Igneous Rocks and Magma Composition

Magma Composition and Properties

Magma is composed of liquid, solid (mineral crystals), and gas. If magma erupts to the surface, it is called lava.
The composition of magma is primarily controlled by its source.
Pahoehoe basalt flow is an example of magma at the surface.

Magma Classification by Silica Content

Magmas and lavas are largely subdivided by their silica ( $SiO_2$ ) content.
As silica content increases, iron (FeO) and magnesium (MgO) content decreases.
Lighter elements such as sodium ( $Na<em>2O$ ) and potassium ( $K</em>2O$ ) follow the silica trends.
Elemental composition is described in terms of oxide composition due to common bonds with oxygen.

Viscosity

Viscosity is the resistance to flow of a magma.
Viscosity is controlled by silica content and temperature.
Higher silica content and lower temperature lead to higher viscosity.
Viscosity units are graphed using a logarithmic scale, where each unit change is a 10-fold increase.
Aa basaltic lava flow is an example of cooled and degassed lava.

Basaltic Magma Formation

Basaltic magma forms at four different tectonic settings.
It is always derived from a partial melt of the asthenosphere.
Partial melting occurs at a depth of 100-350 km where the geothermal gradient intersects the melting temperature curve for upper mantle rock (garnet peridotite).
The geothermal gradient depends on pressure (depth).
The melting temperature curve depends on pressure (depth) and composition.
Basaltic magma is a dry melt (little dissolved water).
Its melting temperature decreases with decreasing pressure as the magma rises.
As basaltic magma melts further, its density decreases, causing it to rise until it reaches the surface.
The temperature of rising magma is ~200°C higher than its melting temperature at the surface.

Properties of Basaltic Lava

Basaltic lava has a relatively low viscosity and flows great distances from its vent.
It is dark-colored due to its mafic mineral content (pyroxene and Ca-rich plagioclase).

Pahoehoe vs. Aa Basalt Flows

Pahoehoe basalt flows have a lower viscosity than aa flows.
Pahoehoe flows have a ropey texture.
Aa flows have a blocky texture and are degassed and cooled.
Pahoehoe flows are smooth, while aa flows are rough.

Granitic Magma Formation

Granitic magma forms from a partial melt of continental crust.
Continental crust contains dissolved water.
Dissolved water reduces the melting temperature of magma with increasing pressure.
The melting temperature curve for wet granitic melt increases with decreasing pressure (opposite of basaltic dry melt).
Melting occurs at a depth of 35-45 km within continental crust.
As granitic magma rises, it solidifies because its melting temperature increases while the geothermal gradient decreases.
Granitic magmas rarely reach the surface as volcanic rhyolite flows because of the high water content.

Granitic Magma at Continental Collision Margins

Granitic magma is produced at continental collision margins.
As the continental crust thickens, it begins to partially melt at depth.
Igneous intrusions (plutons) form below mountain belts.
Volcanism is rare in continental collision boundaries.
As collisional tectonic mountain ranges are uplifted, overlying sedimentary and metamorphic rocks erode, exposing granitic plutons.
Granitic rocks of New Hampshire and Vermont represent old granitic plutons that formed when the Appalachian Mountains formed 300 million years ago during the collision of North America and a proto-European continent.

Granitic Magma in Yellowstone

Granitic magma reaches the surface in Yellowstone Park because the continental crust is heated closer to the surface (5-10 km) by upwelling basaltic magma from an asthenosphere hotspot.

Yellowstone Caldera and Rhyolite Flows

The Yellowstone Caldera formed following a large eruption ~600,000 years ago.
Rhyolite flows are very viscous, and internal gas pressures can be very high.
The explosivity of a volcanic eruption is related to the viscosity and gas content of the magma.

Rhyolite/Dacite Flows

Rhyolite/dacite flows retain steep slope fronts because of their high viscosity (related to high $SiO_2$ content and low temperature).

Intermediate Composition Magma

Intermediate composition magma can crystallize below the surface beneath subduction zones.
This creates large plutonic bodies composed of coarse-grained igneous rock.
Compositions range from granite to diorite.
El Capitan in the Sierra Nevada is an example of an intrusive complex that formed over 90 million years ago when a subduction zone existed along the margin of California.
The plutonic bodies comprising the Sierra Nevada are similar in origin to the plutonic bodies forming under the modern Cascades.
Granodiorite rock is found in the Sierra Nevada.

Andesitic Magma Formation

Andesitic magma is produced from a partial melt of oceanic crust along subduction zones.
Introduction of water from the subducting plate lowers the melting temperature of the upper mantle.
The mantle rises and partially melts the overlying crust.
In an ocean-continental convergent margin, magma may mix with partially melted continental crust, increasing the magma’s silica content.
Mount St. Helens dacites are more silica-rich than Mt. Rainier andesite, likely due to a continental source.

