l>Volcanoes and also Volcanic Eruptions
EENS 1110

Physical Geology

ptcouncil.net University Prof. Stephen A. Nelson
Volcanoes and Volcanic Eruptions

Magmas and Lava

Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and also dissolved gas) expelled top top the Earth"s surface, we"ll first review the characteristics of magma the we extended previously. Three simple types the magma: Mafic or Basaltic-- SiO2 45-55 wt%, high in Fe, Mg, Ca, short in K, Na intermediate or Andesitic-- SiO2 55-65 wt%, intermediate. In Fe, Mg, Ca, Na, K Felsic or Rhyolitic-- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na.

You are watching: Magma would probably cause the most explosive eruption.

Gases - At depth in the Earth nearly all magmas save on computer gas. Gas gives magmas your explosive character, because the gas increases as press is reduced.

greatly H2O with some CO2 Minor amounts of Sulfur, Cl , and also F Felsic magmas generally have higher gas contents than mafic magmas.

Temperature of Magmas Mafic/Basaltic - 1000-1200oC Intermediate/Andesitic - 800-1000oC Felsic/Rhyolitic - 650-800oC. Viscosity the Magmas

Viscosityis the resistance to flow (opposite of fluidity). Counts on composition, temperature, & gas content.

greater SiO2 contents magmas have higher viscosity than reduced SiO2 contents magmas reduced Temperature magmas have higher viscosity than higher temperature magmas.

Thus, basaltic magmas often tend to be relatively fluid (low viscosity), yet their viscosity is quiet 10,000 come 100,0000 times an ext viscous 보다 water. Rhyolitic magmas have tendency to have actually even greater viscosity, ranging in between 1 million and also 100 million times much more viscous than water. (Note that solids, also though they show up solid have a viscosity, but it really high, measured as trillions time the viscosity that water). Viscosity is vital property in identify the eruptive actions of magmas.

Summary Table
Magma Type Solidified volcanic Rock Solidified Plutonic Rock Chemical Composition Temperature Viscosity Gas Content
Mafic or Basaltic Basalt Gabbro 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC Low Low

or Andesitic

Andesite Diorite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC Intermediate Intermediate
Felsic or Rhyolitic Rhyolite Granite 65-75 SiO2 %, short in Fe, Mg, Ca, high in K, Na 650 - 800 oC High High

The assets of volcanic Eruptions

Lava flows

When magma will the surface of the earth, it is referred to as lava. Because it the a liquid, it flows downhill in an answer to gravity together a lava flows. Different magma species behave in different way as lava flows, depending on their temperature, viscosity, and gas content.

Pahoehoe operation - Basaltic lava flows through low viscosity begin to cool as soon as exposed to the short temperature the the atmosphere. This causes a surface skin come form, although the is still an extremely hot and also behaves in a plastic fashion, qualified of deformation.Such lava flows the initially have actually a smooth surface are referred to as pahoehoe flows. Initially the surface skin is smooth, but often inflates v molten lava and expands to form pahoehoe toe or roll to form ropey pahoehoe. (See figure 9.3d in your text). Pahoehoe flows tend to be slim and, because of their low viscosity take trip long ranges from the vent. A"A" Flows - higher viscosity basaltic and andesitic lavas additionally initially construct a smooth surface ar skin, however this is quickly broken up by circulation of the molten lava within and by gases that continue to escape indigenous the lava. This creates a rough, clinkery surface that is properties of one A"A" circulation (see number 9.3e in your text). Lava tube - as soon as the surface skin i do not care solid, the lava can proceed to circulation beneath the surface ar in lava tubes. The surface skin insulates the warm liquid lava type further cooling. Once the eruption ends, liquid lava regularly drains leave an open cave (see number 9.3 in her text). Pillow Lavas - When lava erupts top top the sea floor or other body the water, the surface skin creates rapidly, and, like with pahoehoe toes inflates v molten lava. Eventually these inflated balloons of magma fall off and stack up like a heap of pillows and are called pillow lavas. Old pillow lavas are conveniently recognizable because of their shape, their glassy margins and also radial fractures that formed throughout cooling (see number 9.4b in your text). Columnar Jointing - When thick basaltic or andesitic lavas cool, castle contract. The contraction outcomes in fractures and often times results in a kind of jointing called columnar jointing. The columns space usually hexagonal in shape. This frequently happens once lavas pool in depressions or deep canyons (see figure 9.4a in your text). Siliceous Lava flows - High viscosity andesitic and rhyolitic lava flows, since they can’t flow an extremely easily, form thick stubby flows the don’t move far from the vent. Lava Domes or volcanic Domes - an outcome from the extrusion of very viscous, gas bad andesitic and rhyolitic lava. Due to the fact that the viscosity is so high, the lava go not circulation away native the vent, yet instead piles up over the vent.Blocks of practically solid lava break turn off the external surface of the dome and roll under its flanks to kind a breccia roughly the margins of domes.

The surface of volcanic domes room generally really rough, with plenty of spines that have actually been propelled up by the magma native below.


