l>Volcanoes and also Volcanic Eruptions
EENS 1110

Physical Geology

medtox.org University Prof. Stephen A. Nelson
Volcanoes and Volcanic Eruptions

Magmas and also Lava

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

You are watching: _______ magma causes powerful and explosive volcanic eruptions

Gases - At depth in the Earth almost all magmas save on computer gas. Gas provides magmas their explosive character, because the gas expands as press is reduced.

largely H2O v some CO2 Minor quantities of Sulfur, Cl , and F Felsic magmas typically have greater 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). Depends on composition, temperature, & gas content.

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

Thus, basaltic magmas have tendency to be reasonably fluid (low viscosity), but their viscosity is tho 10,000 come 100,0000 times an ext viscous than water. Rhyolitic magmas tend to have actually even greater viscosity, ranging between 1 million and also 100 million times much more viscous than water. (Note the solids, even though they appear solid have actually a viscosity, however it very high, measured as trillions time the viscosity of water). Viscosity is vital property in determining the eruptive actions 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 %, 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 products of volcanic Eruptions

Lava flows

When magma will the surface ar of the earth, it is called lava. Because it its a liquid, it operation downhill in an answer to gravity as a lava flows. Different magma species behave in different ways as lava flows, relying on their temperature, viscosity, and gas content.

Pahoehoe flows - Basaltic lava flows through low viscosity begin to cool when exposed to the low temperature the the atmosphere. This reasons a surface skin to form, although the is still an extremely 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 through molten lava and also expands come form pahoehoe toe or roll to type ropey pahoehoe. (See figure 9.3d in your text). Pahoehoe flows often tend to be slim and, since of their low viscosity travel long distances from the vent. A"A" Flows - higher viscosity basaltic and also andesitic lavas likewise initially develop a smooth surface skin, however this is quickly broken up by flow of the molten lava within and by gases that continue to escape native the lava. This create a rough, clinkery surface that is characteristics of one A"A" circulation (see figure 9.3e in your text). Lava tubes - as soon as the surface ar skin becomes solid, the lava can proceed to flow beneath the surface ar in lava tubes. The surface ar skin insulates the warm liquid lava type further cooling. When the eruption ends, liquid lava regularly drains leave an open cavern (see figure 9.3 in her text). Pillow Lavas - When lava erupts on the sea floor or other body that water, the surface skin develops rapidly, and, choose with pahoehoe toe inflates v molten lava. At some point these inflated balloons that magma drop off and stack up like a pile of pillows and also are referred to as pillow lavas. Ancient pillow lavas are conveniently recognizable due to the fact that of their shape, your glassy margins and also radial fractures that formed during cooling (see number 9.4b in her text). Columnar Jointing - When thick basaltic or andesitic lavas cool, lock contract. The contraction outcomes in fractures and often times results in a type of jointing referred to as columnar jointing. The columns space usually hexagonal in shape. This regularly happens once lavas swimming pool in depressions or deep canyons (see number 9.4a in your text). Siliceous Lava flows - High viscosity andesitic and rhyolitic lava flows, due to the fact that they can’t flow very easily, kind thick stubby flows that don’t move much from the vent. Lava Domes or volcanic Domes - an outcome from the extrusion of very viscous, gas negative andesitic and also rhyolitic lava. Due to the fact that the viscosity is therefore high, the lava go not flow away native the vent, but instead piles up over the vent.Blocks of practically solid lava break turn off the outer surface that the dome and also roll down its flanks to kind a breccia approximately the margins of domes.

The surface ar of volcano domes space generally very rough, with numerous spines that have been pushed up by the magma indigenous below.


Pyroclastic product

If the magma has actually high gas content and also high viscosity, the gas will broaden in an explosive fashion and also break the liquid into clots the fly v the air and cool follow me their path through the atmosphere. Additionally it blast the end solid pieces of rock that once created the volcano edifice. All of these pieces are described as Pyroclasts = hot, damaged fragments. Loosened assemblages of pyroclasts referred to as tephra. Relying on size, tephra deserve to be classified together bombs. Blocks, lapilli, or ash.

Tephra and also 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 were solid as soon as ejected. Bombs have actually an aerodynamic form indicating they were liquid as soon as ejected. Bombs and also lapilli that consist mainly of gas bubbles (vesicles) result in a low density highly vesicular rock fragment dubbed pumice. Rock developed by accumulation and cementation the tephra called a pyroclastic rock or tuff. Welding, compaction and also deposition of various other grains cause tephra (loose material) to be converted into pyroclastic rock.

Volcanic Landforms

Volcanic landforms are regulated by the geological procedures that type them and act on castle after they have actually formed. Thus, a provided volcanic landform will certainly be characteristics of the species of product it is make of, which consequently depends on the front eruptive actions of the volcano. Below we talk about the major volcanic landforms and how castle are formed Most that this product will be debated with referral to slides displayed in class that illustrate the crucial features of every volcanic landform.

Shield Volcanoes

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

Shield volcanoes room composed virtually entirely of fairly thin lava flows collected over a main vent.

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

The low viscosity that the magma permits the lava to take trip down steep on a gentle slope, but as the cools and its viscosity increases, that is thickness build up on the reduced slopes providing a somewhat steeper lower slope.

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

Very little pyroclastic product is uncovered within a shield volcano, other than near the eruptive vents, where little amounts that pyroclastic product accumulate as a an outcome of fire fountaining events.

Stratovolcanoes (also called Composite Volcanoes) have steeper slopes than shield volcanoes, v slopes the 6 to 10o low 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 do not travel far down steep from the vent.


The gentler slopes near the basic are due to accumulations of material eroded indigenous the volcano and to the build-up of pyroclastic material.

Stratovolcanoes show inter-layering that lava flows and also pyroclastic material, i m sorry is why they are sometimes referred to as composite volcanoes. Pyroclastic material can comprise over 50% the the volume the a stratovolcano.

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

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

Stratovolcanoes sometimes have actually a crater at the summit the is created by explosive ejection of product from a central vent. Occasionally the craters have been fill in by lava flows or lava domes, periodically they room filled v glacial ice, and also less frequently they room filled v water.

lengthy periods the repose (times the inactivity) lasting for hundreds to countless years, do this type of volcano specifically dangerous, since many time they have presented no historical activity, and also people are reluctant to heed warnings about possible eruptions.

Cinder Cones

Cinder cones are small volume cones consisting predominantly of ash and scoria that an outcome from mildly explosive eruptions. They normally consist of basaltic come andesitic material. They are actually autumn deposits that are constructed surrounding the eruptive vent. Slopes the the hat are regulated by the edge of repose (angle of secure slope for loose unconsolidated material) and also are commonly between around 25 and 35o.

They show an inner layered structure because of varying intensities of the explosions the deposit different sizes the pyroclastics.

on young cones, a depression at the peak of the cone, referred to as a crater, is evident, and represents the area above the vent from which product was explosively ejected. Craters are usually eroded away on larger cones.

If lava flows are emitted indigenous tephra cones, they room usually emitted indigenous vents top top the flank or close to the basic of the cone during the later stages the eruption.

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

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An excellent instance of cinder cone is Parícutin Volcano in Mexico. This volcano was born in a farmers corn field in 1943 and erupted for the next 9 years. Lava operation erupted from the base of the cone eventually covered two towns.

Craters and Calderas

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