up from the inferno: magma and igneous rocksvolatiles (gases) – magmas contain abundant dissolved...

Up from the Inferno: Magma and Igneous Rocks

Friday, January 25, 13

Igneous Rocks Solidified molten rock that freezes at high temp.

1,100°C to 650°C. Mineral Composition depends on cooling

temperature. Earth is mostly igneous rock.

Magma – Subsurface melt. Lava – Melt at the surface.

Volcanoes erupt magma.


Igneous Rocks Melted rock can cool above or below ground.

Intrusive igneous rocks – Cool slowly underground. Extrusive igneous rocks – Cool quickly at the surface.

Lava – Cooled liquid.

Pyroclastic debris – Cooled fragments.

Volcanic ash.

Fragmented lava. Many types of igneous rocks.

All oceanic crust. Most continental crust.


Sources of Heat Sources of heat to the early

Earth: Planetesimal and meteorite

accretion. Gravitational compression. Decay of radioactive

minerals. Tidal pull of the Sun and

Moon. The crust does NOT float on a

sea of molten rock.


Magma Formation Magma forms in special settings that melt existing

rocks. Partial melting occurs in the crust and upper mantle. Melting is caused by…

Decompression. Volatile addition. Heat transfer.


Magma Formation Geothermal gradient – The Earth is hot inside.

Crustal temperature (T) averages 25°C / km of depth.

At the base of the lithosphere T ~ 1280°C. The geothermal gradient varies from place to place.


Magma Formation Pressure release.

Base of the crust is hot enough to melt mantle rock. Due to high pressure, the rock does not melt. A drop in pressure initiates “decompressional melting.”

Pressure drops when hot rock

rises to shallower depths.


Addition of Volatiles

Volatiles lower the melting T of a hot rock.

Common volatiles include water and carbon dioxide.

Subduction introduces water to the mantle, melting rock.


Magma Formation

Heat transfer. Rising magma carries

mantle heat with it. This raises the T in

nearby crustal rock, which then melts.


What is Magma Made of? Magmas have three components (solid, liquid and gas).

Solid – Solidified mineral crystals are borne by the melt. Liquid – The melt itself is comprised of mobile ions.

Dominantly Si and O; lesser Al, Ca, Fe, Mg, Na, and K.

Other ions present to a lesser extent. Different mixes of elements yield different magmas.


What is Magma Made of? Volatiles (Gases) – Magmas contain

abundant dissolved volatile gas.Dry magma – Scarce volatiles.

Wet magma – To 15% volatiles.

Water vapor (H2O)

Carbon dioxide (CO2)

Sulfur dioxide (SO2)

Nitrogen (N2)

Hydrogen (H2)


Magma Compositions There are four major magma types based on silica (SiO2)

percentage.

Felsic (feldspar and silica) 66 to 76% SiO2.

Intermediate 52 to 66% SiO2.

Mafic (Mg and Fe-rich) 45 to 52% SiO2.

Ultramafic 38 to 45% SiO2.


Magma Compositions Composition controls magma density, T, and viscosity.

The most important factor is silica (SiO2) content.

Silica-rich magmas are thick and viscous.

Silica-poor magmas are thin and “runny.” These characteristics govern eruptive style.

Type Density Temperature Viscosity

Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions.

Intermediate Low Low High: Explosive eruptions.

Mafic High High Low: thin, hot runny eruptions.

Ultramafic Very high Very high (up to 1300°C) Very low.Friday, January 25, 13

Magma Variation The source of the melt dictates the initial composition.

Mantle source – Ultramafic and mafic magmas. Crustal source – Mafic, intermediate, and felsic

magmas.


Partial Melting Upon melting, only a portion of

the rock melts. Silica-rich minerals melt first. Silica-poor minerals melt last.

Partial melting, then, yields a silica-rich magma.

Removing a partial melt from its source creates… Felsic magma. Mafic residue.


Fractional Crystallization

The original melt As early-formed

As magma cools, mineral crystals settle by gravity. Melt composition changes as a result.

Fe, Mg, and Ca are removed in early settled solids. Si, Al, Na, and K remain in melt and increase in

abundance.


Bowen’s Reaction Series N. L. Bowen, in the 1920s, ran experiments with melts. Settling removed elements from the remaining melt. He discovered that minerals solidify in a specific series.

Continuous – Plagioclase changed from Ca-rich to Na-rich. Discontinuous – Minerals that solidify in a narrow T range.

OlivinePyroxene AmphiboleBiotite


Tabular Intrusions Dikes and sills modify invaded country rock.

They cause the rock to expand and inflate. They thermally alter the country rock.

Dikes Cut across preexisting layering (bedding or foliation). Spread rocks sideways. Dominate in extensional settings.


Tabular Intrusions Sills

Are injected parallel to preexisting layering. Are usually intruded close to the surface.

Both dikes and sills exhibit wide variability in... Size. Thickness (or width). Lateral continuity.


Plutonic Activity Most magma is emplaced

at depth in the Earth. A large, deep, igneous

body is called a pluton. Plutonic intrusions modify

the crust. Push aside preexisting

rock. Add new material. Add heat.


Intrusive and Extrusive Intrusive and extrusive

rocks commonly co-occur.

Magma chambers feed overlying volcanoes.

Magma chambers may cool to become plutons.

Many igneous geometries are possible.


Intrusive and Extrusive With erosion, progressively deeper features are

exposed. Dikes. Sills. Laccoliths – Mushroom-shaped intrusions.


up from the inferno: magma and igneous rocksvolatiles (gases) – magmas contain abundant dissolved...

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