Pyroclastic product

If the magma has high gas content and also high viscosity, the gas will expand in one explosive fashion and also break the liquid right into clots the fly with the air and cool follow me their route through the atmosphere. Conversely it blast out solid pieces of rock that once developed the volcanic edifice. Every one of these pieces are described as Pyroclasts = hot, damaged fragments. Loose assemblages that pyroclasts called tephra. Depending on size, tephra deserve to be classified together bombs. Blocks, lapilli, or ash.

Tephra and also Pyroclastic Rocks

Average bit Size (mm) Unconsolidated material (Tephra) Pyroclastic Rock
>64 Bombs or Blocks Agglomerate
2 - 64 Lapilli Lapilli Tuff
Blocks are angular pieces that to be solid once ejected. Bombs have actually an aerodynamic shape indicating they to be liquid when ejected. Bombs and also lapilli that consist mostly of gas bubbles (vesicles) result in a low thickness highly vesicular absent fragment dubbed pumice. Rock formed by buildup and cementation the tephra called a pyroclastic rock or tuff. Welding, compaction and also deposition of various other grains reason tephra (loose material) to it is in converted right into pyroclastic rock.

Volcanic Landforms

Volcanic landforms are managed by the geological processes that kind them and act on castle after they have actually formed. Thus, a offered volcanic landform will certainly be characteristics of the types of material it is do of, which consequently depends ~ above the former eruptive actions of the volcano. Here we comment on the major volcanic landforms and how lock are developed Most that this product will be debated with reference to slides shown in course that highlight the crucial features of every volcanic landform.

Shield Volcanoes

A shield volcano is defined by gentle top slopes (about 5o) and also somewhat steeper reduced slopes (about 10o).

Shield volcanoes room composed virtually entirely of relatively thin lava flows accumulated over a main vent.

Most shields were created by short viscosity basaltic magma the flows easily down slope away from the summit vent.

The short viscosity the the magma allows the lava to take trip down slope on a gentle slope, however as the cools and its viscosity increases, the thickness builds up top top the lower slopes giving a somewhat steeper lower slope.

Most shield volcanoes have a approximately circular or oval form in map view.

Very tiny pyroclastic material is found within a shield volcano, other than near the eruptive vents, where little amounts of pyroclastic material accumulate as a result of fire fountaining events.

Stratovolcanoes (also called Composite Volcanoes) have actually steeper slopes than shield volcanoes, through slopes of 6 come 10o short on the flanks come 30o close to the top. The steep slope near the summit is due partially to thick, quick viscous lava operation that execute not travel much down steep from the vent.


The gentler slopes near the base are because of accumulations of product eroded from the volcano and also to the build-up of pyroclastic material.

Stratovolcanoes present inter-layering the lava flows and pyroclastic material, i m sorry is why they space sometimes referred to as composite volcanoes. Pyroclastic product can make up over 50% of the volume the a stratovolcano.

Lavas and also pyroclastics are usually andesitic come rhyolitic in composition.

Due to the greater viscosity of magmas erupted from this volcanoes, they room usually an ext explosive 보다 shield volcanoes.

Stratovolcanoes sometimes have a crater in ~ the summit the is developed by explosive ejection of product from a main vent. Sometimes the craters have been to fill in through lava operation or lava domes, sometimes they room filled through glacial ice, and also less commonly they are filled through water.

lengthy periods of repose (times of inactivity) lasting because that hundreds to countless years, make this kind of volcano specifically dangerous, because many times they have displayed no historic activity, and people space reluctant come heed warnings about possible eruptions.

Cinder Cones

Cinder hat are small volume cap consisting mostly of ash and also scoria that an outcome from mildly explosive eruptions. They usually consist that basaltic come andesitic material. They are actually loss deposits the are built surrounding the eruptive vent. Slopes the the hat are regulated by the edge of repose (angle of steady slope for loose unconsolidated material) and also are commonly between about 25 and 35o.

They present an inner layered structure due to varying intensities of the explosions that deposit different sizes the pyroclastics.

top top young cones, a depression in ~ the optimal of the cone, referred to as a crater, is evident, and also represents the area above the vent native which product was explosively ejected. Craters room usually eroded away on older cones.

If lava flows space emitted indigenous tephra cones, they space usually emitted from vents ~ above the flank or near the basic of the cone during the later stages of eruption.

Cinder and tephra cones typically occur approximately summit vents and flank vents the stratovolcanoes.

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An excellent instance of cinder cone is Parícutin Volcano in Mexico. This volcano to be born in a farmers corn field in 1943 and erupted because that the following 9 years. Lava operation erupted native the base of the cone eventually covered 2 towns.

Craters and Calderas

Craters room circular depressions, usually less than 1 km in diameter, that type as a result of explosions that emit gases and also ash. Calderas space much bigger depressions, circular to elliptical in shape, with diameters varying from 1 kilometres to 50 km. Calderas kind as a an outcome of please of a volcanic structure. The collapse outcomes from evacuation the the underlying magma chamber.