VOLCANOES Introductory Geology GES 101 Dr. Bergslien


For this class we will use a classification based on six basic kinds of volcanoes (though volcanologists have a minimum of 26 different classifications.) These basic six account for probably >90% of all volcanoes:

Shield volcano: a broad, slightly dome-shaped structure formed primarily of basaltic lava. Shield volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, 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. Are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock) lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Shield volcanoes are the largest volcanoes on Earth that actually look like volcanoes (i.e. not counting flood basalt flows or MORs). The Hawaiian shield volcanoes are the most famous examples. Shield volcanoes are almost exclusively basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep (you can't pile up a fluid that easily runs downhill). Eruptions at shield volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosivity fountaining that forms cinder cones and spatter cones at the vent, however, 90% of the volcano is lava rather than pyroclastic material. Shield volcanoes are the result of high magma supply rates; the lava is hot and little-changed since the time it was generated. Shield volcanoes are the common product of hotspot volcanism but they can also be found along subduction-related volcanic arcs or all by themselves.

- Examples of shield volcanoes are Kilauea and Mauna Loa (and the other Hawaiian islands), Fernandina (and its Gal'apagos friends), Karthala, Erta Ale, Tolbachik, Masaya, and many others.

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 as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings because of their short eruption history. Cinder cones are rarely found alone, usually there are in groups (Monogenetic fields - collections of sometimes hundreds to thousands of separate vents and flows.) or are vents on other, larger volcanoes. For example, Pu'u 'O'o is one of hundreds of vents on Kilauea, and it happens to be a cinder cone.

- Puu Lilinue and Pu'u'O'o in Hawaii, and Craters of the Moon National Monument are examples of Cinder cones.

Composite Cones or Stratovolcanoes: Stratovolcanoes comprise the largest percentage (~60%) of the Earth's individual volcanoes and most are characterized by eruptions of andesite and dacite - lavas that are cooler and more viscous than basalt. These more viscous lavas allow gas pressures to build up to high levels (they are effective "plugs" in the plumbing), therefore these volcanoes often suffer explosive eruptions. Stratovolcanoes are usually about half-half lava and pyroclastic material, and the layering of these products gives them their other common name of composite volcanoes. The lava at stratovolcanoes occasionally forms 'a'a, but more commonly it barely flows at all, preferring to pile up in the vent to form volcanic domes. Some stratovolcanoes are just a collection of domes piled up on each other. Stratovolcanoes are commonly found along subduction-related volcanic arcs, and the magma supply rates to stratovolcanoes are lower. This is the cause of the cooler and differentiated magma compositions and the reason for the usually long repose periods between eruptions.

- Examples of stratovolcanoes include Mt. St. Helens, Mt. Rainier, Pinatubo, and Mt. Fuji.

Mid-Ocean Ridge: This is sometimes considered to be one ~70,000 km-long volcano. Here, the plates are pulled apart by convection in the upper mantle, and lava intrudes to the surface to fill in the space. Or, the lava intrudes to the surface and pushes the plates apart. Or, more likely, it is a combination of these two processes. Either way, this is how the oceanic plates are created. The lava produced at the spreading centers is basalt, and is usually abbreviated MORB (for Mid-Ocean Ridge Basalt). MORB is by far the most common rock type on the Earth's surface, as the entire ocean floor consists of it. We know that spreading occurs along mid-ocean ridges by two main lines of evidence: 1) the MORB right at the ridge crest is very young, and it gets older on either side of the ridge as you move away; and 2) sediments are very thin (or non-existent) right near the ridge crest, and they thicken on either side of the ridge as you move away. Mid-ocean ridges are also the locations of many earthquakes, however, they are shallow and generally of small magnitude.

