Bowen’s Reaction SeriesBowen’s Reaction Series

Different minerals crystallize from magmas at different temperatures

Melting

Melting

Crystallization

Crystallization

HotHot

ColdCold

Daily QuestionDaily Question

The core (center) of the Black Hills of South Dakota is composed of granite. The Columbia River Plateau of Washington and Oregon is composed of basalt. Using a Venn Diagram, compare and contrast the two locations highlighting the composition of the rocks, the texture of the rock, and the location (depth) where the rocks formed.

Magmatic DifferentiationMagmatic Differentiation Formation of more than one magma from a single

parent magma

Magmatic DifferentiationMagmatic Differentiation Crystal Settling: crystallized minerals have a density

greater than the magma and settle to the bottom due to gravity

Because Fe and Mg are first removed, melt becomes rich in SiO2, Na, and K

Marbles analogy

Magmatic DifferentiationMagmatic Differentiation Assimilation: magma reacts with the “country rock” which

is adjacent to the magma chamberMagma composition is altered according to the composition of the assimilated country rockInclusions are rocksIncompletely melted chunks of country rock

Magmatic DifferentiationMagmatic Differentiation Magma Mixing: Magmas of different

compositions are mixed togetherResulting magma is of a composition intermediate between the parents

Magma MixingMagma Mixing

Magma MixingMagma Mixing

Magma MixingMagma Mixing

Fig. 7.21

Igneous ActivityIgneous Activity

Viscosity Controls Violent Nature of Volcanic Eruptions

Viscosity Controls Violent Nature of Volcanic Eruptions

Fig. 6.4

Controls of ViscosityControls of Viscosity Magma composition – silica content

High silica content – high viscosityLow silica content – low viscosity

TemperatureHigh temperature – low viscosityLow temperature – high viscosity

Dissolved gasLow viscosity – gases escapeHigh viscosity – gases cannot escape (pressure builds up)

Magma ProperitesMagma ProperitesCompositionComposition Silica Silica

ContentContentViscosityViscosity Gas ContentGas Content Tendency to Tendency to

form form PyroclasticsPyroclastics

Mafic Least (< 50%) Least Least (1-2%) Least

Intermediate Intermediate (~60%)

Intermediate Intermediate (3-4%)

Intermediate

Felsic Most (~70%) Greatest Most (4-6%) Greatest

Materials Extruded During an EruptionMaterials Extruded During an Eruption

Lava flows – low silica content of basalt lava allows it to flow up to 30 km/hour, typically 10 to 300 m/hour

Gases – reduction of confining pressure allows gases to escape

70% water vapor15% carbon dioxide5% nitrogen5% sulfur dioxideTrace amounts of chlorine, hydrogen, and argon

Pyroclastic material – welded ash, dust, and lava

Nuée ardenteNuée ardente Glowing

avalanche of ash, steam, gas, and pyroclastics

Travels at speeds of 200 km/ hr (125 mph)

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