What are Volcanoes?

These surface expressions of subsurface igneous activity are named after the Roman god of fire, Vulcan. Volcanoes are typically described in terms of their eruptive history as:

1. Active - have erupted in historical time.
2. Dormant - have not erupted in historical time, but are capable of renewed activity.
3. Extinct - have not erupted historically, show major erosion, and have no signs of activity.

Magmas can vary in composition from basic to silicic and carry some percentage of gaseous materials. The magmatic products of volcanism can include:

• Lava is molten rock material that has reached the surface. Basaltic lavas can be separated into three types:

 
1. Pahoehoe - Fluid, gas-rich lava having a ropy surface.
2. Aa - Viscous, gas-poor lava having a broken surface.
3. Pillow Lava — Lava, typical of submarine mid-ocean ridge basalts (MORBs), extruded into water and quickly chilled.


    Some of the surface and internal features shown by these lavas include:
 

1. Pressure ridges are buckled areas on the surface of lava flows.
2. Spatter cones are steep-sided cones built around the vent where escaping gases throw globs of lava into the air.
3. Columnar joints are parallel fractures that form normal to the lava surface, as the lava flow cools and contracts. They form polygonal (4-6 sided) cracks on the surface of the flow and columns perpendicular to the cooling surface.

• Volatiles are the dissolved gaseous materials in magma which can produce explosive volcanic activity when magma nears the Earth's surface and pressure is released. Typical volcanic gases include:

 
1. water
2. carbon dioxide
3. nitrogen and sulfur oxides
4. carbon monoxide
5. hydrogen chloride
6. halogen gases (fluorine, chlorine)

 
• Pyroclastic debris represents hot volcanic fragments blown out of a vent with explosive force. Pyroclastic materials are subdivided on the basis of fragment size:

 
1. Ash - Fine (<2 mm) particles that may settle at considerable distance around the vent. Volcanic dust can remain airborne for long periods.
2. Lapilli - 2-64 mm pieces of lava; basaltic fragments are commonly called cinder.
3. Blocks - >64 mm angular pieces that were solid when ejected.
4. Bombs - >64 mm rounded masses that were molten when ejected; the term pumice describes frothy pieces of rhyolite glass that can float on water.
 
        Accumulations of these fragmental materials form several types of deposits that include:
 
1. Tephra - a general term for pyroclastic debris that accumulates through vertical airfall.
2. Pyroclastic flows - avalanches of incandescent ash and gas (nuees ardentes).
3. Lahars - hot mudflows formed when water mixes with hot pyroclastic debris.

A number of different large-scale structures can be associated with eruption of magma onto the Earth’s surface:

Shield Volcano are most often found in oceanic areas and characterized by features that include:

1. many thin basaltic flows with little to no pyroclastic material.
2. fluid lavas of high temperature (900-1200ûC), low silica (basaltic) magma that undergo relatively quiet eruptions.
3. broad and low profiles (slopes < 10 degrees).
4. flank eruptions.

• Composite Volcano (Stratovolcano) are typically found on continents and volcanic arcs and characteristized by:
 
1. alternating layers of lava flows, pyroclastic material, and lahars.
2. lower temperature (2950ûC) viscous lava of andesitic to granitic composition that typically experience fairly violent eruptions.
3. high and steep cones (up to 30 degree slopes).
 
• Cinder Cones may exist separately or on the flanks of larger volcanoes. These pyroclastic mounds have characteristics that include:
 
1. steep slopes (up to 45 degrees) and low heights (usually < 400 m).
2. often symmetric shape around the vent.
 
• Lava Domes have a bulbous surface structure formed when a viscous mass of magma is extruded. Domes, which grow slowly in size, are:
 
1. steep-sided.
2. generally felsic in composition, but also may be intermediate.
3. often associated with extremely explosive eruptions.
 
• Fissures represent fractures or cracks through which magma erupts in areas where tectonic plates pull apart (rift). Typical fissure eruptions occur on the flanks of shield volcanoes, they are characteristized by:
 
1. large volumes of lava flows.
2. fluid lava covers large areas and may form basaltic plateaus.
 
• Pyroclastic Sheet Deposits result from massive pyroclastic flows erupting from fissures, rather than a central vent. They cover vast areas, are associated with caldera formation, and have characteristics that include:
 
1. felsic composition.
2. thicknesses ranging from a few meters to 100s of meters.

Volcanic landforms may be characterized by several negative structures that include:

1. Vent - an opening through which volcanic material passes.
2. Crater - a steep-walled circular depression (generally <1 km in diameter) at the vent area.
3. Caldera - a large summit depression (>1 km in diameter) caused by subsidence or explosion.
4. Fumarole - a vents that expels only gas.

Most volcanoes occur in well-defined belts, but some isolated volcanoes also exist. The major concentrations of volcanoes occur within:

1. Circum-Pacific Belt - More than 60% of all active volcanoes are in the circum-Pacific belt that encircles the margins of the Pacific Ocean basin. These are mostly composite volcanoes, and consist largely of intermediate to felsic lava flows and pyroclastic layers.
2. Mediterranean Belt - About 20% of all active volcanoes are in the Mediterranean belt. Most of these are composite volcanoes of intermediate to felsic composition.
3. Mid-Oceanic Ridges - Most of the remaining active volcanoes on Earth are located at mid-oceanic ridges, such as the Mid-Atlantic Ridge and East Pacific Rise. Volcanism consists of mostly basaltic fissure eruptions.

Plate tectonics explains the existence of volcanic belts and the origin of different magma compositions:

1. Igneous Activity at Spreading Ridges - New lithosphere is produced by igneous activity as plates move away from each other at either Mid-Oceanic Ridges or Continental Rift Valleys. Magmas originate as basaltic magma is produced by partial melting of the underlying mantle peridotite. Temperature increases with depth (geothermal gradient averages 25°C/km), but increasing pressure tends to keep rocks from melting. Melting is produced at spreading centers by the release of confining pressure (rifting) or by the presence of hot spots/mantle plumes (possibly produced by concentrations of radioactive elements which release heat as they decay). Most of the magmas form gabbroic plutons, but some reach the surface to erupt as basaltic lava. In continental regions, more silicic minerals of Bowen's Reaction Series (quartz and alkali-feldpars), that melt at low temperatures, occur within the crust and melt producing magmas that are more silica-rich than the rocks from which they are derived.
2. Igneous Activity at Subduction Zones - Melting at subduction zones beneath the leading margin of the overriding plate produces island arcs along oceanic-oceanic convergent boundaries or continental arcs along oceanic-continental convergent boundaries. Intermediate to felsic magma are produced by partial melting of the subducted oceanic plate and silica-rich continental shelf sediments or the mantle overlying the subduction zone where release of water from descending wet oceanic crust enhances melting (wet rock melts at a lower temperature than dry rock). As magmas rise, they may be affected by assimilation of continental crust. Most magma crytallizes as plutons, but some volcanism also occurs.
3. Intraplate Volcanism - Igneous activity typically results from tectonic plates moving slowly over hot spots (mantle plumes).

Besides eruptions of volcanic material, other profound effects that volcanism can have on society and on the environment include:

1. Earthquakes - movement of magma underground causes earthquakes as it forces its way upward. Depth of earthquake activity varies but may gives clues to the depth of the magma chamber feeding the volcano.
2. Lahars (Mudflows) - caused by volcanic debris mixed with rainwater, stream water, or melted snow and ice.
3. Climate Effects - dust in upper atmosphere may block solar radiation.
4. Tsunamis - tidal waves generated by underwater earthquakes and volcanic explosions.
5. Gas Clouds - can cause suffocation.