l>Volcanoes and Volcanic Eruptions
EENS 1110

Physical Geology

naipublishers.com University Prof. Stephen A. Nelson
Volcanoes and Volcanic Eruptions

Magmas and also Lava

Since volcanic eruptions are resulted in by magma (a mixture of fluid rock, crystals, and dissolved gas) expelled onto the Earth"s surface, we"ll an initial review the attributes of magma that we spanned previously. Three simple types the 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: Dissolved gases may comprise up to a few percent by weight of a magma.

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

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

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

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

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

Thus, basaltic magmas have tendency to be reasonably fluid (low viscosity), yet their viscosity is quiet 10,000 to 100,0000 times much more viscous 보다 water. Rhyolitic magmas often 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 show up solid have a viscosity, yet it an extremely high, measured together trillions time the viscosity the water). Viscosity is crucial property in identify 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, 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 products of volcano Eruptions

Lava flows

When magma reaches the surface ar of the earth, it is called lava. Because it its a liquid, it flows downhill in response to gravity together a lava flows. Different magma species behave in different way as lava flows, depending on their temperature, viscosity, and also gas content.

Pahoehoe operation - Basaltic lava flows v low viscosity start to cool as soon as exposed come the low temperature the the atmosphere. This reasons a surface skin to form, although that is still very 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. At first the surface skin is smooth, yet often inflates v molten lava and expands to form pahoehoe toes or roll to kind ropey pahoehoe. (See number 9.3d in her text). Pahoehoe flows have tendency to be thin and, because of your low viscosity take trip long distances from the vent. A"A" Flows - greater viscosity basaltic and andesitic lavas also initially build a smooth surface ar skin, yet this is quickly broken up by flow of the molten lava within and also by gases that continue to escape from the lava. This create a rough, clinkery surface ar that is characteristics of one A"A" flow (see figure 9.3e in her text). Lava tubes - once 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 form further cooling. Once the eruption ends, liquid lava often drains leave an open cavern (see number 9.3 in your text). Pillow Lavas - When lava erupts ~ above the sea floor or various other body the water, the surface skin forms rapidly, and, choose with pahoehoe toe inflates with molten lava. Ultimately these inflated balloons the magma autumn off and also stack up like a heap of pillows and also are called pillow lavas. Ancient pillow lavas are conveniently recognizable because of your shape, your glassy margins and radial fractures that formed throughout cooling (see number 9.4b in your text). Columnar Jointing - When special basaltic or andesitic lavas cool, lock contract. The contraction results in fractures and often times results in a form of jointing called columnar jointing. The columns are usually hexagonal in shape. This regularly happens as soon as lavas swimming pool in depressions or deep canyons (see figure 9.4a in your text). Siliceous Lava flows - High viscosity andesitic and rhyolitic lava flows, due to the fact that they can’t flow an extremely easily, kind thick stubby flows the don’t move far from the vent. Lava Domes or volcano Domes - result from the extrusion of highly viscous, gas poor andesitic and also rhyolitic lava. Due to the fact that the viscosity is for this reason high, the lava does not flow away from the vent, but instead piles up over the vent.Blocks of practically solid lava break off the outer surface the the dome and roll under its flanks to form a breccia about the margins of domes.

The surface of volcano domes room generally an extremely rough, with numerous spines that have actually been propelled up by the magma indigenous below.


Pyroclastic material

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

Tephra and also Pyroclastic Rocks

Average fragment Size (mm) Unconsolidated product (Tephra) Pyroclastic Rock
>64 Bombs or Blocks Agglomerate
2 - 64 Lapilli Lapilli Tuff
Blocks space angular fragments that to be solid when ejected. Bombs have an aerodynamic shape indicating they were liquid when ejected. Bombs and lapilli that consist largely of gas balloon (vesicles) an outcome in a low thickness highly vesicular rock fragment called pumice. Rock developed by accumulation and cementation of tephra dubbed a pyroclastic rock or tuff. Welding, compaction and also deposition of various other grains cause tephra (loose material) to be converted right into pyroclastic rock.

Volcanic Landforms

Volcanic terrain are controlled by the geological processes that type them and act on them after they have actually formed. Thus, a provided volcanic landform will certainly be properties of the varieties of material it is do of, which subsequently depends top top the former eruptive behavior of the volcano. Here we comment on the major volcanic landforms and also how they are created Most that this material will be debated with referral to slides displayed in class that show the essential features of each volcanic landform.

Shield Volcanoes

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

Shield volcanoes room composed practically entirely of reasonably thin lava flows collected over a central vent.

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

The low viscosity of the magma permits the lava to take trip down steep on a tenderness slope, however as the cools and its viscosity increases, that is thickness builds up ~ above the lower slopes offering a rather steeper reduced slope.

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

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

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


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

Stratovolcanoes show inter-layering that lava flows and pyroclastic material, i beg your pardon is why they room sometimes called composite volcanoes. Pyroclastic material can comprise over 50% the the volume that a stratovolcano.

Lavas and pyroclastics space usually andesitic to rhyolitic in composition.

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

Stratovolcanoes sometimes have actually a crater at the summit that is created by explosive ejection of material from a main vent. Periodically the craters have been fill in by lava flows or lava domes, periodically they space filled through glacial ice, and less generally they space filled with water.

lengthy periods of repose (times that inactivity) lasting for hundreds to thousands of years, make this type of volcano an especially dangerous, since many times they have displayed no historical activity, and also people room reluctant come heed warnings about feasible eruptions.

Cinder Cones

Cinder hat are little volume hat consisting primarily of ash and also scoria that an outcome from mildly explosive eruptions. They typically consist that basaltic come andesitic material. They are actually fall deposits that are constructed surrounding the eruptive vent. Slopes that the cones are controlled by the angle of repose (angle of secure slope for loose unconsolidated material) and are generally between around 25 and also 35o.

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

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

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

Cinder and tephra cones generally occur roughly summit vents and also flank vents the stratovolcanoes.

See more: Find The Volume Of The Solid That Is Enclosed By The Cone And The Sphere

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 operation erupted indigenous the basic of the cone at some point covered two towns.

Craters and 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 space much bigger depressions, circular to elliptical in shape, with diameters ranging from 1 kilometres to 50 km. Calderas form as a an outcome of fallen of a volcano structure. The collapse results from evacuation that the basic magma chamber.