizes: small, medium, large, extra large, giant economy size, etc. Giant eruptions can literally affect the whole world. On the other hand, small eruptions may affect only a single hillside or valley. Let's look at the characteristics and effects of volcanic eruptions of different sizes, starting with small and working our way up.

A small volcanic eruption may consist of a single small burst of steam and volcanic ash, such as the initial eruption of Mount St. Helens, or a single lava flow like those that make the local evening news in Hawaii. By exercising some caution, you can view many small eruptions from a reasonable distance. What is reasonable? It all depends on the type of eruption. Explosive eruptions (high-water, high silica), even small ones, are best seen from a distance of miles. On the other hand, effusive eruptions (low-water, low-silica) can be viewed from quite close if you position yourself properly. Large flows of very fluid lavas can attain speeds of 50 to 60 mph. However, with care and proper clothing, these flows can be observed and even sampled from the side at a distance of a few feet.

Some flows are very slow, and even the fastest flows eventually cool, slow, and stop. "Slow" here means human walking speed or less. You can observe slow flows from any vantage point - front, side, even on top (in some cases). G. A. McDonald tells of a volcanologist who had observed a slow-moving "blocky" flow through a long, hot morning in Hawaii. By noon, the flow had appeared to stop, and the volcanologist decided to have lunch on the top of the flow. After sitting and eating for awhile, the volcanologist noticed that the surrounding scenery was moving past! The flow he was sitting on was still creeping along and was not "dead" after all.

There is no universally accepted scale, comparable to the Richter Scale for earthquakes, for classifying the sizes of volcanic eruptions. However, one useful comparison is the volume of new volcanic rock blasted out by an eruption. Volumes of new volcanic rock are represented on this page in two ways: by the figure at right, which pictorially compares ejected volumes, and by the table below, which shows similar, numerical data. The designations of "small," "large," "major," and "great" eruptions are simply descriptive and are used only to draw attention to the enormous range in the sizes of volcanic eruptions.

* Volumes are approximate. 1 mi³ = 4.168 km³. The preceeding table was complied using data from the following sources: R. L. Smith (1979) GSA Special Paper 180, pp. 5-27; R. B. Smith and L. W. Braile (1994) J. Volc. Geotherm. Res. 61:121-187; J. J. Dvorak, C. Johnson and R. I. Tilling (1992) Sci. Am., August, pp. 46-53; J. M. Rhodes (1988) J. Geophys. Res. 93:4453-4466; F. Press and R. Siever (1974) Earth, W. H. Freeman & Co.  

Small volcanic eruptions-- The volume of volcanic material tossed out in the bursts or flows of small eruptions is relatively small: a few football stadiums full (equal to a few million cubic feet or a few ten thousandths of a cubic mile). No worries, mate! After all, the inhabitants of the island of Stromboli in Italy simply ignore the virtually constant thumps of red-hot lava "bombs" being tossed out of the vent of their resident volcano. Of course, just how big or important a particular eruption appears depends largely on how close you are to it at the time. Even a small steam explosion seems huge if you happen to be standing right next to the vent when it "blows." And a single lava flow seems catastrophic if it happens to go right through your bedroom!

Large volcanic eruptions-- Large eruptions often make national and even international headlines, so you have probably heard of some of these: El Chichon, Mount Redoubt (which nearly nailed a passing jetliner!), and Mount St. Helens. Large eruptions affect areas as far as tens of miles away and typically eject a few tenths of a cubic mile of volcanic rock and lava (figure). Depending on the local population distribution, large eruptions can be very destructive of life and property. Potential hazards in these eruptions include huge blasts of superheated steam and pulverized rock, rock avalanches, massive clouds of volcanic ash, asphyxiating gases, and, in the cases of volcanoes with snow-caps or glaciers, giant mudflows. The eruption of Vesuvius (79 AD) buried the towns of Pompeii and Herculaneum in Italy and killed thousands of people with falling ash and poisonous gases. The main blast of steam and rock from the 1902 eruption of Mount Pelee on the Caribbean island of Martinique destroyed St. Pierre in less than two minutes, killing all but two of the city's 30,000 inhabitants. Ironically, warnings about the dangers of an imminent eruption were down-played by public officials anxious to keep the voting populace in town for an upcoming election. Unfortunately, the volcano exploded 3 days before the scheduled election, annihilating both electors and potential electees. Mudflows triggered by a 1985 eruption of Nevado del Ruiz in Columbia swept through a small town and killed 25,000 people. In contrast, only 57 people were killed in the 1980 Mount St. Helens eruption, largely because of its remote location and the effectiveness of public warnings.

