Mt.
St Helens - pre-1980 
Mt.
St. Helens - post-1980
Lecture 5 Volcanoes and mountain building
I. Types of volcanoes
II. Locations of volcanism
III. Edifice building and dissection
IV. Volcano educational sites
I. Volcano types
Mt.
St Helens - pre-1980 Mt.
St. Helens - post-1980
Figure. Mount St. Helens, a Cascade volcano that lies in the Pacific Northwest above the subducting Juan de Fuca plate, before (left) it May 18, 1980 eruption and two years after (right) the eruption. Note the small dome or plug that has built itself inside the crater of the volcano (right). This plug of sticky magma sets the stage for future eruptions by plugging the throat of the volcano. A close-up of the plug is shown below (from JPL).
Post-eruption
dome in crater of Mount St. Helens, Oregon
In the following figure, which shows Volcan Colima in
western Mexico, you can see the ongoing construction of a stratovolcano.
Loose ash and pyroclasts
are accumulating on the steep flanks of the volcano, and are covered
in some areas by lava flows. The volcano thus has a unstable foundation
for an edifice that in this instance is roughly 14,000 feet high.
Colima Volcano - western Mexico
More than 500 stratovolcanoes have erupted in historic times and in any given year, several dozen are active.
Shield volcanoes gained their name from the fact that their profile against the horizon looks like a shield laying on the ground. The flanks of a shield volcano have low slopes, sometimes only a few degrees from horizontal. Shield volcanoes are the largest volcanic features on earth - the largest shield volcano, Mauna Loa in the Hawaiin Islands, has a volume 300 times greater than that of the largest stratovolcano, Mount Fuji in Japan. Shield volcanoes are created through the eruption of basaltic magmas, which flow easily and thus are capable of flowing over the landscape many miles from the central fissures or vents.
Distorted image of shield volcanoes in the Galapagos island chain, Ecuador.
Close-up
of shield volcano summit. Note profile of another shield volcano in
the distance.
This type of volcano consists almost entirely of tephra, which is another term used for a pyroclast. From a distance, cinder cones resemble piles of sand because they are made almost entirely of loose material that has rained down from their central vent. These cones usually erupt lava flows, too; however, the flows erupt through the side because the loosely piled cinders that make up the cone are easily pushed outwards toward the side as magma attempts to move up the central vent toward the peak.
La Poruna monogenetic cinder cone, northern Chile
Monogenetic cinder cones are products of single eruptions that last from several hours to several years. They are typically several hundred meters high and have distinct geometric forms with slopes of 33 degrees. See the train in the shadow of the cone for scale.
Click here for excellent additional information about cinder cones
In total, Earth has approximately 10,000-20,000 active cinder cones, many of which are clustered on or near other types of active volcanoes.
Dome volcanoes typically form from sticky viscous magmas that are squeezed onto the surface. Small dome volcanoes often extrude up the throat of a stratovolcano after it has a major eruption (see the figure above of the dome volcano inside the Mt. St. Helens crater).
Fissure eruptions occur along linear rifts in the surface. Fissure eruptions on land are rare outside of Hawaii and Iceland; however, new seafloor is created through fissures that are located along the axis of a seafloor spreading center. More volcanic rock is extruded each year by fissure eruptions along seafloor spreading centers than by any other type of volcanism.
II. Volcano locations
Ignoring volcanism that occurs along mid-ocean ridges, volcanoes appear in a variety of geologic settings. Volcanoes occur most often above subducting slabs, and additionally occur in zones of crustal extension where faulting presumably provides a conduit for magma to reach the surface. In ocean basins, thousands of seamounts have been formed over the past 200 million years as oceanic plates have drifted over assorted mantle hotspots. These volcanoes mark the trace of a plate as it drifts steadily over the hotspot, which is fixed (or nearly so) in the mantle beneath the plate.
Most volcanoes that are located above subducting plates are stratovolcanoes, of which several excellent examples are shown above.
Finally, flood basalts are voluminous outpourings of sheets of basaltic magmas that are believed to originate when the head of a mantle hotspot (or plume) first arrives beneath the base of a plate. Examples include the Columbia River Plateau basalts and the Deccan Trap basalts of southern India. There are presently no active flood basalt provinces.
III. Edifice building and dissection
An active volcano can build itself in a few thousand years (or longer). Given that the highest stratovolcanoes are more than 22,000 feet high (in the Andes), the rate at which they increase in height is rather impressive. By their nature, most volcanoes build themselves from the top down. Material is ejected from the central vent at or near the volcanic peak, and gravity brings it down the slopes. The processes by which a volcano builds its edifice depends on the type of volcano. For example, stratovolcanoes and shield volcanoes build from the top downwards. In contrast, lava that flows from cinder cones is emitted from its sides or near its base (for reasons described above). Volcanic debris is transported onto the surrounding area through direct eruption, wind and water, and landslides from the flanks of the volcano.
For example, in the following figure, its hard to avoid the conclusion that this volcano (Volcan Colima) has spread itself over the countryside (try to visualize the countryside without Volcan Colima, which is the volcano in the foreground).
- Taking it down
The lifetime of a volcano on the landscape is less than that of most mountains for two reasons.
1) Stratovolcanoes often self-destruct. During large eruptions, a volcano sometimes ejects so much material from its interior magma chamber that too little material is left inside to support the edifice of ejected material that comprises the material. In these instances, the edifice can collapse downwards, i.e. the volcano caves in. Such a collapse forms a caldera. These collapses can occasionally remove thousands of feet of material from the top of a volcanic edifice.
In addition to caldera formation, mass wasting in the form of landslides can also remove significant material from the flank of a volcano. Such landslides can be initiated by volcanic or tectonic earthquakes prior to an eruption, which appears to be what happened in the 1980 eruption of Mount St. Helens, or they can initiate an eruption since they suddenly remove large amounts of material that sustained pressure on the magma chamber (this process is analagous to removing a champagne cork - pressurized gas in the champagne explodes once the "plug" is removed).
2) Most volcanoes by their very nature are poorly constructed and top-heavy. They are built from the top down of loose pyroclasts interleaved with lava flows. Consequently, they are gravitationally unstable and prone to mass wasting events if they are rained on or destabilized by a small eruption or earthquake. For example, 4300 years ago,the ancestor to the volcano shown in the figure above collapsed downward onto the surrounding countryside. Its collapse spawned the largest known subaerial debris flow in history. Volcanic material from the landslide traveled 75 miles and buried parts of the countryside to depths of 650 feet. In all, the slide covered approximately 900 square miles, an area the size of Dane County. The volcanic edifice has rebuilt itself; however, hundreds of thousands of people now live directly on the land slide that once devastated the countryside.
Not surprisingly, what remains of a volcano long after the poorly consolidated material ejected out of the cone has eroded away is its erosion-resistant central plug. Ship Rock in New Mexico and Devil's Tower in Wyoming are excellent examples.
Ship Rock, NM
Devil's Tower, WY
In many senses, volcanoes are short-lived geological features, with lifetimes that typically measure hundreds of thousands or a few millions of years. Nonetheless, at any given time, the Earth's landscape is dotted with thousands of volcanoes and they are amongst the most fascinating and memorable features on the Earth's surface.