Flood Basalts: Flood basalts are yet another strange type of "volcano." Some parts of the world are covered by thousands of square kilometers of thick basalt lava flows - individual flows may be more than 50 meters thick, and individual flows extend for hundreds of kilometers. The old idea was that these flows went whooshing over the countryside at incredible velocities (e.g., like a flash flood). The new idea is that these flows are emplaced more like flows, namely slow moving with most of the great thickness being accomplished by injecting lava into the interior of an initially thin flow. The most famous US example of a flood basalt province is the Columbia River Basalt province, covering most of SE Washington State and extending all the way to the Pacific and into Oregon. The Deccan Traps of NW India are much larger and the Siberian Traps are even larger than that (but poorly understood). The Ontong Java plateau may be an oceanic example of a flood basalt province.

Rhyolite Caldera Complexes: Rhyolite caldera complexes are the most explosive of Earth's volcanoes but often don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure. The collapsed depressions are large calderas, and they indicate that the magma chambers associated with the eruptions are huge. In fact, layers of ash (either ash falls or ash flows) often extend over thousands of square kilometers in all directions from these calderas. Fortunately we haven't had to live through one of these since 186 AD when Taupo erupted in New Zealand (which was uninhabited at the time). Many rhyolite caldera complexes, however, are the scenes of small-scale eruptions during the long reposes between big explosive events. The vents for these smaller eruptions sometimes follow the ring faults of the main caldera but most often they don't. The origin of these rhyolite complexes is still not well-understood. Many folks think that Yellowstone, for example, is associated with a hotspot. However, a hotspot origin for most other rhyolite calderas doesn't work; they occur in subduction-related arcs. Examples of rhyolite caldera complexes include Yellowstone, La Primavera, Rabaul, Taupo, Toba, and others.

Chemical Composition of Magma The wide range of different types of volcanoes and volcanic products is due to differences in magma composition, which control the viscosity of the magma, and to the magmas gas content. VISCOSITY is the resistance of a fluid to flow.

Magma is less dense than solid rock, so it migrates upwards towards the surface. As it moves up the pressure decreases and gas becomes less soluble. The gas begins to escape from the magma causing explosions. The more gas trapped in the magma the more explosive the eruption. The gaseous portion of most magmas makes up between 1-6 percent of the total weight. The most common gas is water vapor


The more viscous the magma the harder it is for gas to escape and the more explosive the eruption. Magma rich in silica (SiO4 - most important rock forming molecule - quartz is the most common example) is much more viscous then magma poor in silica

Other terms:

Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.

Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.

Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles. (think Yellowstone).

Hot Spot: A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs and may not be associated with ocean ridges.

Lava: Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.

Lava Dome: Mass of lava, created by many individual flows, that has built a dome-shaped pile of lava.

Lava Flow: An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet-like body formed by outpouring lava.

Lava Fountain: A rhythmic vertical fountainlike eruption of lava.

Lava Lake (Pond): A lake of molten lava, usually basaltic, contained in a vent, crater, or broad depression of a shield volcano.

Lava Shields: A shield volcano made of basaltic lava.

Lava Tube: A tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away.

Magma: Molten rock beneath the surface of the earth.

Magma Chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.

Pumice: Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.

Pyroclastic material is another name for a cloud of ash, lava fragments carried through the air, and vapor. Such a flow is usually *very* hot, and moves *rapidly* under it's own power due to buoyancy provided by the vapors. Pyroclastic flows can extend miles from the volcano, and devastate life and property within their paths.

Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air. This includes ash, lava bombs, lava blocks, and cinders. (This is a general term which can be applied to anything that is erupted airborn instead of flowing)

More than 50 volcanoes in the United States have erupted one or more times in the past 200 years. The most volcanically active regions of the Nation are in Alaska, Hawaii, California, Oregon, and Washington. Volcanoes produce a wide variety of hazards that can kill people and destroy property. Large explosive eruptions can endanger people and property hundreds of miles away and even affect global climate. Some of the volcano hazards described below, such as landslides, can occur even when a volcano is not erupting.

