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There are many types of triggers which may lead to a volcanic eruption.  The requirement for all eruptions is that pressure in the magma chamber is high enough to create instability in the volcanic system.  This excess pressure is known as overpressure.

4.1A:  Magma Mixing

When two types of magma mix the results can be explosive. This is a result of rapid heating and the release of volatiles which occurs by this newly mobilized magma. Magma type is defined by its chemical composition and particularly silica content.  High silica (felsic) magmas are viscous and slow moving and cause explosive eruptions such as Mt. Saint Helens.  Low silica (mafic) magmas flow easily and relatively quickly resulting in less catastrophic eruptions like at Kilauea,  Hawaii.  Both magma types, however, become more mobile at higher temperatures.  If these two very different molten rocks come in contact, the mafic magma can heat and mobilize the felsic magma which may lead to an eruption. Magma mixing likely occurred at the 1991 eruption of Mount Pinatubo which you can read more about Here.  It is proposed that this mixing was initiated by a magnitude 7.7 earthquake 100 km away the year before (see 4.1I).  Also magma mixing is believed to be the main trigger at Mount Hood in Oregon which is an active and potentially dangerous volcano in western Oregon.  Read about it Here.

Simplified model of magma mixing from USGS Source:

4.1B:  Nearby/Internal Earthquakes

Moderate to high magnitude earthquakes which occur in close proximity to or within volcanoes can trigger eruptions.  As the seismic waves shake the magma chamber and ground, they can agitate or stir the magma resulting in pressure buildup.  Earthquake fractures can extend beneath or through volcanoes resulting in a pressure drop which can release magma from its deep-seated roots.  Also, earthquakes can trigger large landslides or even flank collapse (4.1E) which results in a rapid drop in confining pressure potentially causing an eruption. An earthquake beneath Mt. Saint Helens occured on May 18, 1980 and resulted in the flank collapse which lead to its eruption.

4.1C:  Decompression

A decrease in pressure over the magma chamber can result in magma movement and an eruption.  The mass of rock above the chamber created by previous eruptions and deformed crust acts as a plug preventing the magma from reaching the surface.  If that plug is removed (decompression) there is nothing to prevent the magma from exploding from its buried source.  Decompression can be caused by erosion and landslides, but these often have minimal affect on the load applied to the magma chamber. Glacial melt and flank collapse, on the other hand, can result in rapid decompression of volcanoes.  In some cases when pressure is released, rapid crystallization occurs which actually heats the remaining liquid magma.  This makes it more fluid and moves it toward the surface faster than normal, potentially resulting in an eruption within hours to weeks.  This type of trigger can hopefully be used at many locations around the world such as Vesuvius in Naples, Italy. Read about it Here.  If large amounts of pressure are released, as in a flank collapse or rapid glacial melting, eruption may occur almost instantly.  This can be seen in the video posted beneath Section 4.1E at Mt. Saint Helens.  Read more about Mt. Saint Helens flank collapse Here.

4.1D:  Glacial Melt/Outwash

Glacial melt can cause volcanic eruptions by decreasing confining pressure and through phreatomagmatic triggers.  Rapid melting of huge ice and snow bodies can occur as magma nears the surface and heats the ground.  Slower melting may occur as a result of global warming.  If a glacial lake is present in the volcanic crater, extra water from glacial melt can cause overflow or a breach which causes rapid decompression.  This melt-water can seep into the ground increasing pore water pressure which destabilizes the slopes and can even come near enough to the magma chamber to instantly vaporize.  This is called a phreatic or phreatomagmatic eruption depending if just water or water and magma are erupted.  This is explored further below in Section 4.1G.

Snow cover and rising magma resulted in a phreatomagmatic eruption at Redoubt volcano. Source:

4.1E:  Landslides/Flank Collapse

Landslide and flank collapse triggers may cause eruption by decompression as mentioned above.  Large enough events to destabilize a volcano are usually the result of nearby or internal earthquakes, or magma intrusion.  Heavy rain or glacial melt can also destabilize steep volcanic surfaces by increasing the pore water pressure.  Additionally, if magma nears the surface enough to cause phreatomagmatic eruptions, the expansion of water can rapidly destabilize a slope.  For more information see Big Idea #9 and read Here.

The video below shows the flank collapse which occurred immediately before the 1980 eruption of Mount St. Helens.



4.1F:  Escape of Volatiles

As magma moves toward the surface, gasses (volatiles) dissolved in the magma come out of solution due to decreased pressure.  As the pressure decreases, the amount of gasses which can be dissolved decreases and the magma becomes saturated with gas. Once the saturation level is passed, these gasses are able to expand rapidly creating overpressure in the system. A good analogy for this is a shaken bottle of pop.  The dissolved carbon dioxide (soda bubbles) are stable as long as the cap remains on.  Once the seal is broken, or magma moves near enough to the surface in a volcano, the gas expands rapidly and the soda sprays out of the opening.  This type of eruption occurs often (approximately twice per year) at Stromboli in small scale, but much larger eruptions are possible.  Read more Here.

4.1G:  Contact with Ground or Surface Water

Magma moving toward the surface may come in contact with ground or surface water causing rapid vaporization and a steam eruption (phreatomagmatic eruption).  This type of eruption does not actually involve the release of magma directly, but is instead caused by the instant expansion of water to water vapor resulting in incredible pressures in the subsurface.  Magma may or may not be carried with the water vapor and debris which differentiates a phreatomagmatic from a phreatic eruption respectively.  Read about phreatomagmatic eruptions at White Island, New Zealand Here.

The video below shows an underwater eruption near Tonga in 2009



Less Likely Trigger Mechanisms

4.1H:  Earth Tides

Much the same as the gravitational pull of the sun and moon affect the oceans, they affect the solid earth. Earth tides are weak and result in only minimal ground deformation; however it may sometimes be enough to upset an unstable volcano.  The affect is greater on the more liquid mafic magmas and at active volcanoes an increase in activity is sometimes seen during times of high tide. Also active volcanoes at locations with lower crust thickness may be more susceptible to eruption at high tide.  Read about earth tides at Kilauea, Hawaii Here and crustal variations Here.

4.1I:  Distant High Magnitude Earthquakes

Currently up for debate is whether distant high magnitude earthquakes can cause increased volcanic activity. It is believed that seismic waves traveling through the mantle can stir magma resulting in increased movement.  Possibly earthquake associated activity has been recorded up to 500 km away, but affects could potentially extend beyond that.  This type of trigger can take up to a year to cause eruption due to the slow movement of magma.  The 1991 eruption of Mount Pinatubo occurred shortly after a magnitude 7.7 earthquake around 100 km away. Also after the great Chilean earthquake of 1960 (the largest quake in recorded history) activity at 7 volcanoes was reported.  Read more about these and other events  Here and Here.

4.1J:  Continental Deglaciation

Historical data implies that as continental glaciers melt, there may be a significant effect on the periodicity of volcanic eruptions.  With decreased pressure on the crust from these massive ice sheets up to kilometers in thickness, mantle temperatures can increase and magma may rises more freely to the surface due to decreased confining pressure.  Locations for such events have been identified in Iceland, Chile, and the Sierra Nevadas; all places previously covered by continental glaciers.  This is not currently a threat to many location on earth, but as global temperatures continue to rise, it may affect areas such as Greenland and Antarctica. Read more Here and Here.

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