Volcanoes are all over the Earth.  Until one erupts and sends up a spray of lava or a cloud of smoke and ash they go largely unnoticed.  Those that have not erupted in the age of television are looked at as just another mountain.  We should be careful about that, however, because volcanoes can lie dormant for thousands of years before suddenly bursting forth with a blast of molten rock, caustic dust and poison gas.  Let’s explore these sleeping giants to better understand where they are, what they are, and the mechanisms by which they can cause immense damage.

The potentially bad news is that there are lots and lots of volcanoes.  The good news is that we know pretty well where they are, and most of them are “dormant”—not likely to erupt soon.


Volcanoes form as the result of one of three geological processes, all related in one way or another to plate tectonics.  The surface of the Earth (including the surface of the Earth under the sea), called the lithosphere, consists of about 8 large “plates” or huge, but relatively thin, masses of rock, and a large number of small plates with a thin layer of dirt or “crust” on top.  The rock making up the plates varies widely in thickness.  Continent plates are often 120 miles thick.  Those under the oceans are generally thinner—in some places as thin as 3 miles.  All plates ride on the uppermost part of the Earth’s mantle called the asthenosphere.  It is a hot, viscous, plastic layer of molten rock that allows the plates to move about in response to heat convections through it from the deeper mantle. 

The Earth's Tectonic Plates

A Section Through The Earth Showing Its Layers

The plates are much like the stitched-together pieces of the covering of a volley ball or soccer ball.  However, the plates are of dramatically different sizes and varying shapes.  More importantly, unlike the surface of a sports ball, the plates move around.  In fact, over many, many millions of years the movement of the tectonic plates has rearranged the Earth’s surface and its continents many times.  Places that are now tropical rain forests on the equator were once at the South Pole and vice-versa.  Click here to see an animation of the movement of the tectonic plates over millions of years.

Not so long ago, geologically speaking, the entire dry surface of the Earth was compressed into one large continent—called by geologists—Pangaea.  Pangaea was assembled out of tectonic plates that broke up from a previous super continent—Gondwanaland.  Gondwanaland was preceded by yet another almost super-continent—Rodinia, but we don’t have the data to go back much farther than that.

Pangea, The Last Super Continent


The plates move.  The plates collide.  The plates separate after a collision.  As the collisions occur some plates are pushed beneath other plates.  Some plates, mostly under ocean plates, are splitting down the middle, separating and getting wider while molten magma pushes up along the split to fill the gap.  Earth’s plate tectonics are the biggest train wreck in the Solar System.  (Given the dynamics of the orbital motion of planets and asteroids in the mature Solar System, collisions of those bodies are unlikely for billions of years.)  It is a train wreck in extreme slow motion and exquisite silence.  Slow motion to us because humans do not live long enough to see much visible evidence of the motion, at best minor displacement along active faults. Accurate measuring instruments, especially on satellites, do record the travels of the plates.  It is also a silent train wreck, but once again instruments can record the groans of the Earth as the plates work against one another emitting sounds at frequencies far too low to be heard by human ears.  Persons in earthquakes, however, have reported hearing loud groans seemingly from the Earth just before and during quakes. 

This is all important to us because the collisions of the plates are the principal cause of volcanoes.  When plates collide, things happen.  First, some of one plate gets pushed UNDER the other.  Second, the upper of the two often scrapes up the crust off the lithosphere of the other just like a bulldozer scrapes up dirt with its blade.  Well, actually, the crust mostly gets scraped up from off of a thin layer of rock called the Mohorovicic discontinuity, or “Moho” for short, named for its Croatian discoverer.  The Moho separates the crust and the lithosphere and represents a poorly understood, but significant change in the properties of the rock at that level.  The scraped-up dirt and rocks pile up against the leading edge of the upper tectonic plate, sometimes very deeply, and make hills and mountains.  The best examples of the scraping effect are the Swiss Alps, pushed up by the collision of the African Plate with the Eurasian Plate, and the Himalayas, pushed up by the collision of the Indian Plate with the Eurasian Plate. 

