l>Volcanoes and Volcanic Eruptions
EENS 1110

Physical Geology University Prof. Stephen A. Nelson
Volcanoes and Volcanic Eruptions

Magmas and also Lava

Since volcanic eruptions are brought about by magma (a mixture of liquid rock, crystals, and also dissolved gas) expelled onto the Earth"s surface, we"ll first evaluation the qualities of magma that we extended formerly. Three basic types of 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%, low in Fe, Mg, Ca, high in K, Na.

You watching: The nature of a volcanic eruption is determined largely by the ________ of the magma.

Gases - At depth in the Planet almost all magmas contain gas. Gas gives magmas their explosive character, because the gas broadens as push is decreased.

Mostly H2O with some CO2 Minor quantities of Sulfur, Cl , and F Felsic magmas usually have actually greater gas contents than mafic magmas.

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

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

Higher SiO2 content magmas have actually higher viscosity than lower SiO2 content magmas Lower Temperature magmas have better viscosity than better temperature magmas.

Hence, basaltic magmas tfinish to be sensibly fluid (low viscosity), yet their viscosity is still 10,000 to 100,0000 times more viscous than water. Rhyolitic magmas tfinish to have even greater viscosity, ranging between 1 million and also 100 million times more viscous than water. (Keep in mind that solids, also though they appear solid have actually a viscosity, however it exceptionally high, measured as trillions time the viscosity of water). Viscosity is a crucial building in determining the eruptive actions of magmas.

Outline 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 reaches the surface of the earth, it is called lava. Due to the fact that it its a liquid, it flows downhill in response to gravity as a lava flows. Different magma types behave in a different way as lava flows, depending upon their temperature, viscosity, and gas content.

Pahoehoe Flows - Basaltic lava flows via low viscosity start to cool once exposed to the low temperature of the environment. This reasons a surchallenge skin to develop, although it is still incredibly warm and also behaves in a plastic fashion, capable of deformation.Such lava flows that initially have a smooth surconfront are dubbed pahoehoe flows. Originally the surconfront skin is smooth, however often inflates through molten lava and also broadens to form pahoehoe toes or rolls to develop ropey pahoehoe. (See number 9.3d in your text). Pahoehoe flows tfinish to be thin and, bereason of their low viscosity travel lengthy ranges from the vent. A"A" Flows - Higher viscosity basaltic and andesitic lavas likewise initially construct a smooth surconfront skin, yet this is quickly damaged up by flow of the molten lava within and also by gases that proceed to escape from the lava. This creates a stormy, clinkery surface that is characteristic of an A"A" circulation (view figure 9.3e in your text). Lava Tubes - Once the surface skin becomes solid, the lava can continue to circulation beneath the surconfront in lava tubes. The surchallenge skin insulates the hot liquid lava kind even more 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 on the sea floor or other body of water, the surface skin creates rapidly, and, like through pahoehoe toes inflates via molten lava. Ultimately these inflated balloons of magma drop off and also stack up favor a pile of pillows and also are called pillow lavas. Ancient pillow lavas are readily recognizable because of their shape, their glassy margins and also radial fractures that developed during cooling (view number 9.4b in your text). Columnar Jointing - When thick basaltic or andesitic lavas cool, they contract. The contractivity results in fractures and regularly times outcomes in a type of jointing referred to as columnar jointing. The columns are typically hexagonal in shape. This frequently happens when lavas pool in depressions or deep canyons (check out number 9.4a in your text). Siliceous Lava Flows - High viscosity andesitic and also rhyolitic lava flows, because they can’t circulation incredibly conveniently, develop thick stubby flows that don’t move much from the vent. Lava Domes or Volcanic Domes - result from the extrusion of highly viscous, gas poor andesitic and also rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, yet instead piles up over the vent.Blocks of virtually solid lava break off the outer surface of the dome and also roll dvery own its flanks to develop a breccia about the margins of domes.

The surconfront of volcanic domes are mainly incredibly stormy, with numerous spines that have been puburned up by the magma from listed below.


Pyroclastic Material

If the magma has actually high gas content and also high viscosity, the gas will expand in an explosive fashion and also break the liquid right into clots that fly with the air and also cool alengthy their route through the atmosphere. Conversely it blast out solid pieces of rock that when created the volcanic edifice. All of these fragments are referred to as Pyroclasts = hot, damaged fragments. Loose assemblages of pyroclasts dubbed tephra. Depfinishing on dimension, tephra deserve to be classified as bombs. blocks, lapilli, or ash.

