l>Volcanoes and Volcanic Eruptions
EENS 1110

Physical Geology

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

Magmas and Lava

Since volcano eruptions are led to by magma (a mixture of liquid rock, crystals, and also dissolved gas) expelled ~ above the Earth"s surface, we"ll an initial review the features of magma the we spanned previously. Three basic 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: A lava solidifies into nonvesicular basalt instead of vesicular basalt because:

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

largely H2O with some CO2 Minor quantities of Sulfur, Cl , and also F Felsic magmas generally have higher gas components 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 flow (opposite of fluidity). Relies on composition, temperature, & gas content.

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

Thus, basaltic magmas often tend to be reasonably fluid (low viscosity), however their viscosity is quiet 10,000 to 100,0000 times an ext viscous than water. Rhyolitic magmas often tend to have even greater viscosity, ranging in between 1 million and also 100 million times an ext viscous than water. (Note that solids, even though they appear solid have a viscosity, yet it really high, measured together trillions time the viscosity that water). Viscosity is vital property in determining the eruptive habits 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 %, short in Fe, Mg, Ca, high in K, Na 650 - 800 oC High High

The commodities of volcano Eruptions

Lava operation

When magma reaches the surface ar of the earth, the is dubbed lava. Due to the fact that it that a liquid, it operation downhill in an answer to gravity as a lava flows. Various magma types behave in different way as lava flows, relying on their temperature, viscosity, and gas content.

Pahoehoe flows - Basaltic lava flows through low viscosity begin to cool once exposed to the short temperature the the atmosphere. This causes a surface skin to form, although the is still really hot and also behaves in a plastic fashion, qualified of deformation.Such lava flows the initially have a smooth surface ar are referred to as pahoehoe flows. Initially the surface skin is smooth, yet often inflates v molten lava and also expands to form pahoehoe toe or rolfes to type ropey pahoehoe. (See figure 9.3d in her text). Pahoehoe flows tend to be thin and, due to the fact that of your low viscosity take trip long ranges from the vent. A"A" Flows - higher viscosity basaltic and also andesitic lavas also initially develop a smooth surface ar skin, but this is quickly damaged up by flow of the molten lava within and also by gases that proceed to escape native the lava. This creates a rough, clinkery surface that is properties of one A"A" circulation (see number 9.3e in her text). Lava tubes - as soon as the surface ar skin i do not care solid, the lava can proceed to circulation beneath the surface ar in lava tubes. The surface ar skin insulates the hot liquid lava kind further cooling. Once the eruption ends, fluid lava frequently drains leave an open cave (see figure 9.3 in your text). Pillow Lavas - When lava erupts on the sea floor or other body the water, the surface skin forms rapidly, and, like with pahoehoe toes inflates v molten lava. Eventually these inflated balloons the magma autumn off and also stack up favor a pile of pillows and also are dubbed pillow lavas. Ancient pillow lavas are easily recognizable because of your shape, their glassy margins and radial fractures the formed throughout cooling (see figure 9.4b in her text). Columnar Jointing - When special basaltic or andesitic lavas cool, castle contract. The contraction results in fractures and also often times outcomes in a kind of jointing referred to as columnar jointing. The columns room usually hexagonal in shape. This frequently happens when lavas pool in depressions or deep canyons (see number 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 an extremely easily, form thick stubby flows the don’t move far from the vent. Lava Domes or volcano Domes - an outcome from the extrusion of highly viscous, gas negative andesitic and rhyolitic lava. Because the viscosity is so high, the lava go not circulation away indigenous the vent, however instead piles up over the vent.Blocks of almost solid lava break turn off the external surface that the dome and also roll under its flanks to kind a breccia approximately the spare of domes.

The surface of volcano domes are generally really rough, with numerous spines that have been thrust up by the magma from below.


Pyroclastic product

If the magma has actually high gas content and also high viscosity, the gas will expand in one explosive fashion and break the liquid right into clots that fly v the air and cool along their course through the atmosphere. Alternatively it blast out solid pieces of rock the once formed the volcano edifice. All of these fragments are referred to as Pyroclasts = hot, damaged fragments. Loosened assemblages of pyroclasts called tephra. Depending upon size, tephra deserve to be classified as 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 are angular pieces that to be solid once ejected. Bombs have an aerodynamic form indicating they were liquid when ejected. Bombs and also lapilli the consist largely of gas bubbles (vesicles) result in a low thickness highly vesicular rock fragment referred to as pumice. Rock formed by build-up and cementation that tephra dubbed a pyroclastic rock or tuff. Welding, compaction and deposition of various other grains cause tephra (loose material) to it is in converted right into pyroclastic rock.

Volcanic Landforms

Volcanic terrain are managed by the geological procedures that type them and also act on them after they have actually formed. Thus, a provided volcanic landform will certainly be characteristics of the types of product it is make of, which in turn depends ~ above the prior eruptive behavior of the volcano. Below we comment on the major volcanic landforms and how lock are developed Most that this material will be questioned with recommendation to slides shown in course that illustrate the important 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 virtually entirely of fairly thin lava flows built up over a central vent.

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

The short viscosity the the magma enables the lava to travel down steep on a gentle slope, but as that cools and its viscosity increases, its thickness build up on the lower slopes providing a somewhat steeper lower slope.

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

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

Stratovolcanoes (also called Composite Volcanoes) have steeper slopes than shield volcanoes, through slopes of 6 to 10o low on the flanks come 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 slope from the vent.


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

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

Lavas and pyroclastics room usually andesitic come rhyolitic in composition.

Due to the greater viscosity that magmas erupted from this 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 product from a central vent. Sometimes the craters have actually been filled in by lava operation or lava domes, sometimes they space filled through glacial ice, and less generally they room filled v water.

long periods that repose (times of inactivity) lasting for hundreds to thousands of years, make this kind of volcano particularly dangerous, because many time they have presented no historic activity, and people are reluctant come heed warnings about feasible eruptions.

Cinder Cones

Cinder cap are little volume cones consisting predominantly of ash and also scoria that result from mildly explosive eruptions. They usually consist of basaltic come andesitic material. They are actually fall deposits the are built surrounding the eruptive vent. Slopes that the cap are regulated by the edge of repose (angle of stable slope for loose unconsolidated material) and also are commonly between around 25 and 35o.

They show an internal layered structure because of varying intensities that the explosions that deposit various sizes the pyroclastics.

~ above young cones, a depression in ~ the height of the cone, called a crater, is evident, and represents the area above the vent indigenous which product was explosively ejected. Craters are usually eroded away on larger cones.

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

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

See more: How To Say Merry Christmas In Scottish Gaelic, How To Say Merry Christmas In Scottish

An excellent instance of cinder cone is Parícutin Volcano in Mexico. This volcano to be born in a farmer corn field in 1943 and erupted because that the following 9 years. Lava operation erupted indigenous the base of the cone ultimately covered 2 towns.

Craters and also Calderas

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