Subduction Of One Plate Beneath Another

Click here to see the Indian Plate Collide with the Eurasian Plate 


Volcanoes generally don’t result directly from the formation of scraped up mountains, but when one plate gets pushed under another—the “subduction”—volcanoes form from the heating that happens when cool rock from near the Earth’s surface is subducted down to near the Earth’s blistering hot mantle.  The subducted plate melts.  If it contains significant quantities of water, such as the parts of a plate originally under an ocean might, the water can become super hot, but it does not turn to steam because of the immense weight and pressure of the overlying plate.  Instead, it remains a super hot liquid.  Molten rock and super hot water have a greater volume than the same mass (weight) of rock and water when they are cold.  The molten rock and water are lighter per volume unit than the overlying cooler rock.  Like a hot air balloon the molten rock and expanding steam push upward through the over lying layers of cooler rock and form—right!—volcanoes, often starting with an explosive rush of lava, ash and superhot steam sent high into the sky.

The Pacific Plate moves a lot and has caused large numbers of volcanoes around its edges.  The so-called “Ring of Fire” that surrounds the Pacific Ocean is made up of volcanoes caused by subduction.  Along the west coast of North America where the Pacific Plate and related smaller plates have pushed under the North American Plate, the Sierra Nevada, Cascade Range in California, Oregon and Washington, the Canadian Rockies and the Aleutians in Alaska are volcanic in origin.  Where the Pacific Plate is subducted under the South American Plate, the Andes contain many active volcanoes.  The volcanoes of the South Pacific such as the recently violent Mount Merapi in Indonesia result from the Pacific Plate-Australian Plate collision.  Those in the Philippines, including active Mount Pinatubo, in Japan with the mostly dormant Mount Fuji, and Karymsky on the Kamchatka Peninsula in far western Russia all result from the Pacific Plate versus Eurasian Plate collision.

The Pacific Ring Of Fire 

Mount Fuji

Some mountains formed by volcanic action resulting from plate collision are not obviously volcanic in origin.  California’s Sierra Nevada are not obviously volcanic like the adjacent Cascades, where Mt. Saint Helens forcefully identified itself as a volcano not long ago (for eruption video click here), and as did Mount Lassen in 1914, and the volcanoes of Mexico, like Colima and Popocatepetl, close to Mexico City and both active within the last few years.  The Sierra Nevada were largely formed by the elevation of humongous molten granite balls, called plutons, some as much as 100 miles in diameter, pushing their way up through the lithosphere to near the surface.  The plutons formed from molten rock formerly part of the Pacific Plate subducted under the North American Plate.  Erosion has revealed much of them in places like Yosemite; in others the sedimentary crust they elevated along with their rise remains on top.  Volcanic action is visible in the Sierra Nevada, however.  On their eastern flank hot springs abound; dead cinder and pumice cones, are all around, and one of the potentially most dangerous volcanoes in North America hides.  The Long Valley Caldera where molten rock sometimes rises to within a few miles of the surface is covered by a placid lake masking the potential violence below.  (See the article on the Eastern Sierra on this web site.)  The Long Valley Caldera erupted 760,000 years ago, covering most of the western US with ash several feet deep.

Mt. Lassen Erupting in 1915 provided by Wikipedia Commons

The Sierra's Long Valley Caldera provided by Wikipedia Commons


The second aspect of plate tectonics that causes volcanoes is the separation of two plates where they are pressed together or the division of one plate near its middle.  Where they separate or divide the lithosphere becomes very thin and magma from the asthenosphere can rise through the thinned surface to form a volcano or a zone where lava pours out along a lengthy split.  These occur most frequently at mid ocean ridges.  Volcanoes are forming as you read this along the mid-Atlantic separation.  The North American and South American plates are moving west and the Eurasian and African plates are moving east, leading to a mid Atlantic rift.  Some volcanologists think it is manifesting itself in a more visible way in the form of Iceland, one of the busiest places on Earth volcanically speaking.  Others think Iceland is a hot spot, discussed below.  

The volcanic Islands of Tenerife and the Azores are of plate separation volcanic origin.  More commonly, a plate separation evidences itself with minor volcanic activity—vents of super hot water and minerals showing that magma is just below the surface of the ocean floor. 

