Volcanoes – shapes, sizes, eruptive styles

Q: My daughter saw a photo of a volcano throwing fiery things into the night sky. Do all volcanoes do that?
-Jo S

A: There are as many different kinds of volcanoes and volcanic features as there are different bears in the forest. And like bears and humans, each has a different “personality” – and even this evolves over time. This is one reason why volcanic eruptions are not always easy to predict. Volcanoes can be classified by their shape – but their eruption styles can be classified also.
In the Pacific Northwest there are roughly 3,500 different volcanic features, ranging from small spatter-cones to visually arresting, conical-shaped composite “stratovolcanoes” like Mount Hood and Mount Shasta. However, there are many other kinds of volcanic features, including cinder cones, lava domes, shield volcanoes, supervolcanoes, and large igneous provinces. There are more types of different volcanic features than I can describe in a single chapter; in my lifetime I’ve seen many but not all of them. An issue with describing volcanoes is that they change in shape as they evolve, and then subsequent erosion further erases key features over time. Roughly following increasing size, volcanoes can be broken up into five to seven main categories:

Cinder cones:
These are the simplest and most common type of volcanic product you might see. These are built up from blobs and spatterings of congealing lava – molten material spit out (under gas pressure) from a single vent. As the still-liquid lava is ejected energetically into the air, it breaks into small fragments that solidify and come down as cinders around the source vent. These form a generally circular cone or mound, often with a divot taken out of the top where the original vent once was. However, sometimes these cones form as strings of volcanic material strung out along a fissure on the Earth’s surface; I’ve seen these in Alaska, New Mexico, and Saudi Arabia. Rarely are cinder cones found that rise more than about 300 meters above their surroundings (Paracutín, which began erupting in a farmer’s corn field in 1943 in Mexico, is a notably larger exception). Cinder cones are numerous in western North America, in western Saudi Arabia, and throughout other volcanic regions of the world.

Lava domes:
These are formed by relatively small, blob-like masses of lava too viscous (in other words, too loaded with sticky silica) to flow any great distance; consequently, they extrude as lava piles over and around a source vent. Domes typically grow by expansion from within; a great example is seen inside the crater formed by the eruption in 1980 of Mount St Helens. As a dome grows its outer surface cools, hardens, then shatters, dumping loose fragments down its sides. Some domes appear as craggy knobs, sometimes as spines over the source volcanic vent. Before Mont Pelée erupted the lesser Antilles in 1902, a huge spine was seen rising over a period of several days above the nearby coastal town of St. Pierre. When the dome blew open, it didn’t kill by lava flowing or even pyroclastic (hot rock fragment) flow – it unloaded an incandescent, high-velocity flow of gas and ash (called a nuée ardent) that killed nearly all of the 30,000 inhabitants almost instantly, and burned all the ships remaining in the harbor.

Stratovolcanoes (sometimes called composite volcanoes):
Some of the Earth’s most visually spectacular mountains are composite volcanoes—sometimes called stratovolcanoes. They are typically large, steep-sided cones built of alternating layers of lava, volcanic ash, cinders, lava blocks and airborne bombs. They 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, and these include Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount Rainier in Washington.
Most but not all composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through gaps 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 against erosion and earthquakes.
An essential feature of a composite volcano is a conduit system through which magma makes its way from a reservoir deep in the Earth’s crust and then rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.
A dormant composite volcano begins to erode at once. As the cone is weathered away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, but they too are reduced by erosion. Ultimately, all that is left is the central plug and radiating dike complex projecting above the land surface. Shiprock, in New Mexico, is an example of the radiating dikes dominating the final eroded result. A good example of a small composite volcano remnant that left a plug is something you can still climb. Beacon Rock in Washington State sticks up on the edge of the Columbia River like a giant thumb (it is discussed elsewhere in this book).

Technically, a caldera (the word means “cauldron” in Spanish) is the life-cycle end product of some stratovolcanos or supervolcanos. Sometimes a volcanic edifice is totally destroyed by a paroxysmal blast during an eruption, and this leaves a crater behind, something that inevitably fills with water. A visually arresting example is Crater Lake in west-central Oregon. An ancient composite volcano, probably like Mount Shasta or Mount Rainier, was blown open by a tremendous explosion about 6,800 years ago. The gaping hole later filled with water, and is now completely occupied by a beautiful lake that attracts visitors from around the world. A last gasp of the original paroxysmal eruption left a small cinder cone, which rises above the modern lake surface as Wizard Island. Depressions such as Crater Lake, formed by the collapse of volcanoes, are generally but not always called calderas. They are usually large, steep-walled, bowl-shaped depressions formed by the collapse of the “lid” over the original magma chamber. Calderas range in form and size from roughly circular or elliptical depressions, and can be as much as 100 km across. Modern Yellowstone is an example. When it erupted 640,000 years ago, a sunken giant caldera 1,500 square miles in area was left. It released over 1,000 cubic kilometers of material; earlier eruptions were even larger!

Shield Volcanoes:
Shield volcanoes don’t look like a volcano to an uninformed visitor (me, for instance, the first time I saw one). Mauna Loa in Hawai’i did not look at all like a Cascade volcano to me. Instead, it appeared more like a gently-rounded bulge above the Earth’s surface – in shape like a Greek soldier’s shield, hence its odd description. Mauna Loa, however, rises over 30 kilometers above the ocean floor that it rests on and in, and almost all of it is hidden from view. Shield volcanoes are built almost entirely of low-viscosity basalt lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping dome-shaped monster. These flows typically cool as many thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the original volcano. Some of the largest volcanoes in the world are shield volcanoes; the floor of the ocean around the Hawai’ian Islands is more than 5,000 meters deep. In addition to this, Mauna Loa reaches nearly 4,200 meters above sea level. At 9,200 meters it is taller than Mount Everest!

