l>Volcanoes and also Volcanic Eruptions
EENS 1110

Physical Geology

julianum.net University Prof. Stephen A. Nelson
Volcanoes and also Volcanic Eruptions

Magmas and also Lava

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

You are watching: Which of these substances is usually not present in explosive volcanic eruptions?

Gases - At depth in the Earth virtually all magmas save on computer gas. Gas gives magmas your explosive character, since the gas broadens as pressure is reduced.

largely H2O with some CO2 Minor amounts of Sulfur, Cl , and F Felsic magmas typically have higher gas components than mafic magmas.

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

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

greater SiO2 content magmas have higher viscosity than lower SiO2 content magmas lower Temperature magmas have greater viscosity than greater temperature magmas.

Thus, basaltic magmas tend to be fairly fluid (low viscosity), however their viscosity is tho 10,000 to 100,0000 times much more viscous than water. Rhyolitic magmas tend to have actually even greater viscosity, ranging between 1 million and also 100 million times more viscous 보다 water. (Note that solids, even though they appear solid have actually a viscosity, but it very high, measured as trillions time the viscosity the water). Viscosity is critical property in determining 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 %, intermediary in Fe, Mg, Ca, Na, K 800 - 1000 oC Intermediate Intermediate
Felsic or Rhyolitic Rhyolite Granite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na 650 - 800 oC High High

The assets of volcanic Eruptions

Lava operation

When magma reaches the surface ar of the earth, the is referred to as lava. Due to the fact that it the a liquid, it operation downhill in solution to gravity together a lava flows. Various magma species behave in different ways as lava flows, depending on their temperature, viscosity, and also gas content.

Pahoehoe flows - Basaltic lava flows v low viscosity begin to cool as soon as exposed come the low temperature of the atmosphere. This causes a surface skin come form, although that is still very hot and behaves in a plastic fashion, capable of deformation.Such lava flows that initially have actually a smooth surface are called pahoehoe flows. At first the surface ar skin is smooth, yet often inflates with molten lava and also expands to form pahoehoe toe or rolls to type ropey pahoehoe. (See number 9.3d in your text). Pahoehoe flows tend to be slim and, due to the fact that of your low viscosity travel long ranges from the vent. A"A" Flows - greater viscosity basaltic and also andesitic lavas also initially construct a smooth surface ar skin, yet this is quickly damaged up by flow of the molten lava within and by gases that proceed to escape indigenous the lava. This create a rough, clinkery surface that is properties of one A"A" flow (see figure 9.3e in your text). Lava tubes - as soon as the surface ar skin i do not care solid, the lava can continue to flow beneath the surface in lava tubes. The surface skin insulates the warm liquid lava form further cooling. When the eruption ends, liquid lava frequently drains leaving an open cave (see figure 9.3 in your text). Pillow Lavas - When lava erupts top top the sea floor or other body that water, the surface ar skin creates rapidly, and, like with pahoehoe toes inflates with molten lava. Eventually these inflated balloons of magma drop off and stack up like a pile of pillows and are dubbed pillow lavas. Old pillow lavas are conveniently recognizable since of your shape, their glassy margins and radial fractures the formed during cooling (see figure 9.4b in her text). Columnar Jointing - When thick basaltic or andesitic lavas cool, castle contract. The contraction outcomes in fractures and also often times outcomes in a type of jointing referred to as columnar jointing. The columns are usually hexagonal in shape. This frequently happens when lavas pool in depressions or deep canyons (see number 9.4a in your text). Siliceous Lava operation - High viscosity andesitic and also rhyolitic lava flows, since they can’t flow really easily, form thick stubby flows the don’t move much from the vent. Lava Domes or volcano Domes - result from the extrusion of highly viscous, gas bad andesitic and also rhyolitic lava. Since the viscosity is for this reason high, the lava go not flow away from the vent, yet instead piles up over the vent.Blocks of almost solid lava break turn off the external surface of the dome and roll under its flanks to type a breccia approximately the spare of domes.

The surface ar of volcanic domes space generally an extremely rough, with plenty of spines that have been driven up by the magma indigenous below.