Volcanic eruptions come in many different forms. Shield volcanoes typically spew lava accompanied only by hot gases. These lavas flow slowly down the mountain with speeds of 15 miles per hour or slower. Composite volcanoes can put forth lava accompanied by clouds of ash, bombs, and lava fragments (crystallized, glassy material) as well as hot gases. In some eruptions, ash and lava is buoyed by hot vapors and pours down the slopes of a volcano very rapidly, with speeds up to 100 miles per hour. This is called pyroclastic flow. In other cases hot material from the volcano can melt snow and ice at the volcano summit and the whole mass of mud and lava can sweep rapidly down the mountain, destroying everything in its path. This type of flow is called a lahar (mudflow).

Eruption Columns and Clouds: An explosive eruption blasts solid and molten rock fragments (tephra) and volcanic gases into the air with tremendous force. The largest rock fragments (bombs) usually fall back to the ground within 2 miles of the vent. Small fragments (less than about 0.1 inch across) of volcanic glass, minerals, and rock (ash) rise high into the air, forming a huge, billowing eruption column.

Eruption columns can grow rapidly and reach more than 12 miles above a volcano in less than 30 minutes, forming an eruption cloud. The volcanic ash in the cloud can pose a serious hazard to aviation. During the past 15 years, about 80 commercial jets have been damaged by inadvertently flying into ash clouds, and several have nearly crashed because of engine failure. Large eruption clouds can extend hundreds of miles downwind, resulting in ash fall over enormous areas; the wind carries the smallest ash particles the farthest. Ash from the May 18, 1980, eruption of Mount St. Helens, Washington, fell over an area of 22,000 square miles in the Western United States. Heavy ash fall can collapse buildings, and even minor ash fall can damage crops, electronics, and machinery.

Volcanic Gases: Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to reach the surface through small openings called fumaroles. More than ninety percent of all gas emitted by volcanoes is water vapor (steam), most of which is heated ground water (underground water from rain fall and streams). Other common volcanic gases are carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulfur dioxide gas can react with water droplets in the atmosphere to create acid rain, which causes corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low areas in concentrations that are deadly to people and animals. Fluorine, which in high concentrations is toxic, can be adsorbed onto volcanic ash particles that later fall to the ground. The fluorine on the particles can poison livestock grazing on ash-coated grass and also contaminate domestic water supplies.

Cataclysmic eruptions, such as the June 15, 1991, eruption of Mount Pinatubo (Philippines), inject huge amounts of sulfur dioxide gas into the stratosphere, where it combines with water to form an aerosol (mist) of sulfuric acid. By reflecting solar radiation, such aerosols can lower the Earth's average surface temperature for extended periods of time by several degrees Fahrenheit (°F). These sulfuric acid aerosols also contribute to the destruction of the ozone layer by altering chlorine and nitrogen compounds in the upper atmosphere.

Lava Flows and Domes: Molten rock (magma) that pours or oozes onto the Earth's surface is called lava and forms lava flows. The higher a lava's content of silica (silicon dioxide, SiO2), the less easily it flows. For example, low-silica basalt lava can form fast-moving (10 to 30 miles per hour) streams or can spread out in broad thin sheets up to several miles wide. Since 1983, Kilauea Volcano on the Island of Hawaii has erupted basalt lava flows that have destroyed more than 200 houses and severed the nearby coastal highway.

In contrast, flows of higher-silica andesite and dacite lava tend to be thick and sluggish, traveling only short distances from a vent. Dacite and rhyolite lavas often squeeze out of a vent to form irregular mounds called lava domes. Between 1980 and 1986, a dacite lava dome at Mount St. Helens grew to about 1,000 feet high and 3,500 feet across.

Pyroclastic Flows / Pyroclastic Surge : High-speed avalanches of hot ash, rock fragments, and gas can move down the sides of a volcano during explosive eruptions or when the steep side of a growing lava dome collapses and breaks apart. These pyroclastic flows can be as hot as 1,500 °F and move at speeds of 100 to 150 miles per hour. Such flows tend to follow valleys and are capable of knocking down and burning everything in their paths. Lower-density pyroclastic flows, called pyroclastic surges, can easily overflow ridges hundreds of feet high. Damage from pyroclastic flows can occur by impact of rock fragments moving at high speeds or burial of the surface with ash and coarser debris a foot or more thick. Hot pyroclastic surges may start fires and kill or burn people and animals.