In the case of subduction volcanoes, nothing can live in the 1300- to 2400-degree Fahrenheit temperatures of magma from subduction volcanoes, but sub-ocean volcanoes have to warm a lot of water when they erupt.  That water may be at only a few hundred degrees Fahrenheit and a wide variety of life can survive there.  They live on nutrients created by chemical synthesis carried out by bacteria capable of utilizing the chemicals and heat pouring out of the volcano.  They in turn are consumed by ever larger animals until those recognizable to us as life emerge—shrimp, worms, shellfish, and corals.  All are fed indirectly by the volcano as no sunlight penetrates even a small fraction of the distance from the surface of the sea to where these creatures live. 

A "Black Smoker" Where the Super Hot Water of an Undersea Volcano Emits Dissolved Chemicals, Fueling Both Microscopic Life As Well As Small Ocean Creatures

A "Mat" of Bacteria Fed By An Undersea Volcanic Vent


The third way volcanoes form is not directly connected to plate tectonics, but tectonics manifests itself in where volcanoes of this third type are located.  In a few places around the Earth circulation of the molten rock in the Earth’s core causes a narrow column of very hot rock to puncture both the asthenosphere and the lithosphere and reach the surface.  Often these narrow columns of lava, or “mantle plumes,” spread out when they approach the surface and stay subterranean.  In others they push through and manifest themselves as “hot spots” creating volcanoes on top of the column. 

The Hawaiian Islands are the product of a mantle plume.  Mauna Loa and the southeast end of the Big Island lie just northwest of the place where the plume is currently breaking through the surface of the lithosphere.  The exact spot is deep in the ocean about 22 miles off the Big Island, but still on the flank of Mauna Loa, where the plume is starting to build a volcano and a eventually a new island called by scientists Lo’ihi, which is a Hawaiian word for “long.”  The plume is mountain building just as it built Mauna Loa and Mauna Kea and Haleakala in the past, pushing out lava which runs down the side of the mountain, building a taller and taller cone until it breaks the water’s surface and climbs higher and higher.  The reason the plume is pushing out lava at the bottom of the sea rather than under Mauna Loa is that, despite their name, hot spots do not stay in one spot.  Actually, they do stay in one spot relative to the asthenosphere and the mantle, but not with respect to the surface of the lithosphere which, due to plate tectonic motion (with us along for the ride) is slowly moving to the northwest relative to the hot spot, causing the mantle plume to appear to have moved southeast.

This Is Where Hawaii's Newest Volcano Is Located.

And This Is Lo'ihi Herself.

The Hawaiian Islands are only the most recent manifestation of the Hawaiian mantle plume.  Leading off to the northwest from Hawaii is a string of islands and atolls that were the previous surface locations of the hot spot.  The Emperor Sea Mounts, worn down by millions of years of erosion, barely break the surface, but are the remains of previous “Hawaiian” island volcanoes. 

Iceland is thought by some to be a hot spot, but as it lies close to the division between the North American Plate and the Eurasian Plate there is a good deal of debate about Iceland’s volcanism.  But hotspot or plate separation, Iceland sure has a lot of volcanoes.  During the 20th century there were 39 recorded eruptions on and around Iceland.  In 984 (yes-the Vikings had colonized Iceland by then) the world's largest basaltic eruption ever witnessed pushed out of Mount Katla.).  In 1783 Mount Laki had the world’s second largest observed basaltic eruption, and we all saw the videos of the spectacular 2010 eruption of Eyjafjallajökull. 

Eyjafjallajökull Erupting

No surprise probably, but volcanoes are so common in Iceland that there is a local mythology about many of them.  For example, the caldera of the volcano pictured to the left--Hekla--is said by locals to be so deep that it reaches the netherworld.  One of The Real Mr. Science's correspondents reports that those Icelanders who live near Hekla say that if you climb the volcano and stand quietly on the rim, you can hear the voices of the souls in hell crying for help.  The locals also say the voices are speaking Danish, which probably says more about the relations between Iceland and Denmark than the actual depth of the Hekla caldera.  

Other recently-active hot spots include the Galapagos Islands, Yellowstone, and Réunion Island.  When the Réunion hotspot was under what is now India about 65 million years ago (Remember that the Indian Plate is moving north to collide with the Eurasian Plate and build the Himalayas), it erupted off and on for perhaps 30,000 years, covering over a million square miles with lava thousands of feet deep.

