Stratovolcanoes

Perhaps one of the most notable eruptions in modern history of an oceanic stratovolcano–also known as composite volcano–is the eruption of Krakatau–more commonly but incorrectly known as Krakatoa—on August 27, 1883. It is said that the simultaneous eruptions of the three volcanic peaks, Perboewaton, Danan and Rakata on the island of Krakatau were so powerful that they sent a column of hot ash, dust and gas, known as tephra, 17 miles up into the air. The sound from a succession of four eruptions that occurred that day from the three volcanic peaks was so loud, it could be heard over 2,000 miles away in a desert of Australia. The third eruption was the most powerful and loudest of the four, and it has been estimated that the explosive power of that eruption was the equivalent to 200 megatons of TNT or 13,000 Hiroshima bombs. The type of eruption produced by a startovolcano is known as a Plinian (or Vesuvian), named after Pliny the Younger, who wrote a very detailed account of the eruption of Mt. Vesuvius–a continental stratovolcano.

Stratovolcnaoes are easily identified by their appearance. They have the classic triangular or conic shape with steep sides normally associated with volcanoes.

Schematic representation of the internal structure of a typical composite volcano.

Courtesy of the USGS.

The pyroclastic flow traveling atop the water at 200 mph killed many thousands of inhabitants on the southern coast of Sumatra on the mainland, 20 miles away from Krakatau. So much magma was released during the successive eruptions, that the rock walls of all three volcanic peaks could no longer support their own weight and consequently collapsed into the sea and thus Krakatau was transformed into what is called a caldera. The resulting tsunami from the collapse killed thousands of people more on the southern coast of Sumatra–a force of nature most definitely to be reckoned with. Of all the types of volcanoes, stratovolcanoes are the most deadly.

Seismogram of a Plinian Eruption of Mount Redoubt in Alaska

Courtesy of the Alaska Volcano Observatory (AVO)/Geophysical Institute/USGS

Anak Krakatau–meaning child of Krakatau—like the Phoenix reborn from it’s own ashes, arose from the depths of the sea floor, where the caldera of it’s parent, Krakatau is located. The latest seismic data as well as its prodigious growth rate, seems to indicate that Anak Krakatau may very well be on its way to becoming as powerful and as dangerous as it’s parent Krakatau.

Ice core samples, taken from Antarctica, have sulfuric acid contained within them—evidence that Krakatau had erupted many times in history. It is believed that some eruptions of Krakatau prior to 1883, based on the evidence from the ice cores, were even more powerful than the one of August 27, 1883. It is quite possible that Anak Krakatau may take after its extremely powerful and deadly parent.

So, what causes such explosive volcanoes? The answer lies in mixed magmas.

Andesite and Mixed Magmas

Andesite has more felsic constituents in it than basalt. What I mean by this is that it has a higher silica content than the mafic basalt, which has a high iron and magnesium content. It is considered an intermediate rock or magma, consisting of both mafic and felsic constituents in the Bowen’s reaction series. Mafic magmas also have a high calcium content and are associated with calcium-rich plagioclase feldspars (e.g. anorthite), whereas felsic rocks and magma have a high sodium and aluminum content which produce more sodium-rich plagioclase feldspars (e.g. albite).

Mafic magmas have a higher melting point than felsic magmas. But, in subduction zones where basalt and its intrusive counterpart, gabbro are saturated with ocean water, the melting point is lowered by the presence of water in the rock. It’s not clearly understood what exactly happens to the basalt after it has sunken into the upper mantle and has become molten. But it is believed by many that where continental stratovolcanoes exist, as it rises up to the continental crust–which is made of felsic rock with a lower melting point–the crustal material then is made molten by the white hot basalt. Being less dense than the basalt, the felsic magma tends to float on the mafic magma. In composite volcanoes, whether continental or oceanic you do have a certain amount of magma mixing. All composite volcanoes, whether continental or oceanic, contain mostly andesitic magma.

Now, because mafic magmas have a higher melting point than felsic magmas, the mafic magmas will crystallize first while still underground. In the discontinuous side of Bowen’s reaction series, the mineral pyroxene, which is found in mafic igneous rocks such as basalt and gabbro, is the first mineral to crystallize as the magma cools. As the magma cools even further pyroxene reacting with the surrounding magma at a specific temperature changes into amphibole, which in turn is changed into the mineral biotite when it reacts with the magma at an even lower specific temperature. In the continuous side of Bowen’s reaction series, you have the calcium-rich plagioclase feldspars slowing changing into the sodium-rich plagioclase feldspars. Now because mafic minerals are denser than felsic minerals they tend to sink to the bottom of a magma chamber, separating out the mafic from the felsic constituents. This process is known as differentiation. Felsic minerals at the top will begin to crystallize and in some cases even form a plug at the top of the main vent in a composite volcano, thus preventing the volcano from erupting. This in turn creates greater pressures that build up inside the volcano and makes for an even more explosive eruption down the road.

Porphyritic Andesite

Because certain minerals are crystallizing slowly underground, they have time to form larger crystals than they would if they were to have crystallized above ground. Consequently, you end up getting relatively large crystal in the magma before it is extruded in a volcanic eruption. These larger crystals are what we call phenocrysts. When the volcano finally erupts, as the lava flows–in this case andesite–it carries along with it the phenocrysts that were formed while the molten rock was still underground. Because the phenocrysts bearing lava cools much quicker as a result of being exposed to the much cooler air around it, it crystallizes around the phenocrysts into a matrix or groundmass, with crystals too small to be seen by the unaided eye and is said to have an aphanitic texture. This combination of larger crystals contained in a groundmass with an aphanitic texture is what geologists call a porphyry or porphyritic rock–in this case porphyritic andesite.

Tephra and Pyroclastic Flow

When a stratovolcano first erupts and the pressure that has built up inside of the volcano has been released, the column of tephra in a classic Plinian eruption, because of its tremendous mass and hence tremendous weight, drops back down for a moment. When this happens the material at the top of the vent is pushed downward onto the outside surface of the volcanic mountain. It is this downward force from the shear weight of the material that causes it to start descending down the outside of the volcano. It is this material that we call pyroclastic flow. That’s the difference between tephra, which is going up into the stratosphere and pyroclastic flow which is going down the mountain and then horizontally once it reaches a level surface. Both pyroclasic flow and tephra are made up of exactly the same material, namely extremely hot volcanic ash, dust, rock fragments and gas.

Courtesy of J.W. Vallance, USGS

Copyright © 2010 Eric F. Diaz

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