Up from the Inferno: Magma and Igneous Rocks

Igneous Rocks Solidified molten rock that freezes at high temp.

1,100°C to 650°C.

Temp depends on composition.

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.

Magma Variation Why are there different magma compositions?

Magmas vary chemically due to…

Initial source rock compositions.

Partial melting.

Assimilation.

Fractional crystallization.

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, rocks rarely dissolve completely.

Instead, 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.

Magma melts the country rock it passes through.

Blocks of country rock (xenoliths) fall into magma.

Assimilation of these rocks alters magma composition.

Assimilation

Magma Migration Magma moves by…

Injection into cracks.

Melting overlying rocks.

Squeezed by overburden.

Low viscosity eases movement. Lower viscosity from…

Higher T.

Lower Silica content.

Higher volatile content.

Magma Migration Viscosity depends on T, volatiles, and silica.

Temperature:

Hotter – Lower viscosity

Cooler – Higher viscosity.

Volatile content:

More volatiles – Lower viscosity.

Less volatiles – Higher viscosity.

Silica (SiO2) content:

Less SiO2 (Mafic) – Lower viscosity.

More SiO2 (Felsic) – Higher viscosity.

Cooling Rates Cooling rate – How fast does magma cool?

Depth: Deep is hot; shallow is cool.

Deep plutons lose heat very slowly and cool slowly.

Shallow flows lose heat rapidly and cool quickly.

Shape: Spherical bodies cool slowly; tabular bodies cool faster.

Ground water: Ground water removes heat.

Fractional Crystallization

The original melt As early-formed

As magma cools, early 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.

Fractional Crystallization

Felsic magma can evolve from mafic magma.

Progressive removal of mafics depletes Fe, Mg, and Ca.

Remaining melt becomes enriched in Na, K, Al, and Si.

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

Igneous Environments Two major categories – Based on cooling site.

Extrusive settings – Cool at or near the surface.

Cool rapidly.

Chill too fast to grow big crystals.

Intrusive settings – Cool at depth.

Lose heat slowly.

Crystals grow large.

Most mafic magmas extrude.

Most felsic magmas don’t.

Tabular Intrusions Tend to have a uniform thickness.

Can be traced laterally.

Two major subdivisions.

Sill – Parallels rock fabric.

Dike – Crosscuts rock fabric.

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.

Interlocking or crystalline texture

Igneous Textures The size, shape, and arrangement of the minerals.

Interlocking – Mineral crystals fit like jigsaw puzzle pieces.

Fragmental – Pieces of preexisting rocks, often shattered.

Glassy – Made of solid glass or glass shards.

Texture directly reflects magma history.

Glassy texture Fragmental texture

Crystalline Igneous Textures Texture reveals cooling history.

Aphanitic (finely crystalline).

Rapid cooling.

Crystals do not have time to grow.

Extrusive.

Phaneritic – (coarsely crystalline).

Slow cooling.

Crystals have a long time to grow.

Intrusive.

Crystalline Textures

Texture reveals cooling history.

Porphyritic texture – A mixture of coarse and fine crystals.

Indicates a two-stage cooling history.

Initial slow cooling creates large phenocrysts.

Subsequent eruption cools remaining magma more rapidly.

Pegmatitic

Coarse mineral crystals found in dikes.

Large crystals are not due to slow cooling.

Instead, pegmatites form from water-rich melts.

Many unusual minerals are found in pegmatites.

Some pegmatites are rich in prized minerals.

Crystalline Textures

Glassy textures

Form by very rapid cooling of lava in water or air.

Glassy textures are more common in felsic magmas.

They often preserve gas bubbles (vesicles).

Underwater, basalt lava quenches into “pillows.”

Crystalline Textures

Glassy Classification

Vesicular.

escaping volatiles cause frothing of lava

rapid cooling then solidifies the lava producing rock with many vesicles

Fragmental Textures

Preexisting rocks that were shattered by eruption.

After fragmentation, the pieces fall and are cemented.

Fragmental Classification

aka Pyroclastic – Fragments of violent eruptions.

Tuff – Volcanic ash that has fallen on land and solidified.

Volcanic breccia – Made of larger volcanic fragments.

Igneous Activity Distribution

Igneous activity tracks tectonic plate boundaries.