Deccan Traps photographed by Lazlo Keszthelyi

Volcanoes are generally classified into four basic kinds depending on how they are formed and their resulting shapes:  cinder cones, composite volcanoes, shield volcanoes and lava domes.  Their shapes are determined largely by the type, temperature and viscosity of the rock that pours out of them.


Lava domes are similar in shape to a shield volcano, but usually much smaller and a lot messier.  When a relatively small mass of almost, but not quite, solidified lava is forced to the surface by magma below, it is too thick to flow very far from the vent.  It tends to pile up around, and frequently on top of, the vent.  Continued pressure from below forms a dome.  Its surface cools and hardens, but as the lava below continues to be pushed up, the surface of the dome cracks into pieces that roll down the dome’s sides.  This cycle can be repeated frequently, the dome being pushed higher and higher from within, cracking into pieces and forming a field of irregular rubble.  When the pressure below subsides, the dome usually collapses, leaving a more or less round caldera of rubble with a depressed interior.  Once again eastern California shows us some lava domes.  The so called Mono Craters, a group of five craters, are lava domes as is Obsidian Dome, so named because the lava that formed it is rich in silicon, which solidifies as obsidian.  

Mono Craters Near Mono Lake In California's Eastern Sierra.  These are lava domes.  Courtesy of the US Geological Survey


Shield volcanoes, like this one in Iceland, which look like a gigantic Viking’s shield without a point, lying on the ground, are built by numerous fluid lava flows.  These liquid rock flows spread out from the volcano’s vent in all directions, but because they are very liquid (as a result of being very hot basalt) they spread evenly and thinly.  Hundreds or thousands such eruptions slowly build the shield.  A shield volcano becomes taller closer to its vent because both large and small eruptions deposit lava near the vent building up the area around it, while only large eruptions produce enough lava to flow down to and build up the lower slopes.  The quintessential shield volcano is Mauna Loa.  Many, many thousands of eruptions slowly built it from the sea floor, 15,000 feet below the surface, to over 13,000 feet above sea level, a total height of 28,000 feet.  It is the largest shield volcano and the largest volcano of any type on Earth.  Check back in a few million years to see if Lo’ihi has surpassed Mauna Loa in size.


Cinder cones form from the cinders that result when particles and masses of congealing lava are blown out of the volcanic vent.  Expanding gasses force these particles high in the air where they solidify from rapid cooling and fall as cinders onto the area around the vent.  Naturally, more fall near the vent and fewer farther away, so that the area close to the vent is filled higher.  This forms a generally symmetrical dome.  The slope of the ground on which the volcano forms or a strong prevailing wind may cause the cone to assume a lopsided shape.  Cinder cones can be quite small, perhaps as little 50 or 100 feet high, but Paricutin in Mexico, which erupted from 1943 to about 1952, built a cinder cone 1,200 feet high covering many square miles.  Cinder cones frequently erupt by explosively emitting a red or brown rock, full of small gas pockets called “cinders.”  Other cinder cones emit pumice, a grayer material of similar consistency.  Geologically active areas display many cinder cones.  For example, the Owens Valley, east of the Sierra Nevada Mountains in California, has many cinder cones, some only 50 or 100 feet tall, but unmistakably cinder cones.

A cinder cone located on the flank of Mauna Loa.  It is about 300 feet tall.


The fourth type—composite volcanoes, sometimes called Stratovolcanoes—are what most of us think of as a volcano.  They have a triangular profile with steep sides sloping and curving up to a small central vent.  Picture Japan’s Mt. Fuji, Italy’s Mt. Vesuvius, Mt. Shasta in California, Mt. Rainier in Washington, Mt. Cotopaxi in Ecuador and you will have a good picture of a composite volcano.  These are complex creatures.  They build their forms using lava, cinders, ash, pumice—more or less anything that can be melted and blown out of the vent, hence the name “composite.”  The composite volcano is the shape it is because it erupts through a conduit system.  Often a large pipe like vent rises from a deep magma chamber and carries the lava, cinders and ash to the mouth at the mountain’s top, but the “pipe” is not solid, and cracks and side channels develop.  Some erupt on the sides of the cone; others carry lava to near the top.  As the lava in these side channels cools and hardens they provide support for the immensely tall cone, hence the name Stratovolcano. 

