Volcanoes and Igneous Activity Earth - Chapter 4

Chapter 5 Volcanoes and Other Igneous Activity

Koko Head Crater, Hawaii

The nature of volcanic eruptions Characteristics of a magma determine the violence or explosiveness of a volcanic eruptionComposition TemperatureDissolved gases The above three factors actually control the viscosity of a given magma

The nature of volcanic eruptionsViscosity is a measure of a materials resistance to flow (how sticky)

Factors affecting viscosityTemperature - Hotter magmas are less viscousComposition - Silica (SiO2) contentHigher silica content = higher viscosity (e.g., felsic lava such as rhyolite)Felsic = granitic composition = rock is composed almost entirely of light colored silicatesLower silica content = lower viscosity (e.g., mafic lava such as basalt)

Mafic = Basaltic = mostly dark silicates and feldspar

Mafic vs. FelsicDenotes compostition of silicate minerals, magmas and rocks with heavy elements.

Ma from magnesium and FIC from latin word for iron.

Dark

Produced at spreading centers- rifts

Examples include, olivine, pyroxene, amphibole, biotite mica, and plagioclase feldspars.

Denotes composition of silicate minerals, magma, and rocks with low amount of heavy elements.

More silica, oxygen, aluminum, and potassium

Fel for feldspar, Sic from silica

Light in color

Examples: Quartz, muscovite mica, orthoclase feldspars, granite

What do the terms mafic and felsic mean?These are both made up words used to indicate the chemical composition of silicate minerals, magmas, and igneous rocks.Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color and have relatively high specific gravities (greater than 3.0). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro. (Please note that some geologists with questionable motives switch the order of the magnesium and iron and come up with the term "femag." This term is not to be confused with Femag, the dull-witted henchman of the Diabolical Dr. Saprolite.)Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silica and oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color and have specific gravities less than 3.0. Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. The most common felsic rock is granite, which represents the purified end product of the earth's internal differentiation process.It is important to note that there are many intermediate steps in the purification process, and many intermediate magmas which are produced during the conversion from mafic to felsic. We call the magmas associated with these intermediate stages "intermediate." Clever, huh?

5Rhyolite-light colored igneous rock.

Are these mafic or felsic?Basalt Dark colored igneous rock

Remember the oceanic crust is made mostly of basaltWhat type of igneous rock is this? Would you say Rhyolite or Basalt?

The nature of volcanic eruptionsDissolved gasesGas content affects magma mobilityGases expand within a magma as it nears the Earths surface due to decreasing pressureThe violence of an eruption is related to how easily gases escape from magmaIn summaryBasaltic lavas = mild eruptions (less silica)Rhyolitic or andesitic lavas = explosive eruptions (more silica)

Comparison of eruptionsViolentLess ViolentCompositionMore silicaRhyolite Felsic

Gas ContentCannot escape easily

TemperatureCompositionLess silicaBasalt - MaficGas Content Cannot escape easily

TemperatureMaterials extruded from a volcanoLava flowsBasaltic lavas exhibit fluid behaviorTypes of basaltic flowsPahoehoe lava (resembles a twisted or ropey texture)Aa lava (rough, jagged blocky texture)

Dissolved gases1% - 6% by weightMainly H2O and CO2

A pahoehoe lava flow

Figure 5.5 AAa lava flow

Figure 5.5 B Materials extruded from a volcanoPyroclastic materials fire fragmentsTypes of pyroclastic debrisAsh and dust - fine, glassy fragmentsPumice - porous rock from frothy lavaCinders - pea-sized materialLapilli - walnut-sized material Particles larger than lapilliBlocks - hardened or cooled lavaBombs - ejected as hot lavaA volcanic bombBomb is approximately 10 cm long

Figure 5.7VolcanoesGeneral featuresOpening at the summit of a volcanoCrater - summit depression < 1 km diameterCaldera - summit depression > 1 km diameter produced by collapse following a massive eruptionVent surface opening connected to the magma chamberFumarole emit only gases and smokeVolcanoesTypes of volcanoesShield volcanoBroad, slightly domed-shapedGenerally cover large areasProduced by mild eruptions of large volumes of basaltic lavaExample = Mauna Loa on Hawaii

Anatomy of a shield volcano

Figure 5.10VolcanoesCinder coneBuilt from ejected lava (mainly cinder-sized) fragmentsSteep slope angleSmall sizeFrequently occur in groupsArenal Volcano, Costa Rica - 1997

Notice the vent at the topArenal Volcano Costa Rica

Discussion question; why would people live so close to Arenal?Features of ArenalStratovolcanoVentBasalt FlowSteep SlopesGases easily escape

Concepcion Volcano, Nicaragua

Another stratovolcano

Cinder cone volcano

Figure 5.14Stratovolcano or composite volcano

Stratovolcanoes are common in subduction zones, forming chains along plate tectonic boundaries where oceanic crust is drawn under continental crust (Continental Arc Volcanism, e.g. Cascade Range, central Andes) or another oceanic plate (Island arc Volcanism, e.g. Japan, Aleutian Islands). The magma that forms stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. The water freed from the rock lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of igneous rock). When the magma nears the surface, it pools in a magma chamber under the volcano. There, the relatively low pressure allows water and other volatiles (CO2, S2, Cl) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened. Once a critical volume of magma and gas accumulates, the obstacle provided by the volcanic cone is overcome, leading to a sudden explosive eruption.[citation needed]

25VolcanoesComposite cone (stratovolcano)Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)Large, classic-shaped volcano (1000s of ft. high and several miles wide at base)Composed of interbedded lava flows and pyroclastic debrisMost violent type of activity (e.g., Mt. Vesuvius)

Poas Volcano, Costa Rica

Poas Volcano, Costa Rica

Is this a crater or a caldera?It is a Crater! Crater - summit depression < 1 km diameterCaldera - summit depression > 1 km diameter produced by collapse following a massive eruption

StratovolcanosCommon in subduction zones

The magma that forms stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab.

