l>Volcanoes and Volcanic Eruptions
EENS 1110

Physical Geology

gaianation.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 fluid rock, crystals, and also dissolved gas) expelled top top the Earth"s surface, we"ll first review the qualities of magma that we covered previously. Three straightforward types the magma: Mafic or Basaltic-- SiO2 45-55 wt%, high in Fe, Mg, Ca, low 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.

You are watching: Which list places the magma types in order of decreasing viscosity?

Gases - At depth in the Earth practically all magmas save gas. Gas provides magmas their explosive character, since the gas broadens as press is reduced.

mostly H2O v some CO2 Minor amounts of Sulfur, Cl , and also F Felsic magmas usually have greater gas components than mafic magmas.

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

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

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

Thus, basaltic magmas have tendency to be reasonably fluid (low viscosity), yet their viscosity is quiet 10,000 come 100,0000 times more viscous 보다 water. Rhyolitic magmas tend to have actually even greater viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, also though they appear solid have actually a viscosity, but it an extremely high, measured as trillions time the viscosity that water). Viscosity is vital 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 %, intermediate 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 products of volcanic Eruptions

Lava flows

When magma get the surface of the earth, it is called lava. Because it that is a liquid, it operation downhill in an answer to gravity as a lava flows. Various magma types behave differently as lava flows, relying on their temperature, viscosity, and gas content.

Pahoehoe operation - Basaltic lava flows through low viscosity begin to cool once exposed come the short temperature that the atmosphere. This reasons a surface ar skin to form, although that is still really hot and also behaves in a plastic fashion, qualified of deformation.Such lava flows that initially have actually a smooth surface are dubbed pahoehoe flows. Originally the surface skin is smooth, but often inflates with molten lava and also expands to form pahoehoe toes or roll to kind ropey pahoehoe. (See number 9.3d in her text). Pahoehoe flows tend to be slim and, because of your low viscosity take trip long ranges from the vent. A"A" Flows - greater viscosity basaltic and andesitic lavas additionally initially build a smooth surface skin, however this is quickly damaged up by flow of the molten lava within and also by gases that continue to escape from the lava. This creates a rough, clinkery surface ar that is properties of an A"A" flow (see figure 9.3e in your text). Lava pipe - once the surface skin i do not care solid, the lava can continue to flow beneath the surface ar in lava tubes. The surface skin insulates the hot liquid lava kind further cooling. When the eruption ends, fluid lava frequently drains leave an open cavern (see number 9.3 in your text). Pillow Lavas - When lava erupts top top the sea floor or other body that water, the surface ar skin forms rapidly, and, prefer with pahoehoe toes inflates through molten lava. Eventually these inflated balloons of magma autumn off and stack up prefer a heap of pillows and are dubbed pillow lavas. Old pillow lavas are easily recognizable due to the fact that of their shape, your glassy margins and radial fractures that formed throughout cooling (see figure 9.4b in your text). Columnar Jointing - When special basaltic or andesitic lavas cool, castle contract. The contraction results in fractures and also often times results in a type of jointing dubbed columnar jointing. The columns are usually hexagonal in shape. This frequently happens as soon as lavas pool in depressions or deep canyons (see number 9.4a in her text). Siliceous Lava flows - High viscosity andesitic and rhyolitic lava flows, due to the fact that they can’t flow really easily, form thick stubby flows that don’t move far from the vent. Lava Domes or volcanic Domes - result from the extrusion of extremely viscous, gas bad andesitic and also rhyolitic lava. Because the viscosity is therefore high, the lava go not flow away indigenous the vent, however instead piles up end the vent.Blocks of virtually solid lava break turn off the outer surface that the dome and also roll down its flanks to kind a breccia around the margins of domes.

The surface of volcano domes are generally very rough, with plenty of spines that have been moved up through the magma indigenous below.


Pyroclastic product

If the magma has actually high gas content and high viscosity, the gas will broaden in an explosive fashion and break the liquid right into clots the fly through the air and also cool along their course through the atmosphere. Conversely it blast out solid piece of rock the once developed the volcano edifice. All of these pieces are described as Pyroclasts = hot, broken fragments. Loose assemblages the pyroclasts called tephra. Relying on size, tephra deserve to be classified together 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 room angular pieces that to be solid once ejected. Bombs have an aerodynamic form indicating they were liquid as soon as ejected. Bombs and lapilli the consist largely of gas bubbles (vesicles) result in a low thickness highly vesicular absent fragment referred to as pumice. Rock formed by buildup and cementation of tephra referred to as a pyroclastic rock or tuff. Welding, compaction and deposition of various other grains reason tephra (loose material) to be converted into pyroclastic rock.

Volcanic Landforms

Volcanic landforms are managed by the geological procedures that type them and also act on lock after they have actually formed. Thus, a offered volcanic landform will be characteristic of the types of product it is do of, which in turn depends top top the front eruptive behavior of the volcano. Below we talk about the significant volcanic landforms and how lock are developed Most the this product will be disputed with recommendation to slides shown in course that show the necessary features of each volcanic landform.

Shield Volcanoes

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

Shield volcanoes space composed nearly entirely of relatively thin lava flows collected over a main vent.

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

The low viscosity the the magma allows the lava to travel down slope on a gentle slope, yet as it cools and its viscosity increases, that thickness builds up ~ above the reduced slopes providing a rather steeper lower slope.

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

Very small pyroclastic material is discovered within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events.

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


The gentler slopes close to the basic are as result of accumulations of material eroded from the volcano and to the buildup of pyroclastic material.

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

Lavas and pyroclastics room usually andesitic come rhyolitic in composition.

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

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

long periods that repose (times of inactivity) lasting for hundreds to hundreds of years, do this form of volcano specifically dangerous, because many times they have shown no historical activity, and people space reluctant to heed warnings about feasible eruptions.

Cinder Cones

Cinder cones are little volume cones consisting mainly of ash and also scoria that result from mildly explosive eruptions. They commonly consist the basaltic come andesitic material. They room actually autumn deposits that are built surrounding the eruptive vent. Slopes of the cones are managed by the angle of repose (angle of secure slope for loosened unconsolidated material) and are normally between around 25 and also 35o.

They display an interior layered structure as result of varying intensities that the explosions the deposit various sizes that pyroclastics.

~ above young cones, a depression in ~ the height of the cone, called a crater, is evident, and also represents the area over the vent native which product was explosively ejected. Craters are usually eroded far on older cones.

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

Cinder and also tephra cones generally occur around summit vents and also flank vents that 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 for the next 9 years. Lava flows erupted indigenous the base of the cone ultimately covered two towns.

Craters and also Calderas

Craters room circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and also ash. Calderas are much bigger depressions, circular come elliptical in shape, through diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcano structure. The collapse results from evacuation that the underlying magma chamber.