A Classic Composite Volcano

Mount Shasta in California is a large composite volcano.


Titanic explosions of composite volcanoes are well documented.  Mt. St. Helens in southern Washington exploded in 1980, literally blowing its top off and covering large parts of the west with ash and pumice.  Crater Lake in Oregon was once Mt. Mazama.  About 6,800 years ago it blew up in an even more spectacular way than Mt. St. Helens.  It spewed so much material that its magma chamber was emptied and collapsed, forming a deep caldera that filled with water.  A small cinder cone in the middle of the lake, Wizard Island, formed as Mt. Mazama’s last dying gasp. Picture courtesy of Wikipedia Commons.


Whether a particular volcano develops as a cinder cone, a shield volcano, a stratovolcano or a composite depends on many factors.  These are the primary ones:  First is the material at hand.  What kinds of rock is the magma coming to the surface composed of?  Some rock more easily forms cinders; some wants to stay in a thick bread dough-like form.  Second is temperature.  How hot is the magma?  Generally, the hotter the magma the more liquid it is and the more likely to pour out uniformly into a gentle shield.  Third, how much material is there?  Quantity will affect both size and shape.  Fourth, how much pressure is there pushing the magma up?  Very high pressure often results in explosive eruptions that destroy the volcano's shape and emit large rock masses. High pressure may result in magma being projected into the air, where it cools and forms ash or cinders.  Moderate but steady pressure pushes the magma out uniformly forming a stratovolcano or a shield depending on the viscosity (usually determined by temperature) of the magma. Fifth is the thickness, density and structure of the layers of rock through the magma erupts.  If the rock is dense and little fractured a large magma chamber may form in which pressure builds until there is an explosive eruption, frequently leaving a large caldera.  If there is a single primary fracture through which the magma erupts without exploding, a cinder cone, shield volcano or stratovolcano will form depending on the other factors.  If the rock above the magma is fractured in multiple places, huge lava flows that we often do not call volcanoes form, such as the Deccan Traps in India. Lesser flows through a fractured overlying layer often produce multiple small cinder or pumice cones.  Moreover, the determining conditions change over time.  The plate collisions can fracture otherwise solid rock.  Subducted rock can be pushed deeper and deeper and so become hotter and hotter.  Plate subduction can stop as when a plate makes the equivalent of a left turn, heads off in another direction and, thereby “dries” up the magma source.  Also subduction can speed up, forcing more of a plate under an adjacent one and accelerating the magma production process. 


Volcanoes are not confined to Earth. Both Mars and Venus have volcanoes. On Mars Olympus Mons is a huge shield volcano several times larger than Earth’s Mauna Loa. On Venus Maat Mons appears to be a large composite volcano and bears a resemblance to Earth’s Mount Shasta. Jupiter’s moon Io has sulphur volcanoes heated by the interaction of Io with Jupiter’s powerful magnetic field. Mars’ Olympus Mons and Venus’ Maat Mons were probably created by the heat of their planets’ respective cores just as with volcanoes on Earth, but we cannot be sure. Neither Mars nor Venus is thought to have crustal plates that move and collide.

A recent issue of Sky & Telescope Magazine has a fine article on the volcanoes on other planets and moons: Where The Hot Stuff Is.  Better yet, the author, Rosaly Lopes gives a 15 minute interview about the Solar System's volcanoes that can be heard here: Mountains of Fire.

Olympus Mons on Mars. Picture courtesy of NASA.

Maat Mons on Venus.  Image created by NASA using data from the Magellan space craft's radar.

Volcanoes are interesting beasts.  They can seem so serene and beautiful while beneath their surface molten rock and immense pressures can combine to emit millions and millions of tons of ash and other mineral materials over vast territories.  Recent explosions of Mt. St. Helens, Mt. Pinatubo and Iceland’s Mts. Katla and Eyjafjallajökull should teach us that we need to keep a close eye on these slumbering giants lest they accidentally step on us without even noticing.

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