Tephra and Pyroclastic Rocks

Mean Pshort article Size (mm) Unconsolidated Material (Tephra) Pyroclastic Rock
>64 Bombs or Blocks Agglomerate
2 - 64 Lapilli Lapilli Tuff
Blocks are angular fragments that were solid as soon as ejected. Bombs have an aerodynamic shape indicating they were liquid as soon as ejected. Bombs and lapilli that consist mostly of gas bubbles (vesicles) result in a low density highly vesicular rock fragment referred to as pumice. Rock created by buildup and also cementation of tephra referred to as a pyroclastic rock or tuff. Welding, compactivity and deplace of various other grains cause tephra (loose material) to be converted right into pyroclastic rock.

Volcanic Landforms

Volcanic landdevelops are managed by the geological procedures that develop them and also act on them after they have actually formed. Hence, a given volcanic landform will be characteristic of the forms of product it is made of, which consequently depends on the prior eruptive habits of the volcano. Here we talk about the significant volcanic landforms and also just how they are created Most of this material will certainly be questioned through recommendation to slides presented in course that show the crucial functions of each volcanic landdevelop.

Shield Volcanoes

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

Shield volcanoes are created almost totally of reasonably thin lava flows gathered over a central vent.

Many shields were created by low viscosity basaltic magma that flows conveniently down slope amethod from the summit vent.

The low viscosity of the magma enables the lava to take a trip dvery own slope on a gentle slope, however as it cools and also its viscosity boosts, its thickness builds up on the lower slopes giving a rather steeper reduced slope.

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Most shield volcanoes have a roughly circular or oval shape in map see.

Very bit pyroclastic material is discovered within a shield volcano, other than near the eruptive vents, wright here small amounts of pyroclastic product accumulate as an outcome of fire fountaining occasions.

Stratovolcanoes (likewise called Composite Volcanoes) Have steeper slopes than shield volcanoes, with slopes of 6 to 10o low on the flanks to 30o close to the optimal. The steep slope close to the summit is due partly to thick, brief viscous lava flows that do not take a trip much down slope from the vent.


The gentler slopes close to the base are because of accumulations of product eroded from the volcano and to the build-up of pyroclastic product.

Stratovolcanoes display inter-layering of lava flows and pyroclastic material, which is why they are occasionally called composite volcanoes. Pyroclastic product can consist of over 50% of the volume of a stratovolcano.

Lavas and pyroclastics are usually andesitic to rhyolitic in complace.

Due to the higher viscosity of magmas erupted from these volcanoes, they are normally more explosive than shield volcanoes.

Stratovolcanoes occasionally have a crater at the summit that is created by explosive ejection of material from a main vent. Sometimes the craters have actually been filled in by lava flows or lava domes, periodically they are filled with glacial ice, and also much less commonly they are filled with water.

Long periods of repose (times of inactivity) lasting for hundreds to hundreds of years, make this form of volcano especially dangerous, because many times they have actually shown no historic activity, and world are reluctant to heed warnings around feasible eruptions.

Cinder Cones

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

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

On young cones, a depression at the top of the cone, referred to as a crater, is obvious, and also represents the area over the vent from which material was explosively ejected. Craters are commonly eroded away on older cones.

If lava flows are emitted from tephra cones, they are normally emitted from vents on the flank or close to the base of the cone throughout the later stages of eruption.

Cinder and tephra cones typically occur approximately summit vents and flank vents of stratovolcanoes.

An terrific example of cinder cone is Parícutin Volcano in Mexico. This volcano was born in a farmers corn area in 1943 and also erupted for the next 9 years. Lava flows erupted from the base of the cone ultimately spanned 2 towns.

Craters and Calderas

Craters are circular depressions, usually less than 1 km in diameter, that develop as a result of explosions that emit gases and ash. Caldperiods are much bigger depressions, circular to elliptical in shape, through diameters ranging from 1 kilometres to 50 kilometres. Caldperiods create as a result of collapse of a volcanic framework. The collapse results from evacuation of the underlying magma chamber.