The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths.

The water freed from the rock lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of igneous rock).

When the magma nears the surface, it pools in a magma chamber under the volcano. There, the relatively low pressure allows water and other volatiles (CO2, S2, Cl) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened. Once a critical volume of magma and gas accumulates, the obstacle provided by the volcanic cone is overcome, leading to a sudden explosive eruption.[citation needed]

Central AmericaCascades of North AmericaSouth America Andes

Anatomy of a composite volcano

Figure 5.9Mt. St. Helens prior to the 1980 eruption

Mt. St. Helens after the 1980 eruption

Profiles of volcanic landforms

Figure 5.12VolcanoesNue ardente A deadly pyroclastic flowFiery pyroclastic flow made of hot gases infused with ash and other debrisAlso known as glowing avalanchesMove down the slopes of a volcano at speeds up to 200 km per hourLahar volcanic mudflowMixture of volcanic debris and waterMove down stream valleys and volcanic slopes, often with destructive results

Nuee ardente

A nue ardente on Mt. St. Helens

Figure 5.20Lahar flow

Other volcanic landformsCalderaSteep-walled depressions at the summitGenerally > 1 km in diameterProduced by collapsePyroclastic flowFelsic and intermediate magmasConsists of ash, pumice, and other debrisMaterial ejected at high velocitiesExample = Yellowstone plateauCalderas

Formation of Crater Lake, Oregon

Figure 5.22 Other volcanic landformsFissure eruptions and lava plateausFluid basaltic lava extruded from crustal fractures called fissuresExample = Columbia River PlateauLava domesBulbous mass of congealed lavaAssociated with explosive eruptions of gas-rich magma A lava dome

Figure 5.26Volcano in St. Vincent

St. Vincent volcano shoeing the lava dome

Lava dome Other volcanic landformsVolcanic pipes and necksPipes - short conduits that connect a magma chamber to the surfaceVolcanic necks (e.g., Ship Rock, New Mexico) - resistant vents left standing after erosion has removed the volcanic cone

Formation of a volcanic neck

Figure 5.27Shiprock, New Mexico

Intrusive igneous activityMost magma is emplaced at depth in the EarthOnce cooled and solidified, is called a plutonNature of plutonsShape - tabular (sheetlike) vs. massiveOrientation with respect to the host (surrounding) rockConcordant vs. discordant

Devils Tower, Wyoming

Devils Tower a volcanic neck

Intrusive igneous activityTypes of intrusive igneous featuresDike a tabular, discordant plutonSill a tabular, concordant pluton (e.g., Palisades Sill in New York)LaccolithSimilar to a sillLens or mushroom-shaped massArches overlying strata upward

Igneous structures

Figure 5.28 BA sill in the Salt River Canyon, Arizona

Figure 5.30 Intrusive igneous activityIntrusive igneous features continuedBatholithLargest intrusive bodySurface exposure > 100+ km2 (smaller bodies are termed stocks)Frequently form the cores of mountains

Batholithsof westernNorthAmericaFigure 5.32Practice with PlutonsTake 10 minutes to work with a group to label the following pictures. You may use your book

Discuss with your group wether the numbered volcanic features are discordant or concordant.

Dike exposed after erosion

Plate tectonics and igneous activityGlobal distribution of igneous activity is not randomMost volcanoes are located within or near ocean basinsBasaltic rocks = oceanic and continental settingsGranitic rocks = continental settings Distribution of some of the worlds major volcanoes

Figure 5.34Plate tectonics and igneous activityIgneous activity at plate marginsSpreading centersGreatest volume of volcanic rock is produced along the oceanic ridge systemMechanism of spreadingDecompression melting of the mantle occurs as the lithosphere is pulled apartLarge quantities of basaltic magma are producedPlate tectonics and igneous activitySubduction zonesOccur in conjunction with deep oceanic trenchesPartially melting of descending plate and upper mantleRising magma can form eitherAn island arc if in the oceanA volcanic arc if on a continental marginAssociated with the Pacific Ocean BasinRegion around the margin is known as the Ring of Fire Majority of worlds explosive volcanoes Plate tectonics and igneous activityIntraplate volcanismOccurs within a tectonic plateAssociated with mantle plumesLocalized volcanic regions in the overriding plate are called a hot spotProduces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland)Produces granitic magma sources in continental crust (e.g., Yellowstone Park)

Volcanoes and climateThe basic premiseExplosive eruptions emit huge quantities of gases and fine-grained debris A portion of the incoming solar radiation is reflected and filtered outPast examples of volcanism affecting climateMount Tambora, Indonesia 1815Krakatau, Indonesia 1883

Volcanoes and climateModern examplesMount St. Helens, Washington - 1980El Chichn, Mexico - 1815Mount Pinatubo, Phillippines - 1991

What type of lava is this?

What is this?Crater or Caldera?

The raised mound on the right is called a _______________.

Tell me about this rock.

This spread out, gradually sloped volcano is an example of a ________________volcano.

End of Chapter 5