GEO 102: Igneous Rocks

Rock

• A coherent natural occurring solid 

• Consisting of an aggregate of one or more minerals or a mass of natural glass or organic matter 

Magma

• Molten rock that is located below Earth's surface, within the crust or upper mantle. 

• Magma is stored in magma chambers — pockets of molten rock beneath volcanoes. 

• Because it is under pressure, magma often contains dissolved gases (mainly H₂O, CO₂, and SO₂)

Lava

• Molten rock that has reached Earth's surface through a volcanic eruption. 

• Lava loses much of its dissolved gas as pressure decreases upon eruption. 

• Lava can flow across the surface or be ejected explosively

Factors Controlling Melt

• Temperature: increases melting, raises rock above melting point 

• Pressure: as pressure increases, the melting temperature of a rock also increases 

• Volatiles: any gas or liquid dissolved within the rock (water) 

Partial melting

not all minerals in a rock melt at the same temperature

selective melting means that the resulting magma often has a different chemical composition than the original source rock

Partial melting is one of the primary reasons that different types of magma and ultimately different igneous rocks exist

Plutonic

cools underground (magma) (intrusive) 

Volcanic

cools above ground (lava) (extrusive)

Bowen's Reaction Series

When magma cools, minerals do not all crystallize at the same time. Instead, they crystallize in a predictable sequence based on their melting and freezing temperatures

• The Discontinuous Series (left/mafic branch): This branch involves iron- and magnesium-rich (mafic) minerals that change from one crystal structure to a completely different one as temperature drops

• The Continuous Series (right/feldspar branch): This branch involves the plagioclase feldspar minerals. Rather than abrupt structural changes, the feldspar composition changes gradually and continuously

Mafic rocks

dark in color, High in Iron and Magnesium (Fe Mg)

First to crystalizes

High cooling temp

Intermediate Rocks

light and dark colors, medium concentration of silica (Si)

Felsic Rocks

light in color, high in silica and aluminum (Si Al)

Amorphic

minerals cool so fast they have no crystalline structure (obsidian)

Convergent Plate

plates coming together

Divergent Plates

plates coming apart

Transform Plates

plates sliding against each other

Volatiles

Adding water melts rocks easier 

Assimilation

blocks of wall rock melt into the magma, shifts toward host rock 

Magma Mixing

mafic magma injects beneath silica magma; blends to intermediate 

Fractional Crystallization

early crystals (mafic) settle or stick to the walls; melt grows more silicic 

Felsic Magma Viscosity

rocks have more silica content, making them stickier or more viscous (like honey)

Mafic Magma Viscosity

rocks have less silica content making them runnier, less viscous

Pegmatic

cool from low viscosity, allows fewer crystals to grow large, rather than many small crystals

Glassy

cools super-fast, no crystalline structure formed

Vesicular

lots of holes, caused by trapped gas (pressure)

Dike

discordant magma body (often a raised ridge of rock) (often mafic rocks) (vertical usually)

Sill

parallel/concordant igneous magma body (horizontal usually)

Intrusive (plutonic) rocks

form when magma cools slowly deep within Earth's crust. The slow cooling rate (sometimes spanning millions of years) allows mineral crystals to grow large enough to be seen with the naked eye

Extrusive (volcanic) rocks

form when lava or pyroclastic material cools rapidly at or near Earth's surface. The quick cooling rate does not allow large crystals to grow, resulting in fine-grained or glassy textures.

Porphyritic

rocks contain a two-stage texture: large crystals (called phenocrysts) embedded in a finer-grained matrix (called the groundmass)

Aphanitic

rocks have crystals that are too small to see without a microscope (typically < 1 mm). This is the characteristic texture of extrusive rocks that cooled rapidly

Phaneritic rocks

have crystals large enough to be seen and identified with the naked eye

Decompression melting

At mid-ocean ridges, as plates move apart, hot asthenospheric mantle rock rises to fill the gap.

This upwelling mantle does not melt because of increased heat, it melts because of decreased pressure. As the mantle rock rises from depth, the weight of overlying rock decreases.

At the same temperature, lower pressure allows minerals to melt that would remain solid under higher pressure

mid-ocean ridges

Subduction

At convergent boundaries, two tectonic plates collide. When one plate carries oceanic crust, its greater density causes it to sink, or subduct, beneath the opposing plate

chains of volcanoes; Volcanic Arcs