An extremely destructive eruption of Mount Pelee occurred in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed almost instantly by a pyroclastic flow which swept down the mountain. The cloud of hot ash and gases swept into town at an estimated speed of 100 miles per hour or more. It would only have been 5 minutes for the pyroclastic flow to sweep from the volcano's summit into town.

The climactic eruption of Mount St. Helens on May 18, 1980, generated a series of explosions that formed a huge pyroclastic surge. This so-called "lateral blast" destroyed an area of 230 square miles. Trees 6 feet in diameter were mowed down like blades of grass as far as 15 miles from the volcano.

Volcanic Landslides: A landslide or debris avalanche is a rapid downhill movement of rocky material, snow, and (or) ice. Volcano landslides range in size from small movements of loose debris on the surface of a volcano to massive collapses of the entire summit or sides of a volcano. Steep volcanoes are susceptible to landslides because they are built up partly of layers of loose volcanic rock fragments. Some rocks on volcanoes have also been altered to soft, slippery clay minerals by circulating hot, acidic ground water. Landslides on volcano slopes are triggered when eruptions, heavy rainfall, or large earthquakes cause these materials to break free and move downhill.

At least five large landslides have swept down the slopes of Mount Rainier, Washington, during the past 6,000 years. The largest volcano landslide in historical time occurred at the start of the May 18, 1980, Mount St. Helens eruption.

Lahars: Lahars are mudslides caused by the mixing of volcanic ash and debris with water. They can occur when the heat from a volcano melts snow and ice on the volcano's summit, or if an eruption disturbs a crater lake. These flows of mud, rock, and water can rush down valleys and stream channels at speeds of 20 to 40 miles per hour and can travel more than 50 miles. Some lahars contain so much rock debris (60 to 90% by weight) that they look like fast-moving rivers of wet concrete. Close to their source, these flows are powerful enough to rip up and carry trees, houses, and huge boulders miles downstream. Farther downstream they entomb everything in their path in mud. Lahars can cause great environmental and economic damage. They can cover fertile fields and topple buildings. Trees, boulders, and other debris which lahars pick up can shear off anything they flow by at ground level.

Historically, lahars have been one of the deadliest volcano hazards. They can occur both during an eruption and when a volcano is quiet. The water that creates lahars can come from melting snow and ice (especially water from a glacier melted by a pyroclastic flow or surge), intense rainfall, or the breakout of a summit crater lake. Large lahars are a potential hazard to many communities downstream from glacier-clad volcanoes, such as Mount Rainier. Lahars are very dangerous, and anyone caught in the path of one is in great danger of death from severe crushing injuries.

Lava: Lava is the word for magma (molten rock) which is extruded onto the surface of the Earth. Upon being released from the magma chamber and cooling, lava solidifies into rock. The term lava is used to describe active flows, solidified deposits, and fragments hurled into the air by explosive eruptions. Lava comes in many different forms, among them are:

'A'a - Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.

Pahoehoe - Lava with a smooth, bulbous, or ropy appearance; its highly variable surface texture can lead to bizarre shapes.

Block lava - A solid rock fragment greater than 64mm in diameter which was ejected from a volcano or lava flow.

Bomb lava - Also known as volcanic bombs; lava fragments greater than 64mm in diameter which were ejected while partially molten.

Pillow lava - Lava released underwater forms elongate mounds or pillows.

Ash - Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted.

Pahoehoe flows can evolve into lava tubes. One way that tubes form is by the crusting over of channelized lava flows. As the crust on a flow becomes thicker, it insulates the lava in the interior of the flow. The lava drains down slope and feeds the advancing front or flows into the ocean. When the eruption stops or the vent is abandoned, the lava drains from the tube.

Unlike the advancing front of a pahoehoe flow, which is fed by a lava tube, an advancing aa flow is fed by a channel.