Major volcanic eruptions-- Mount Pinatubo in the Philippines in 1991 and Krakatoa in Indonesia in 1883 (Hollywood even made a movie about that one!) were tens to hundreds of times larger than the eruption of Mount St. Helens and are classified as major eruptions. These eruptions are often caldera-type eruptions. They eject many cubic miles of material and affect areas around them up to hundreds of miles away. Everything about major eruptions must be written in superlatives. Major eruptions are incredibly destructive. The very islands on which Krakatoa, Tambora (Indonesia, 1815), and Santorini (Greece, about 1500 BC) had stood simply disappeared into the sea during the eruptions. Tambora spread thick layers of ash and floating islands of pumice across 2000 miles of Indonesia. Superheated steam and ash from Krakatoa's main blast killed and burned people 30 to 40 miles away on the Sumatra coast. Krakatoa also spawned tidal waves over 100 feet high that swept the coasts of Java and Sumatra, claiming most of the 37,000 lives lost in that eruption. The destruction is so great and widespread in large and especially major eruptions that eyewitnesses often describe the eruptions and their aftermath as "the end of the world."

Major eruptions are incredibly loud. Noises from the eruption of Tambora in Indonesia in 1815 were mistaken for nearby canon fire in European settlements 200 and 750 miles away from the volcano itself. In each community, nervous military leaders sent out ships and soldiers to investigate what were thought to be attacks on nearby outposts! If the main eruption of Mount St. Helens had made as much sheer noise as Tambora, it would have been heard all over the United States.

Major eruptions can affect weather around the world. Clouds of ash and sulfur-rich particles thrown into the stratosphere by large volcanic blasts reflect part of the sunlight reaching Earth, causing cooling of the atmosphere. The larger the blast, the greater the effect. A global cooling of about 1.5 deg. F (detected by modern sensors) continued for about two years after the Mount Pinatubo eruption. Cooling was so severe following the Tambora eruption that 1816 was called "the year without a summer" or "Eighteen hundred and froze to death." Cold temperatures and killing frosts in Europe and New England in America caused extensive crop failures and resulted in famine in post-Napoleonic France. Many farmers in New England left for the frontier in New York in search of better weather, swelling the American "Move West."

Major eruptions have even been responsible at times for literally changing the course of history. The Mataram empire in Java was apparently destroyed by a major eruption of Merapi in 1006 AD. The course of Mayan civilization was changed when a major eruption of Ilopango in El Salvador destroyed the highland Mayas around 300 AD. Subsequent shifts in population, trade routes, and resultant economic power gave a great boost to the development of the "Classic Maya" civilization. And the eruption of Santorini in the middle of Bronze-age Europe apparently caused the destruction of the advanced Minoan civilization on Crete and the surrounding islands and allowed the expansion of the early Greeks. Ash clouds and tidal waves from Santorini may also have reached Egypt about the time the Hebrews were leaving the country and given rise to the stories of the biblical plagues in Egypt.

Lest we become complacent thinking that major eruptions only occur in other parts of the world, just remember that the blast that destroyed Mount Mazama to form the placid-appearing Crater Lake in Oregon occurred only about 7000 years ago. Almost any of the Cascade volcanoes could do a repeat performance at any time.

"Great" Eruptions-- As large and devastating as major eruptions are, they are not the largest volcanic eruptions known. None of these largest or "great" eruptions have occurred in historic times (thank goodness!). We have only gigantic deposits of volcanic rock as eloquent evidence of their reality. Great eruptions emerge from two types of volcanoes: fissure eruption and giant caldera. In both types, the volume of volcanic rock deposited on the surface of the earth during a single great eruption can be many hundreds of cubic miles--hundreds of times larger than the major eruptions mankind has known.

Why are some eruptions gentle and others violent?

Volcanic eruptions could be thought as a continuum between two end members. At one extreme is the gentle effusion of lava. Most Hawaiian eruptions would be a examples of this type of eruption. At the other extreme is the explosive ejection of ash from a vent. The May 18, 1980 Mount St. Helens eruption would be an example of this type of eruption.

The two main factors that influence how a volcano will erupt are viscosity and gas content. Both are related to the composition of the magma. Hawaiian volcanoes tend to erupt basalt, which is low in viscosity and low in gas content (about 0.5 weight percent). The gas that is present can readily escape and little pressure builds up in the magma. At the other extreme, rhyolite magmas are very viscous and can contain a lot of gas (up to 7-8 weight present). As the magma moves into the vent and the pressure drops, the gas wants to escape. The magma is very sticky and resists the expansion of the gas bubbles. Ultimately, enough bubbles grow and expand to blow the magma into ash size fragments and eject them violently into the atmosphere.

What is the approxiamate temperature of an eruption cloud?

The cloud that rises above a volcano is surprisingly cold, probably close to freezing! This is because the gasses are expanding so rapidly.
The temperature of the ash flows that can be erupted, on the other hand, are on the order of hundreds of degrees Celsius.

What are some of the more famous volcanic eruptions?

about 1500 B.C. - the destgruction of Thera in the eastern Mediterranean and the end of the Minoan civilization (which is the basis for the legend of Atlantics)
A.D. 79 - eruption of Vesuvius buried the Roman cities of Pompeii and Herculeneum
1783 - eruption of Laki, Iceland, caused a severe faminine, which resulted in the deaths of 20 percent of the population of Iceland
1815 - Tambora, Indonesia, the largest historical eruption
1883 - Krakatau, Indonesia, resulted in a huge tsunmai, which drowned 36,000 people
1912 - Katmai, Alaska, largest eruption of the 20th century