l>Volcanoes and Volcanic Eruptions
EENS 1110

Physical Geology

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

Magmas and Lava

Since volcanic eruptions are caused by magma (a mixture of fluid rock, crystals, and also dissolved gas) expelled ~ above the Earth"s surface, we"ll an initial review the characteristics of magma the we spanned previously. Three an easy 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%, short in Fe, Mg, Ca, high in K, Na.

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Gases - At depth in the Earth virtually all magmas save gas. Gas provides magmas their explosive character, due to the fact that the gas broadens as pressure is reduced.

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

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

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

greater SiO2 contents magmas have greater viscosity than lower SiO2 content magmas reduced Temperature magmas have greater viscosity than greater temperature magmas.

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

Summary Table
Magma Type Solidified volcano Rock Solidified Plutonic Rock Chemical Composition Temperature Viscosity Gas Content
Mafic or Basaltic Basalt Gabbro 45-55 SiO2 %, high in Fe, Mg, Ca, short 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 commodities of volcano Eruptions

Lava flows

When magma reaches the surface ar of the earth, the is dubbed lava. Since it its a liquid, it operation downhill in an answer to gravity as a lava flows. Various magma varieties behave in different ways as lava flows, relying on their temperature, viscosity, and also gas content.

Pahoehoe flows - Basaltic lava flows through low viscosity start to cool as soon as exposed to the short temperature that the atmosphere. This reasons a surface skin to form, although it is still very hot and also behaves in a plastic fashion, capable of deformation.Such lava flows that initially have actually a smooth surface are dubbed pahoehoe flows. Originally the surface ar skin is smooth, but often inflates v molten lava and also expands come form pahoehoe toe or roll to kind ropey pahoehoe. (See number 9.3d in her text). Pahoehoe flows tend to be slim and, due to the fact that of their low viscosity take trip long distances from the vent. A"A" Flows - higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, yet this is quickly broken 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 characteristic of one A"A" flow (see figure 9.3e in your text). Lava pipe - as soon as the surface ar skin i do not care solid, the lava can proceed to flow beneath the surface ar in lava tubes. The surface ar skin insulates the hot liquid lava type further cooling. As soon as the eruption ends, fluid lava regularly drains leaving an open cave (see number 9.3 in her text). Pillow Lavas - When lava erupts on the sea floor or various other body of water, the surface skin forms rapidly, and, favor with pahoehoe toe inflates v molten lava. Eventually these inflated balloons the magma drop off and also stack up prefer a heap of pillows and also are called pillow lavas. Ancient pillow lavas are conveniently recognizable since of your shape, their glassy margins and radial fractures the formed throughout cooling (see number 9.4b in her text). Columnar Jointing - When special basaltic or andesitic lavas cool, they contract. The contraction results in fractures and also often times outcomes in a type of jointing called columnar jointing. The columns room usually hexagonal in shape. This frequently happens as soon as lavas pool in depressions or deep canyons (see figure 9.4a in her text). Siliceous Lava operation - High viscosity andesitic and also rhyolitic lava flows, due to the fact that they can’t flow really easily, type thick stubby flows that don’t move much from the vent. Lava Domes or volcano Domes - result from the extrusion of very viscous, gas bad andesitic and also rhyolitic lava. Due to the fact that the viscosity is therefore high, the lava go not flow away from the vent, yet instead piles up over the vent.Blocks of practically solid lava break turn off the external surface that the dome and roll under its flanks to type a breccia roughly the margins of domes.

The surface of volcanic domes are generally an extremely rough, with numerous spines that have actually been moved up through the magma native below.


Pyroclastic product

If the magma has high gas content and also high viscosity, the gas will increase in one explosive fashion and also break the liquid into clots that fly through the air and also cool along their path through the atmosphere. Conversely it blast the end solid piece of rock the once formed the volcano edifice. All of these pieces are referred to as Pyroclasts = hot, broken fragments. Loose assemblages of pyroclasts referred to as tephra. Relying on size, tephra have the right to be classified as bombs. Blocks, lapilli, or ash.

Tephra and Pyroclastic Rocks

Average particle Size (mm) Unconsolidated product (Tephra) Pyroclastic Rock
>64 Bombs or Blocks Agglomerate
2 - 64 Lapilli Lapilli Tuff
Blocks are angular fragments that to be solid as soon as ejected. Bombs have actually an aerodynamic form indicating they to be liquid once ejected. Bombs and lapilli the consist greatly of gas balloon (vesicles) result in a low thickness highly vesicular absent fragment called pumice. Rock created by accumulation and cementation that tephra called 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 processes that form them and also act on lock after they have formed. Thus, a given volcanic landform will be properties of the types of product it is made of, which consequently depends ~ above the front eruptive actions of the volcano. Below we comment on the major volcanic landforms and also how castle are developed Most of this product will be discussed with recommendation to slides shown in class that highlight the essential features of every volcanic landform.

Shield Volcanoes

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

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

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

The low viscosity that the magma allows the lava to travel down slope on a gentle slope, however as the cools and its viscosity increases, that is thickness builds up ~ above the lower slopes offering a somewhat steeper lower slope.

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

Very little pyroclastic material is discovered within a shield volcano, other than near the eruptive vents, where small amounts of pyroclastic product accumulate together a result of fire fountaining events.

Stratovolcanoes (also called Composite Volcanoes) have steeper slopes 보다 shield volcanoes, with slopes that 6 to 10o low on the flanks come 30o near the top. The steep slope close to the summit is due partially to thick, brief viscous lava operation that perform not travel much down slope from the vent.


The gentler slopes close to the base are due to accumulations of product eroded indigenous the volcano and to the build-up of pyroclastic material.

Stratovolcanoes present inter-layering of lava flows and pyroclastic material, i m sorry is why they space sometimes called composite volcanoes. Pyroclastic material can comprise over 50% the the volume that a stratovolcano.

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

Due come the greater viscosity of magmas erupted from these volcanoes, they space usually much more explosive 보다 shield volcanoes.

Stratovolcanoes sometimes have actually a crater at the summit the is created by explosive ejection of material from a main vent. Sometimes the craters have been fill in by lava operation or lava domes, periodically they room filled v glacial ice, and less commonly they room filled through water.

lengthy periods of repose (times of inactivity) lasting because that hundreds to hundreds of years, do this kind of volcano an especially dangerous, due to the fact that many time they have displayed no historic activity, and also people space reluctant to heed warnings about possible eruptions.

Cinder Cones

Cinder cones are little volume hat consisting mostly of ash and scoria that result from mildly explosive eruptions. They normally consist the basaltic to andesitic material. They are actually fall deposits that are developed surrounding the eruptive vent. Slopes of the hat are managed by the angle of repose (angle of steady slope for loose unconsolidated material) and also are typically between about 25 and also 35o.

They present an interior layered structure because of varying intensities of the explosions the deposit different sizes the pyroclastics.

on young cones, a depression in ~ the height of the cone, dubbed a crater, is evident, and represents the area above the vent native which material was explosively ejected. Craters room usually eroded away on older cones.

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

Cinder and tephra cones commonly occur around summit vents and also flank vents the stratovolcanoes.

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An excellent example of cinder cone is Parícutin Volcano in Mexico. This volcano to be born in a farmers corn ar in 1943 and also erupted because that the following 9 years. Lava operation erupted indigenous the basic of the cone ultimately covered two towns.

Craters and also Calderas

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