Aa or Pahoehoe

  1. Pahoehoe and aa can be found as parts of the same lava flow, with no significant difference in chemical composition between the two forms.
  2. Pahoehoe may change to aa, but never the reverse.
  3. Most active pahoehoe flows are less viscous and have higher temperatures than aa flows.
  4. Vesicles (air/gas pockets) in pahoehoe tend to be spheroids, whereas those in aa tend to be irregularly shaped, suggesting deformation caused by continued movement during final stages of solidification.
  5. Even though aa tends to be more viscous, molten lava of approximately the same initial viscosity may form either pahoehoe or aa.
  6. In addition to the effects of increasing viscosity, lava tends to change to aa when subjected to flow turbulence and internal shearing, such as during fountaining, flowing down steep slopes or over precipices, or during prolonged flowage for great distances. Scientists have concluded that a critical relationship between viscosity and the amount of internal disturbance due to flowage determines whether pahoehoe or aa is formed.

How close can you get to lava ?
That depends on how active the flow is and what direction the wind is blowing. Sometimes, particularly with pahoehoe flows that have a large percent of their surface being solid or near-solid skin, you can approach close enough to collect a molten sample with a hammer. This is even easier if the wind is at your back. On the other hand, if the wind is blowing on you after passing over the flow, or the flow is 'a'a, with a large percentage of exposed incandescent lava, you can't get closer than about 10 m.

How long will it take for lava to cool enough to walk on ?
Pahoehoe lava will support your weight after only 5-10 minutes once it stops moving. It will still be pretty hot and uncomfortable, and probably might melt your boots, but if you absolutely had to get across such a flow you could.

'A'a flows are a different story. They are the big rough-surfaced flows with a top surface made up of broken and spiny blocks called clinkers. You can walk on an 'a'a flow while it is still moving but it is not very pleasant. 'A'a is not very fluid so you don't have to worry about sinking in but because there are so many spaces between the clinkers for heat to escape, they are really hot. You would only cross a hot one if you absolutely had to.

How fast does lava flow ?
Lava can move in broad flat lava flows, or it can move through constrictive channels or tubes. Lava flows have a large surface area so they tend to cool quickly and flow slowly. The fastest unconstricted lava flows at about 6 mi/hr (an easy jog), but rarely do they exceed speeds of between 2/3 and 1/3 mi/hr.

Lava in channels or tubes on the other hand can move quite quickly. It tends to stay hotter and maintain a lower viscosity. It can typically move up to 23 mi/hr, a sprinter's top speed. It has, however, been clocked at up to 35mi/hr, which is faster than is humanly possible to run.

Pahoehoe, the smooth form of basalt usually travels very slowly-- at speeds of only a few meters per hour, averaged over the whole flow. 'A'a, the rough clinkery form of basalt travels faster, ranging from a few hundred meters per hour up to 10 km per hour on steep slopes.

When you get to lava compositions such as andesite, dacite, and rhyolite, where the viscosities are really high, the lava flows become very slow--perhaps moving at average speeds of only a few meters per day.

More Terms -

Plinian Eruption: An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic.

Strombolian eruptions are characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly. The lava fragments generally consist of partially molten volcanic bombs that become rounded as they fly through the air.

Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits). The intense heat of such material (as high as 1,170° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and bombs. Technically no magma is erupted.

A Vulcanian eruption is a type of explosive eruption that ejects new lava fragments that do not take on a rounded shape during their flight through the air because the lava is too viscous or already solidified. These moderate-sized explosive eruptions commonly eject a large proportion of volcanic ash and blocks. Andesitic and dacitic magmas are most often associated with vulcanian eruptions, because their high viscosity (resistance to flow) makes it difficult for the dissolved volcanic gases to escape except under extreme pressure, which leads to explosive behavior.

Active volcano
Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Scientists also consider a volcano active if it has erupted in “historic time.”

Dormant volcano
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.

Extinct volcano
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For example, since calderas have lifespans sometimes measured in millions of years, a caldera that hasn't produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.

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