Volcanoes and Plate Tectonics

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POSTED: 10 October 2018

Pinatubo vs. Hawaii
The style of volcanism at Mt. Pinatubo (Philippines) is very different from that of the Hawaiian volcanoes. While Pinatubo's eruptions are typically explosive and very violent, the ones on Hawaiian are "peaceful" and, in fact, a major tourist attraction. Eruptions at Kilauea started with spectacular lava fountains at Pu'u O'o, one of its vents, and is still going on, in 2003 though less dramatically. This lecture explains why we find volcanoes at some places and not others and why some are violent while others are not.

• Case Studies

Lectures 8-10 are accompanied by several case studies. The case studies only give examples and the list is by no means complete. These case studies DO NOT imply that a particular feature (e.g. mud flows/lahars) is observed only at a particular volcano or a particular eruption. In fact, most eruptions exhibit a combination of these.

• The Two Principal Types of Volcanoes and Styles of Volcanism

we typically distinguish between two main types of volcanoes (shield and strato volcano) and two styles of volcanism (effusive and explosive):

• shield volcanoes (e.g. Iceland, Hawaii, Mt. Etna): are typically much wider than high; slope angle is often less than 5^o; shield volcanoes are formed when erupting lava has extremely low viscosity and typically for during effusive volcanism; occur at only few places on Earth, typically near hot spots
• strato (or composite) volcanoes (e.g. Mt. St. Helens, Mt. Shasta, Stromboli Mt. Vesuvio, Mt. Fujiyama): are typically only a few times wider than high; the slope angle is often much larger than 5^o, sometimes reaching 35^o; strato volcanoes are typically composed of alternating layers of lava and tephra (ejecta) indicating alternating effusive and explosive activity; vast majority of volcanoes, mostly along subduction zones

• Styles of Volcanism and the Three Vs

the style and vigor of a volcanic eruption depends on the three Vs:

• viscosity of magma (hence SiO₂-content; see next paragraph)
• amount of dissolved gases (volatiles); e.g. the water content determines whether a magma at Hawaii will erupt as lava flow oozing out of the vent or as lava fountain


• in addition to these, the volume of erupted material determines the significance of an eruption (e.g. a volcano oozing out gigantic amounts of low-viscosity lava erupting in an Icelandic type eruption (see below) can become a VEI 4, due to the large volume of erupted lava; a tiny amount of high-viscosity lava may erupt in an explosion small enough to be watched safely relatively close to a volcano)
The following table helps relate the amount of viscosity and volatiles with the two principal styles of volcanism.


Style of Volcanism	viscosity of lava	low in volatiles	high in volatiles
effusive	low	lava flows	lava fountains
explosive	high	small explosions	catastrophic explosions

• Factors Controlling the Type of Volcanism

• Crustal Properties

Review from Lecture 04: continental crust is different from oceanic crust.
• continental crust is thicker, less dense, has light colored rock
• oceanic crust is thinner, denser, has dark colored rock
• continental rock is silica-rich (silicic) while oceanic crust is silica-poor (mafic)
• NB: silica is SiO₂

• Composition of Volcanic Rocks Controlled by

• crustal thickness
• silica-content

• Viscosity

Review from Lecture 03: viscosity is the resistance of a material to flow. More viscous magma causes more violent eruptions.

Factors controlling the viscosity of magma:
• silica-content (increase in silica increases viscosity)
• temperature (increase in temperature decreases viscosity)

• What is Magma and Lava?

both are molten or partially molten rock, but while lava cools at the surface to make volcanic rock, magma cools below ground to make plutonic rock. Above ground, rocks form in an extrusive environment while below the surface rocks form in an intrusive environment.

• What are Lava and Magma made of?

• molten rock
• + dissolved grains
• + up to 15% dissolved volatiles (e.g. water - 50%; CO₂ - 20%; N₂; H₂; SO₂; H₂S)

• The Types of Volcanic and Plutonic Rocks

Volcanic rocks freeze/crystalize from melt above ground, while plutonic rocks crystalize below ground. Plutonic rocks have more time to cool and form larger crystals than volcanic rocks.



Type	Silica content	Volcanic	Plutonic
silicic (or felsic or rhyolitic)	>65%	Rhyolite	Granite
intermediate (or andesitic)	55-65%	Andesite	Diorite
mafic (or basaltic)	< 55%, rich in Fe and Mg	Basalt	Gabbro



• The silica content defines types of magma and igneous rocks
• Si-rich mineral weigh less than mafic minerals
• The rate of cooling determines the grain size, i.e. the texture (the faster the cooling, the finer the grain).

therefore


• mafic rocks are typically darker than silicic rocks
• mafic rocks are denser than silicic rocks
• plutonic rocks are typically coarser than volcanic rocks

• Minerals, Silica and Melting

• different rocks are composed of different minerals
• a rock usually has several different minerals
• Si-rich minerals (e.g. Quartz) are usually found in silicic rocks
• Si-poor minerals (e.g. Olivine) are usually found in mafic rocks
• Si-rich minerals (e.g. Quartz) melt at lower T (650^oC;1200^oF)
• mafic minerals (e.g. Olivine) melt at very high T (>1000^oC;1850^oF).
• Therefore certain types of magma or solid rock can exist only at certain temperatures.
• Si-rich magma is relatively cool while mafic magma is hot
• Partial melting results in a more silicic magma and more mafic residual rock as the more silicic minerals escape into the magma.