Simply put, these are just really big volcanoes; even bigger than Mauna Loa. There are supervolcanoes like Yellowstone and Long Valley in North America, Hudson and Pacana Caldera in South America, Lake Atitlán in Central America, Toba in Indonesia, and Taupo in New Zealand, among numerous others. One of several definitions of a supervolcano is something that releases over 1,000 cubic kilometers of tephra and ash. Another definition of a supervolcano is that it experienced a VEI = 8 eruption (see the following chapter for an explanation of the VEI eruption scale).

Figure 1. World Supervolcanoes, adapted from Wikimedia (https://en.wikipedia.org/wiki/Supervolcano#/media/File:Supervolcano_World_Map.png).

Large Igneous Provinces:
In some eruptions, astonishingly vast amounts of basaltic lava pours out, apparently mostly quietly, from long fissures instead of central vents, and flood the surrounding countryside with lava flow upon lava flow. This literally creates new topography, filling in previous terrain, and generally 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 very well exposed, and are more than 1,700 meters in total thickness.
The largest volcanic features on the planet, in fact, are these Large Igneous Provinces. Examples include the Deccan Traps, which erupted around 65 million years ago and make up the bulk of western India (including far offshore to the west of modern India). Even larger: the Siberian Traps, which cover 2.5 million square kilometers. Yes, that is million. Some calculations of the volume of lava erupted from the Siberian Traps range up to 2,000,000 cubic kilometers! The geologic record indicates that these eruptions were generally effusive – vast horizontal flows instead of blasts – and they created truly vast igneous plateaus.
Incidentally, these huge effusive outpourings of lava can move hundreds of kilometers from their source fissures in rather short periods of time (hundreds of kilometers in just a few days). Basalt found on the western Oregon coast has been traced to eruptive sources (largely self-buried) in eastern Washington and Oregon states. These flows are called the Columbia River Basalts and cover over 200,000 square kilometers, relatively small compared to Siberia, but pretty huge for nearby residents like me. If you are a geologist working in these two states your professional life experience can tend toward “all basalt, all the time.”

There are many other kinds of intermediate to smaller volcanic manifestations, including hydrothermal explosion craters, fumaroles, maars, pillow lavas, geysers, hornitos, skylights (see cover), and obsidian fields (where lava poured out under a cover of water and left, for example, vast tracts of dark green glass near Medicine Lake in northern California). Fumaroles and geysers are the most common (modern) expressions of volcanic activity seen in Yellowstone Park, for instance.

Another way to view volcanoes is by what they produce.

Effusive Eruptions:
Some eruptions can be violent blasts out of the ground, but some can also be effusive – they ooze out and flow over the ground (like in Hawai’i), creating their own new topography, and often whole new lands. In general, the more silica (SiO2) present in the magma, the more viscous (“sticky”) the magma is. Inevitably, this means that gas pressure can build up until it exceeds that sticky viscosity and the lithostatic pressure of the overlying rocks above the reservoir source. This all plays out vividly in the Andes and the Cascades of the Western Hemisphere – and the Pacific Ring of Fire. Subducted oceanic floor is loaded with water, carbon, silica, sulfur, and other light elements, which can then be subjected to partial melting. This hot liquid floats up, much like a lava-lamp, until it breaches the Earth’s surface at some weak point. The weak point may be a pre-existing volcanic conduit (like Mount St Helens in 2004), or simply some place where faults intersect and leave a narrow vertical zone of crustal weakness.

Phreatic eruptions:
Think of adding water to a skillet that is already hot: you often get a steam-flash or explosive reaction. There are some volcanoes that do the same thing when magma reaches a water table. These types of mixing events lead to phreatic eruptions. A phreatic eruption, also called a phreatic explosion, ultra-vulcanian eruption, or steam-blast eruption, occurs when magma heats and drives ground water or surface water to steam. Think of a rising magma that encounters a groundwater table, which then flashes to steam.

Plinian eruptions:
Plinian eruptions are named for Pliny the Elder, who was killed by Vesuvius in 57 BC. His nephew Pliny the Younger actually witnessed the eruption that killed his uncle (and many thousands more). These are gas-driven blasts of ash and tephra that start out going straight up. Inevitably the column will react to gravity and come crashing back down as ash-falls or pyroclastic flows (that word means fire-fragments). Think of a wall of incandescent material rolling down a volcano’s flank at speeds up to 100 km/hr. Not much can stand up to that.

Vulcanian eruptions:
A Vulcanian eruption is a type of volcanic eruption characterized by a dense, ash-laden gas exploding from a volcano and rising high above the peak. These eruptions tend to be very loud, usually short and violent, and can occur when a lava dome ruptures. The term was first used by Giuseppe Mercalli, witnessing the 1888–1890 eruptions on the island of Vulcano in the Tyrrhenian Sea.

Strombolian eruptions:
Strombolian eruptions tend to be milder than the previous types, characterized by the ejection of visibly-incandescent cinders, lapilli, and lava bombs, to heights ranging from tens to a few hundreds of meters. This is sometimes called Fire Fountaining.


These include lahars, tephra, ash, lava, lapilli, etc., but describing them all would take another chapter. This is the shortest single-chapter description of volcanoes that I think is reasonable for a non-specialist to wade through. There is vastly more information about volcanoes available here: https://volcanoes.usgs.gov/index.html