Stories of the Lithosphere
Volcanos and Earthquakes--Part 1, Volcanos
Earth Science Essentials
by Russ Colson
Mt St Helens, Washington, and debris flow, 2004.
Lecture Recap
From the lecture, which one of the following is the most key ingredient in an explosive volcanic eruption?
Consider the topographic profiles of two types of volcanoes below.
Composite volcano magma is often much richer in water than magma in a shield volcano. If you were determined to build a home on the slopes of a volcano, which type of volcano is more likely to produce quiet, less-explosive eruptions with lava flows that you can escape (although your house might not be so lucky)?
The word volcano refers to any fracture in the Earth's crust (or the crust of any other planet) from which gas, molten material, and rock fragment emerge. There are many kinds of volcanoes, not all of which include a mountain or any topographic expression. However, the two most common and recognizable kinds of volcanoes on Earth are the composite volcano and the shield volcano.
Composite Volcanoes
Composite volcanoes get their name from the layered structure of the volcanic mountain that often forms. Composite volcanoes are rich in gases (mainly water vapor) and therefore explosive. At the start of an eruption, gases pulverize emerging lava into volcanic ash, and create scoria and volcanic bombs (pyroclastics). Later, as the gases in the magma chamber are expelled, the eruption becomes quieter and a layer of lava form on top of the pyroclastics. In this way, composite volcanoes are built up from alternating layers of pyroclastics and lava. Because of this layering, these volcanoes are sometimes called strato volcanoes.
Pyroclastics and more silica-rich lava (often andesitic in composition) make the slopes of these volcanoes fairly steep, often over twenty degrees slope with some areas much steeper. This is the kind of volcano that people usually think of when they draw pictures of volcanoes in comic books. They are often picturesque (an entire film company was named after Mt. Fuji in Japan!)
Mt. St. Helens, shown in the picture above, is a composite volcano. You can measure the slope of the volcano with a protractor. The average slope of the left side of the volcano is closest to
Below is another composite volcano in El Salvador (in the background). Its visible slope is closest to
Izalco volcano in El Salvador, 2012. Image courtesy of Fenner Colson
Composite volcanoes are explosive (due to lots of water vapor and silica-rich magma). Lava flows are not the biggest dangers. Flows of ash-gas mixtures called pyroclastic flows or nueé ardente (glowing cloud) can emerge from the volcano and sweep down the slopes at speeds of 100mph or more. The gases are poisonous mixtures of water, carbon dioxide, sulfuric acid, hydrochloric acid and other gases. They are high temperature, often several hundred degrees Celsius (and perhaps as high as 1800°F). The rapid deposition of ash from the cloud can bury anyone caught in it. These dangerous flows move too fast to escape and can kill in multiple ways.
Typically, the most destructive part of an eruption, both for loss of property loss of life, is a lahar. Lahars form when heat from the eruption melts ice and snow at the top of the mountain (or water from rain might mix with the ash). The wet ash then flows down the mountain in wet-concrete-like mudflows that bury and destroy any buildings, bridges, or people in its way.
The Mt St. Helen's explosive eruption in 1980 was so energetic that, even without water, rock and sediment were fluidized into a debris flow that flooded the valley, burying anything before it. As the energy dissipated, the debris flow became a layer of rock and debris hundreds of feet thick. The debris flow is seen in the picture above as the hummocky, bumpy terrain.
You can get some feel for the turbulent energy of this debris flow in the picture below, showing an uprooted tree buried upside down in the rubble.
Mt. St Helens debris flow, 2004, with Mary Colson
Composite volcanoes are often big. Composite volcanoes in the Andes mountains (from which the rock type Andesite gets its name) are often over 20000 feet above sea level. In fact, by one measure of height, Mt Chimborazo, a composite volcano in Ecuador, is the tallest mountain in the world. It is farther from the center of the Earth to the top of Chimborazo than to the top of any other mountain, including Mt Everest. This is because the Earth is slightly flattened by its spin, and this makes Chimborazo, sitting near the equator, a bit farther from the center of the Earth.
Shield Volcanoes
In contrast to composite volcanoes, shield volcanoes are made of runnier, lower-silica lava (typically basaltic). This means that they usually have a lower slope, spreading out in a wide cone that someone imagined looked like a shield.
What is the slope of Mauna Loa, a shield volcano on Hawaii, as seen in the picture below?
