IGNEOUS ROCKS AND IGNEOUS PROCESSES

Introduction

Magma- Molten rock material composed mostly of _____________________.  Magmas may also include dissolved gases and minor amounts of solid minerals.

Magma at Earth's Surface. United States Geological Survey image.

Magmas can occur deep within the Earth, or at Earth's surface.

Igneous rock- A silicate-rich rock that forms when magma solidifies.

There are two types of igneous rocks:

Intrusive/Plutonic- Igneous rock formed when magma solidifies deep underground (includes granites, the main rock of the continents).

Extrusive/Volcanic- Igneous rock formed when magma solidifies at the Earth’s surface as lava (includes basalts, the main rock of ocean floors).

Unlike extrusive volcanism, intrusive igneous activity has never directly witnessed!  However, we can infer much about igneous processing based on indirect evidence.

The Textures of Igneous Rocks

Texture refers to the size, shape and arrangement of crystal grains within a rock.

Under the microscope, the mineral grains of igneous rocks tend to display an interlocking texture that represents the growth of minerals from a melt.

A thin section of gabbro showing plagioclase, clinopyroxene and olivine (GNU Image by Siim Sepp, 2006).

In igneous rocks, crystal size is primarily controlled by _________________________________________________.

Extrusive/volcanic rocks cooled quickly at or near Earth’s surface, giving crystals little time to grow.  These rocks tend to be fine-grained or aphanitic (most crystals <1 mm).

In contrast, intrusive/plutonic rocks cooled slowly deep within the Earth and are coarse-grained or phaneritic (most crystals >1 mm)

To classify igneous rocks, we will need to identify minerals in hand specimen.

Let's review the rock forming minerals, and divide each mineral into "light" and "dark" categories based on color.

The Classification of Igneous Rocks by Color

Igneous rocks can be readily classified into three categories based on color: felsic, mafic, intermediate, and ultramafic.  This color-based classification scheme may seem simplistic, but it turns out that a rock's color tells us much about its mineralogical makeup and overall composition.

Felsic rocks are rich in light-colored minerals (quartz, alkali feldspar, and some plagioclase feldspar).  They are compositionally rich in Si, Na, Al, and K and poor in Fe and Mg (the dark-colored minerals biotite and amphibole are present, but only in minor amounts).

A felsic rock known as granite (R. Welleror of Cochise College, © 2008).

Mafic rocks contain abundant dark-colored minerals (olivine, pyroxene, and plagioclase).  They are compositionally rich in Fe, Mg, and Ca.

A mafic rock known as basalt (R. Welleror of Cochise College, © 2008).

Intermediate rocks contain roughly equal amounts of dark- and light-colored minerals.

An intermediate rock known as andesite (R. Welleror of Cochise College, © 2008).

Ultramafic rocks consist almost exclusively of Fe and Mg-rich minerals from the mantle (olivine and pyroxene, but no plagioclase).  They are compositionally rich in Fe, Mg, and Ca, but poor in Si.

An ultramafic rock known as peridotite consisting chiefly of olivine (R. Welleror of Cochise College, © 2008).

The Classification of Igneous Rocks Based on Color, Texture, and Mineralogy

The most useful system for classifying igneous rocks utilizes color, texture, and mineralogy.

Special Textures in Igneous Rocks

Xenolith: A fragment of rock within an igneous rock that differs compositionally from the host rock.  The host rock and zenolith inclusions formed from different magmas.

Vesicule: A bubble or hole formed by escaping gas (common in basalts).

Olivine xenolith in vesicular basalt, © Dr. Richard Busch

Pegmatite: A very coarse grained igneous rock (crystal sizes > 5 cm) in which crystal growth was enhanced by the presence of fluids.

Pegmatite, © Marli Miller

Porphyritic: Igneous rock with large crystals (called phenocrysts) in a fine-grained matrix.  Porphyritic rocks may represent a two-state cooling history:

1) slow cooling at depth followed by...

2) rapid uplift and fast cooling near Earth's surface.

Porphyritic rock, © Dr. Richard Busch

How Does Magma Form?

1) The temperature of the Earth increases from crust to core at approximately 30 C/km (this is called the geothermal gradient).  The core temperature is > 5000 C, and heat moves upward from the very hot core (where temperatures exceed 5000°C) and melts the upper mantle and crust.

2) Melting can also result from a decrease in pressure. Since pressure favors solids, mineral melting points decrease with decreasing pressure. This decompression melting occurs when hot mantle rock moves upward.