Dacite Composition

Mt. St. Helens is composed of intermediate composition dacitic flows.
Dacite is slightly more felsic (higher silica content) than andesite but more mafic (higher Fe and Mg content) than rhyolite.

Mineral Crystallization

Minerals crystallize at specific temperatures.
Certain minerals will be compatible and form together in igneous rocks (e.g., olivine, pyroxene, and Ca-rich plagioclase).
The crystallization temperature is highest for olivine and becomes progressively lower until quartz forms last from the residual $SiO_2$ melt.

Continuous Reaction Series

Continuous reaction series occur when Ca atoms continually exchange with Na atoms when the melt and solid phases are not separated.
Any proportionality of Na/Ca plagioclase (feldspar) minerals can occur.
The composition of the plagioclase mineral in a crystallizing magma changes continuously, even though the crystal structure remains unchanged.
Zoned feldspar crystals are composed of Ca-rich feldspar in the center and Na-rich towards the outer rim, with intermediate compositions in between.

Discontinuous Reaction Series

Discontinuous reaction series occur when early-formed crystals form entirely new minerals through reaction with the melt.
Olivine crystallizes first (highest melting/freezing temperature), leaving a magma that is slightly more silica-rich and possessing a greater proportion of other elements (Ca).
Olivine will react with the magma to produce an entirely new mineral, pyroxene.
The discontinuous reaction series will occur as long as the melt and crystal phases can react.

Differentiation of Magma

Differentiation of magma can occur from fractional crystallization, where the solid phase is separated from the melt phase.
The solid phase is relatively more mafic than the residual, more silica-rich melt phase.
The reverse reaction process occurs when rock is subjected to partial melting, where the melt phase is separated from the residual solid phase.

Fractional Crystallization

As earlier-formed minerals are removed from the magma by fractional crystallization, a greater proportion of denser elements (i.e., FeO & MgO) are removed.
This leaves a residual melt that is more enriched in $SiO_2$ and lighter elements.
Minerals and rocks that form later will have a greater proportion of lighter elements (i.e., $SiO<em>2$ , $Na</em>2O$ , and $K_2O$ ).

Classification of Igneous Rocks

Igneous rocks are classified based on texture and composition.
Fine-grained (aphanitic) and porphyritic igneous rocks form at the surface of the earth in volcanic settings.
Coarse-grained (phaneritic) igneous rocks form underground in intrusive complexes.
To determine mineral composition, project a line vertically downward and read the percentage numbers along the Y-axis of the classification chart.

Coarse-Grained Igneous Rocks

Coarse-grained igneous rocks crystallize slowly underground.
The composition of the rock depends upon the source of the magma and its cooling history.
Examples include gabbro, diorite, and granite.

Granodiorite

Granodiorite is of intermediate composition between granite and diorite.
It contains quartz, plagioclase, and biotite crystals.

Pink Granite

Pink granite is dominated by potassium feldspar (pink crystals), quartz (gray glassy appearance), Na-rich plagioclase (porcelain white mineral), and biotite (black sheets).

Basalt

Basalt is a fine-grained igneous rock that is erupted along diverse tectonic plate settings.
It has a black color and distinct hardness.

Andesite

Andesite is a porphyritic rock that forms at subduction zones.
It contains large plagioclase phenocrysts and small amphibole phenocrysts.
The gray groundmass is composed of biotite, potassium feldspar, and plagioclase.

Rhyolite

Rhyolite forms from very viscous, silica-rich lava.
It is the fine-grained equivalent of granite.
Eruptions are typically very explosive because of the high silica content and high gas content.

Obsidian

Obsidian forms from the residual melt of a fractionated felsic magma body.
It is composed almost exclusively of silica.
It is an amorphous glass because it does not have a crystalline structure.
Its dark coloration is due to the presence of small amounts of magnetite.
It has a characteristic conchoidal (rounded) fracture.

Porous Textured Igneous Rocks

Pumice (felsic composition) or scoria (intermediate or mafic composition) form when gas bubbles are trapped in rapidly cooling pyroclasts (air fall).
Gas bubbles can also be trapped in solidifying lava flows, such as vesicular basalt.

Pyroclastic Eruptions

Pyroclastic eruptions involve ash and volcanic-derived clasts becoming welded together.
This forms fine-grained tuff or coarse-grained volcanic breccia.

Volcanic Tuff

Volcanic tuff is comprised of welded ash and fine-grained volcanic lithic and pumaceous fragments.

Volcanic Breccia

Volcanic breccia forms from a welded mixture of large, angular volcanic clasts within a matrix of fine ash.