Volcanic Arc Primer

7/27/2019 Volcanic Arc Primer

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Volcanic Arc Primer

1. In the next series of lessons we will look at several National Parks that formed involcanic arcs. Lets start by taking a look at some general principles that apply to all

volcanic arcs. In particular, we want to know how subduction creates magma and what

kind of magma, rock types and volcanoes are formed.

2. This diagram portrays a model of subduction that summarizes my understanding of thisimportant plate boundary. Theres at lot happening here, but we can break down the

generation of the magma by subduction into four fundamental processes.

3. First, water is added to the oceanic crust via hot springs at an ocean ridge, and also to theaccretionary wedge within wet sediments.

4. Now, under progressively higher temperatures and pressures, water moves first into themlange,

5. to facilitate metamorphism of the mlange into blueschist and greenschist-grademetamorphic rocks;

6. then water migrates into the lithospheric mantle 7. where it is vital in the metamorphism these ultramafic rocks into serpentinite. Due to

its water-induced, low density, serpentinite works its way to the surface.

8. Finally water migrates into the asthenosphere where wet melting will occur.9. Progressive dewatering of the oceanic crust turns basalt into amphibolite and amphibolite

into a largely dehydrated, garnet-bearing rock that rarely is exposed at the surface called

eclogite.

10.The addition of water to the asthenosphere lowers its melting point and initiates partialmelting there.

11.Wet partial melting of the ultramafic asthenosphere generates mafic to intermediatemagma

12.while the residual solids remain at the base of the crust to form the lithospheric mantle.13.Partial melting also occurs where the relatively hot mafic to intermediate magma comes

in contact with the base of the continental crust. Partial melting of the continental crust


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creates a felsic magma type that can later crystallize to form granite or rhyolite. The

residual solids from partial melting here are again added to the lithospheric mantle.

14.Driven by its lower density, magma works its way to the surface. In the ductile lowercrust magma is emplaced diapirically,

15. but in the cooler and more brittle upper crust, it must create fractures or exploitpreexisting conduits to reach the surface.

16.Since cooling will accompany the journey to the surface, some crystallization of highermelting point minerals often occurs and thus some differentiation of the magma will take

place. The initially produced mafic to intermediate magma may differentiate into

andesitic and basaltic magma.

17.Although the relatively dense basaltic magma will be less likely to reach the surface, theless dense rhyolitic and andesitic magma will be likely to erupt. Being already silica-

enriched, rhyolitic magma will probably not differentiate further, but andesitic magma

can differentiate to form dacite and basalt,

18. because dacite has more silica than andesite and basalt has less.19.Since the volcano formed from these rocks will have a similar density to the magmas

from which they formed, the density driven rise of magma to the surface often stops

below the volcano and a chamber of ponded magma forms. Differentiation of andesite

into dacite and basalt can also take place in a volcano's magma chamber.

20.In general, the eruption of basaltic magma from volcanic arcs will require significant gaspressure to overcome its relatively high density. Gas pressure is no stranger to volcanic

arcs, as significant amounts of steam originate from all that water released from the

oceanic crust. Thus, if basaltic magma erupts from volcanic arcs, the eruption will usually

be pyroclastic, which implies that gas pressure was sufficient to shoot the magma into the

air where it cooled and solidified before landing on the ground. Such eruptions usually

build cinder cones.

21.On the other hand, rhyolite and dacite are less dense than basalt and thus do not requirehigh gas pressure to reach the surface. They can erupt effusively, which means as some

type of lava flow. High silica content greatly increases lavas viscosity, so such lava

generally piles up near the vent forming volcanic domes.


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22.Although gas pressure is not required for rhyolitic and dacitic magma to erupt, gaspressure is common in volcanic arcs, so pyroclastic eruptions of these magma types are

common. When high gas pressure is present, the high viscosity of high silica magmas

greatly increases eruption violence. High silica, high gas eruptions produce a lot of ash

and are sometimes referred to as Vesuvian,

23. after the type of eruption typical of Mount Vesuvius.24.Andesitic magma may erupt as flows or ash depending on the gas content.25.Because volcanoes fed by subduction generated magma are comprised of both

pyroclastics and flows they are called composite volcanoes.

26.So subduction generated magma is in general wet and variable in compositionrangingfrom basalt, to andesite, dacite and rhyolite.

27.In summary, if there is relatively little gas in the magma, then some type of flow is likely,unless the magma is basaltic, which is unlikely to erupt due to its high density. The

viscosity of the flow is controlled by silica content, such that the high viscosity of dacitic

and rhyolitic magma typically produces domes in low gas eruptions.

28.If the gas content is relatively high, then magma will be hurled into the air where it willsolidify into pyroclastic particles whose size is determined by the silica content of the

magma. Gas will escape from fluid basaltic magma with relative easeproducing coarse

cinders. In viscous, high-silica magma, the more violently expanding gas shreds themagma into fine ash. Typically pyroclastic eruptions will deplete the magma chamber of

gas, leaving relatively low gas magma to erupt at a later time. Continued wet melting

above the subduction zone will replenish the magma chamber with gas-rich magma to

repeat the cycle.

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Volcanic Arc Primer