3) The presence of water vapor reduces the melting point of rock. Wet magma (magma with water vapor) melts at a lower temperature than dry magma (magma with no water vapor). For example, wet granite melts at 700°C whereas dry granite melts at 900°C.

4) Mixtures of minerals always have lower melting points than the pure minerals would.   For example, Quartz melts at ~1650°C and K-Feldspar melts at ~1300°C. However, a 50/50 mixture of these two minerals will melt at ~1150°C.

Magma Crystallization and Melting Sequence

Minerals crystallize in a predictable order over a large temperature range (and melt in the reverse order).  This sequence of mineral crystallization is described by Bowen’s Reaction Series, named after N.L. Bowen who used laboratory experiments to determine the sequence of mineral crystallization.

Lessons from Bowen’s Reaction Series:

1) The chemistry of a magma will determine the type of rock that can form from it.

2) For a given magma composition, the first magmas to solidify will be mafic (rich in Fe, Mg, Ca) such as a basalt or gabbro.

3) Later, more evolved felsic magmas (rich in K, Na, Si, and quartz) will produce rhyolites and basalts.

4) During heating, the order of mineral melting will be reversed from the order of crystallization.

Magma Evolution

Magmas that solidify close to their source rock will be the most like the source rock, whereas magmas that solidify far from the source rock will be changed or evolved.  Magma evolution (called magma differentiation) can occur by 4 different processes:

1) Partial melting produces magmas less mafic than their source rocks, because the first minerals to melt will be felsic in composition.

2) Fractional crystallization involves the changing of magma composition by the removal of denser early-formed ferromagnesian minerals by crystal settling.  The remaining magma becomes more felsic.

3) Assimilation occurs when a hot magma melts and incorporates surrounding country rock.  If mafic magma assimilates more felsic continental crust an intermediate rock will result.

4) Magma mixing involves the mixing of more and less mafic magmas to produce a magma of intermediate composition.

Intrusive Rock Bodies

Intrusive rocks exist in intrusions that penetrate or cut through pre-existing country rock.

Intrusive bodies are given names based on their size, shape and geometric relationship to the country rock.

Two basic types of intrusions are:

A.    Shallow intrusions (formed < 2 km beneath Earth’s surface).  These cool and solidify fairly quickly resulting in fine-grained rocks.

Dike: Tabular structure that cuts across the layering in the country rock.

Igneous intrusions at Alaska's Glacier Bay National Park © Bruce Molnia, Terra Photographics.

Sill: Tabular structure that parallels layering in the country rock.

Basaltic sill near Logan Pass in Montana's Glacier National Park. © Larry Fellows.

Volcanic Neck: Shallow intrusion formed when magma solidifies in the throat of a volcano (i.e., Ship Rock, New Mexico).

 
The volcanic neck, Shiprock. Copyright © Louis Maher

B.    Deep intrusions (formed > 2 km beneath Earth's surface).  These cool and solidify slowly resulting in coarse-grained rocks.

Plutons are large, blob-shaped intrusive bodies formed when rising blobs of magma (diapirs) get trapped within the crust (commonly granite

Summit of Harney Peak in the Black Hills of South Dakota. © Bruce Molnia, Terra Photographics.

Small plutons (exposed over <100 km²) are called stocks, whereas large plutons (exposed over >100 km²) are called batholiths.

The interface between instrusions and country rock are called contacts.

Rapid cooling of igneous rock near the contact (called a chill zone) often results in a smaller crystal size near the contact.

The Link Between Igneous Activity and Plate Tectonics

Igneous activity occurs mainly at or near tectonic plate boundaries.

Mafic igneous rocks commonly form at divergent boundaries.  Here, the low overburden pressure (decompression) contributes to the formation of mafic magmas formed by partial melting of the asthenosphere (upper mantle).

Intermediate igneous rocks commonly form at convergent boundaries.  Partial melting of subducted asthenosphere produces basaltic magma which evolves into intermediate magma by differentiation, assimilation, and magma mixing.

Felsic igneous rocks are also common adjacent to convergent boundaries.  Hot mafic magmas produced near the subducting slab may induce partial melting and assimilation of continental (granitic) crust.

Some ingneous rocks form within plates (not at a plate boundary).  Rising mantle plumes (of controversial origin) can produce localized hotspots and volcanoes as they rise through continental or oceanic crust.