• Melting of Rock/Freezing of Melt

when melting a rock, the silicic minerals are first to melt (e.g. Quartz) when melting a rock, the mafic minerals are last to melt (e.g Olivine) when freezing a melt, the mafic minerals are first to crystallize (e.g. Olivine) when freezing a melt, the silicic minerals are last to crystallize (e.g. Quartz)


NB: since ascending magma in a volcano typically cools, mafic minerals are the first to stay behind and the silicic minerals are more likely to stay in the melt. The erupting lava is therefore typically more silicic than the magma left behind in the magma chamber.

• Partial Melt: Where does Magma come from?

from partially molten rock; the molten grains migrate into the magma while the solid grains stay behind. Different types of grains have different melting temperature so when mantle rock heats to a certain temperature only the ones whose melting temperature is low will melt. Partially molten mantle rock has typically only 2-10% melt. The molten minerals escape into the rising magma while solid minerals stay behind.

• What causes a rock to melt?

• heating (e.g. heat transfer from adjacent hot magma body; e.g. geysers)
• decompression melting
• injection of volatiles

• Earth's Two Principal Melting Processes

decompression melting: the geotherm traces the temperature within the Earth as function of depth (or pressure). Upper mantle temperatures reach the melting temperature of rock at about 100km depth. The rocks are solid however due to increased pressure (e.g. water does not boil in a pressure cooker at 100^oC). If rock ascends from a certain depth pressure decreases, i.e. eventually melting the rock.
injection of volatile: volatile (e.g. water brought into the mantle in subduction zones) break the bonds between atoms at the mineral grain boundaries and the rock begins to melt.

• Where do we find volcanoes and what do we find?

~90% of activity near plate boundaries; the other 10% near hotspots and/or intra-plate rift zones
• mid-ocean ridges: majority of igneous activity (80% of total magma production); as sea-floor spreads new magma from asthenosphere gets to surface and drifts apart (like two divergent conveyor belts); decompression melting leads to partial melting of mafic magma; eruption at ocean floor leads to pillow lava as water chills lava instantaneously, forming a thin skin around lava blobs; hot mineral-rich water escapes in "black smokers"; eruption is submarine, remote and relatively peaceful
• subduction zones: subducting oceanic lithosphere releases volatiles that melt the surrounding material in the asthenosphere; as the silicic minerals have the lowest melting temperatures the rising magma is more silicic than the source it came from
  comes to the surface in continental arcs in continent-ocean subduction zones and island arcs in ocean-ocean subduction zones.
  In ocean-ocean subduction zones, basaltic magma can rise directly to the surface leading to more effusive and less explosive volcanism.
  In continent-ocean subduction zones, rising magma melts a lot of continental crust (low melting T!) and magma becomes more silicic resulting in more explosive volcanism.
• continental rifts: thinning and rifting leads to breaking up of a continent, enabling hot magma to rise and penetrate to the surface, causing volcanism; mantle rock melts through decompression melting; typically very silicic (e.g. East Africa); 1 billion years ago, the Mid-Continent Rift visible from Lake Superior to Kansas almost split North America.
• hot spots: Hot spots are fixed relative to moving plates; magma rises from a stationary mantle plume giving rise to volcanism at the surface; E.g. Hawaii;
  some plumes are thought to rise from the core-mantle boundary; plumes consist mainly of solid rock in the deep mantle and partially molten magma in the shallower mantle as it melts through decompression melting; a chain of volcanoes is created when plate moves over the hot spot; many hot spots are in the oceans, creating basaltic volcanism similar to that along mid-ocean ridges (though slight differences in mineral content); the volcanism at continental hot spots is typically more silicic (e.g. Yellowstone 600,000 years ago).



• large igneous provinces: Some hot spots can be very hot producing more partial melt (10% instead of the usual 4-6%) and large amounts of lava leading to flood basalts (e.g. Columbia River, 17 Mio. yrs ago; Deccan Traps 65 Mio yrs ago; Parana Plateau with Iguacu Falls).
• generally NO volcanic activity near transform boundaries and continent-continent collisions

• Volcanic Hazard Mitigation

Volcanic eruptions cannot be prevented but some measures can reduce the damage and number of fatalities.
• monitoring (see Lecture 10)
• diversion of lava flows (moderately successful); e.g. the residents of Heimaey/Iceland saved their harbor in a 1973 eruption when they slowed down a lava flow by spraying it with water
• draining crater lakes before eruptions: e.g. Mt. Kelut (moderately successful)
• education
• evacuation (often a political rather than scientific problem)
  Case Study:
  Mt. Mayon: an eruption in 1993 killed 75 farmers in a ravine that had been declared off limits. 50,000 residents were evacuated within 10km of the volcano in anticipation of a possible larger eruption. Many residents had to be forcibly removed but returned during the day to tend their fields (without the crop people would starve)

• Are there any Benefits from Volcanic Activity?

• ejecta are extremely rich in minerals so supply fertile soil
• volcanic rock is used as building material
• pyroclastic ejecta used for many purposes (e.g. ash and lapilli on icy streets)
• some activity (e.g. black smokers) create significant mineral resources
• volcanism forms new islands (e.g. Iceland, Japan)
• geothermal activity is used to heat spas, provide electricity (e.g. Iceland, New Zealand)
• hot springs

• Recommended Reading

new activity/unrest and ongoing activity can be checked out at the Smithsonian/USGS Weekly Volcanic Activity Report

1. true
2. false

1. true
2. false

1. extrusive
2. intrusive
3. effusive
4. plutonic

1. Quartz
2. Olivine

1. Olivine
2. Quartz

1. decompression melting
2. additions of volatiles