Mauna Loa, a shield volcano on Hawaii, 1997.
Because of the low slope, shield volcanoes are more massive than composite volcanoes of the same height. In fact, Mauna Loa, the volcano in the picture above, is the most massive mountain in the world if measured from its base at the bottom of the sea to its top.
Measured from its base at the bottom of the sea to the summit, another volcano on Hawaii, Mauna Kea is the tallest mountain in the world.
A volcano on Mars, Olympus Mons, is the both the most massive and tallest mountain or volcano known anywhere. It is more than 3 times taller and 80 times more massive than any mountain on Earth! If we were to set Olympus Mons down in the center of Pennsylvania, it would cover Pittsburgh, New York City, Syracuse, Baltimore, Washington DC, Buffalo, Rochester, Scranton, Ithaca, and many other smaller towns.
Below are three compositions for common igneous lavas: rhyolite, andesite, and basalt. Compositions are given in weight percent of oxides. Basalt is the runniest of the three that makes volcanoes with the shallowest slopes. Which one is basalt (please don't do an internet search to find this—figure it out by what we've learned in this class!)
Because the basaltic lava is runny (low viscosity), the interior of a lava flow can sometimes drain out after the surface crust has frozen, leaving a cave-like lava tube.
Nahuku—lava tube, Hawaii Volcanoes National Park
The runnier lava is also more likely to form a ropy or billowy surface (pahoehoe lava) as seen in the picture below.
Pahoehoe lava flow, Hawaii
Plate Tectonics and Volcanoes :
On Earth, volcanoes are often associated with plate tectonic boundaries. Where two plates are coming together (converging), an ocean crustal plate will be subducted (pushed down into the Earth). In these regions, we find composite volcanoes. Areas with convergent boundaries and subduction include the Andes mountains, the East Indies, many islands in the Caribbean, the Cascade mountains, the Aleutian islands, Japan, the Philippines, and others.
If two plates overlain by continental crust converge, there is no subduction (continents are too buoyant to get pushed back down into the mantle) and little volcanism. Examples of this type of plate tectonic boundary include the Himalayan Mountains and the Alps.
At divergent plate boundaries, the lava is basaltic and we find shield volcanoes. Example locations include the mid ocean ridges--long chains of mountains under the sea--and various continental rift areas, such as the Rio Grande Rift in New Mexico and the East Africa Rift.
Shield volcanoes also often occur where a plume of hot material rising in the mantle causes partial melting in the middle of a tectonic plate. An example of this includes the Hawaiian Islands.
Below is a picture of a 1.1 billion year old lava flow along the North Shore of Lake Superior in Minnesota (bet you didn't know there were once volcanoes in Minnesota!).
1.1 billion year old lava flow in Temperance River State Park, MN.
Predicting Volcanic Eruptions
No, we can't do this yet, but we're working on it. Clues to an impending volcanic eruption mainly come from two factors: Changes due to movement of magma underground and changes due to gases building up underground.
Movement of magma underground can cause small earthquakes. Studies are ongoing to try to recognize the kinds of small earthquakes that signal impending eruptions. Movement of magma underground can also change the flow of heat from the surface or cause bulging or other changes in the surface.
Gases are a key factor in volcanic eruptions, particularly the more explosive ones. So, gases are a key area of research in trying to predict earthquakes. The emission of gases from a volcano often changes in volume or in the composition of the gases prior to major eruptions. These gases can be measured either by collecting samples or by remote sensing. The build-up of gas pressure can cause changes in the regional ground water table. Building up of gases can also cause small earthquakes (kind of like the clanging of steam pipes in a radiator system).
A general evaluation of volcanic risk can be based on past eruption history. The past history can be figured out by radiometric dating of lava from the eruptions or by stratigraphic principles we talked about previously. How often has the volcano erupted in the past? Did those eruptions come at regular intervals? How long has it been since the last eruption?
The problem with this method is that most volcanoes don't erupt at regular intervals. Even those that do erupt at regular intervals, say every 5000 years, vary by enough that making predictions on human time scales is difficult. For example, suppose the volcano erupted after 4600 years one time, and after 5400 years another, and then after 5000 years for a third. The volcano erupts every 5000 years, but with a plus-or-minus of 400 years. It's pretty hard to evacuate a city on the evidence that a volcano is likely to erupt sometime in the next 800 years!
Last updated June 25, 2015. All text and pictures are the property of Russ Colson.
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