Perhaps one of the most notable eruptions in modern history of a stratovolcano or supervolcano, as it is sometimes called, 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 and dust, 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 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 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 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 most definitely to be reckoned with. Of all the types of volcanoes, stratovolcanoes are the most deadly.

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 become 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 volcanoes to be so explosive? Well the answer to that is contained in the next topic of "mixed magmas"

Mixed Magmas

The volcanoes that are the product of convergent plate boundaries usually consist of two types of magmas. The first type, rich in iron, magnesium and calcium has a very high melting point and is much less viscous than the second type. The second type of magma due to it's interaction with water has a much lower melting point and is rich in sodium, silicates and aluminum.

When the first type of magma is extruded in the form of lava and cools, it solidifies and crystallizes into mafic igneous rock (i.e. basalt). When the second type of magma is extruded from a volcano in the form of lava and then cools, it solidifies and crystallizes into felsic igneous rock (i.e. rhyolite).

Now it's only volcanoes that are the product of subduction zones, in which a denser basaltic oceanic plate is subducted underneath a less dense and more boyiant granitic continental plate--a result of plate movement due to plate tectonics--that contain both types of magma, with the less dense felsic magma floating on top of the denser mafic magma. Because the felsic magma has a lower melting point and crystallizes at lower temperatures than the mafic magma, it is more viscous. As a result the felsic magma being more viscous can prevent the mafic magma from being extruded from a volcano. Consequently pressure builds up. And because the felsic magma has a much higher water content than the mafic magma and the mafic magma beneath the felsic magma is much hotter than the felsic magma, the water vapor along with other gases such as sulfur dioxide in the felsic magma, become heated even further thus causing them to expand and create even greater pressure. There is also a zone in which the two magmas actual mix and form what is known as an intermediate magma, which when extruded as lava, crystallizes to form andesite, a rock rich in the minerals amphibole, mica and biotite. Because of the tremendous pressures that build up in these type of volcanoes due to the mixed magmas, these volcanoes tend to be extremely explosive, e.g., Mount St. Helens or the stratovolcano in the Pacific, Anak Krakatau.

But it is because you have these two types of magmas mixing with one another that you end up with felsic igneous rocks such as rhyolite combined with mafic igneous rocks such as basalt.

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