Pyroclastic material

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

Tephra and Pyroclastic Rocks

Average fragment Size (mm) Unconsolidated material (Tephra) Pyroclastic Rock
>64 Bombs or Blocks Agglomerate
2 - 64 Lapilli Lapilli Tuff
Blocks room angular pieces that were solid when ejected. Bombs have an aerodynamic shape indicating they were liquid once ejected. Bombs and lapilli the consist greatly of gas balloon (vesicles) an outcome in a low thickness highly vesicular rock fragment called pumice. Rock developed by build-up and cementation that tephra referred to as a pyroclastic rock or tuff. Welding, compaction and also deposition of other grains cause tephra (loose material) to it is in converted into pyroclastic rock.

Volcanic Landforms

Volcanic terrain are regulated by the geological procedures that form them and also act on castle after they have actually formed. Thus, a offered volcanic landform will certainly be characteristic of the types of product it is do of, which consequently depends ~ above the front eruptive actions of the volcano. Right here we discuss the major volcanic landforms and also how lock are formed Most that this product will be disputed with recommendation to slides displayed in class that illustrate the necessary features of each volcanic landform.

Shield Volcanoes

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

Shield volcanoes room composed almost entirely of relatively thin lava flows built up over a main vent.

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

The low viscosity of the magma permits the lava to travel down slope on a gentle slope, however as it cools and its viscosity increases, the thickness build up top top the reduced slopes providing a rather steeper lower slope.

Most shield volcanoes have actually a about circular or oval shape in map view.

Very small pyroclastic product is uncovered within a shield volcano, except near the eruptive vents, where tiny amounts that pyroclastic product accumulate together a an outcome of fire fountaining events.

Stratovolcanoes (also called Composite Volcanoes) have steeper slopes 보다 shield volcanoes, v slopes the 6 to 10o low on the flanks to 30o near the top. The steep slope near the summit is due partly to thick, short viscous lava flows that carry out not travel far down slope from the vent.


The gentler slopes near the base are because of accumulations of material eroded native the volcano and also to the buildup of pyroclastic material.

Stratovolcanoes present inter-layering that lava flows and also pyroclastic material, i beg your pardon is why they space sometimes referred to as composite volcanoes. Pyroclastic material can comprise over 50% the the volume that a stratovolcano.

Lavas and pyroclastics room usually andesitic to rhyolitic in composition.

Due come the greater viscosity the magmas erupted from these volcanoes, they room usually much more explosive than shield volcanoes.

Stratovolcanoes sometimes have actually a crater in ~ the summit the is formed by explosive ejection of material from a main vent. Occasionally the craters have been fill in by lava operation or lava domes, occasionally they space filled with glacial ice, and less frequently they space filled with water.

long periods of repose (times that inactivity) lasting because that hundreds to thousands of years, do this form of volcano particularly dangerous, since many time they have presented no historic activity, and people are reluctant come heed warnings about feasible eruptions.

Cinder Cones

Cinder cones are little volume hat consisting predominantly of ash and scoria that result from mildly explosive eruptions. They normally consist that basaltic come andesitic material. They space actually autumn deposits the are developed surrounding the eruptive vent. Slopes that the hat are controlled by the edge of repose (angle of stable slope for loosened unconsolidated material) and also are generally between around 25 and also 35o.

They show an internal layered structure as result of varying intensities the the explosions the deposit various sizes the pyroclastics.

~ above young cones, a depression in ~ the peak of the cone, referred to as a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters room usually eroded far on enlarge cones.

If lava flows room emitted native tephra cones, they space usually emitted from vents ~ above the flank or close to the basic of the cone during the later on stages of eruption.

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

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An excellent example of cinder cone is Parícutin Volcano in Mexico. This volcano was born in a farmer corn field in 1943 and also erupted for the following 9 years. Lava operation erupted native the base of the cone ultimately covered two towns.

Craters and also Calderas

Craters space circular depressions, usually less than 1 km in diameter, that type as a an outcome of explosions that emit gases and ash. Calderas are much bigger depressions, circular come elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas type as a an outcome of please of a volcanic structure. The collapse results from evacuation that the basic magma chamber.