Igneous rocks

Geology 101

Igneous rocks’ origin

• Ignis (Latin for fire) – these rocks were crystallized from a molten state. They are not formed by sediment accumulation.

The diversity of volcanic materials

• Vesicular basalt

• Banded rhyolite

Igneous rock classification• The names of different igneous rocks

depends first on if they have a distinctive texture, and second on composition and grain size.

Volcanic rock names are either due to composition or to texture

• Basalt, because the rock is mafic

• Obsidian, because the rock is glassy

Igneous rocks have textures

• Mostly erupted rocks – these textures are all due to extremely rapid cooling, which tends to trap gas bubbles (which results in vesicles), to fuse falling hot rocks into already-fallen rocks (“welding”) and to create glassy surfaces (a vitreous luster).

VesiclesVesicles represent areas of trapped gas frozen in place when the surrounding rock crystallized. Scoria (left) and pumice may differ in composition but they principally differ in density: scoria will sink in water, pumice floats.

WeldingMicrograph of sample on left shows a welded tuff; note the melted, elongated look to many of the crystals.On the right is a sample of a tuff; note that many of the larger crystals look well-shaped and not melted.Welding hardens the rock.

Vitreous – as it formsAs a runny (basaltic) lava erupts, trails of molten rock are thrown into the air and freeze in glassy threads (Pele’s hair) and glassy teardrop shapes (Pele’s tears).

Vitreous – on a large scaleObsidian is a glassy volcanic rock; shown below is snowflake obsidian (the snowflakes are crystals of the mineral cristobalite, a polymorph of quartz). In fact, obsidian is mostly amorphous silica with the same chemical composition as quartz. There can be thick flows of obsidian, as seen in the Big Obsidian Flow at Newberry Caldera in Oregon.

Grain size allows sorting of igneous rocks

• Terminology:

• Volcanic rocks = extrusive = fine-grained (cannot see mineral grains within rocks)

• Plutonic rocks = intrusive = coarse-grained (easily seen mineral grains)

Why the different grain sizes?• Grain size is dependent on the rate of

magma crystallization, which is in turn dependent on the depth at which crystallization occurs

• Deep = slow cooling = coarse grains

Plutonic rocks

• The general structure in which plutonic rocks exist is called, not surprisingly, a pluton

• Types of plutons: batholiths (large), stocks (small), laccoliths, lopoliths, etc.

Volcanoes are complex structures

When the volcano erodes away, ring dikes may remain; example below is

near Odessa

The diversity of volcanic eruptions

• Kilauea (Hawaii)• quiescent

• Vesuvius (Italy)• explosive

More terminology

• Magma = molten rock underground

• Lava = molten rock above ground

• New tectonic setting: the “hotspot” -- small, intense mantle convection cell

Connections

• Tectonic setting leads to type of magma generated leads to type of volcano made leads to behavior of volcano

• Thus, understanding magma generation is the key

Magma generation at subduction

• At a subduction zone, the subducted slab loses volatiles (water, principally) and causes the melting temperature of the rocks above it to lower

Magma generation at rift

• At a rift, as the pressure of overlying layers lessens (crust thins), melting temperature of rock lowers.

But why different rocks?

• Start with mantle rocks, which are very rich in iron and other metals (like aluminum and magnesium)

• Magnesium, Aluminum, iron

(Fe) = mafic

Bowen’s reaction series shows mineral crystallization temperatures

• NL Bowen (Carnegie Institute) in 1915-1922 measured the melting (crystallizing) temperatures of various minerals and also found that there were two distinct progressions of mineral changes as the temperature changed: the continuous and the discontinuous series.

Which is why we can have non-mafic igneous rocks

• Using Bowen’s series, then, the temperature of magma sets its mineral composition, and thus the rock’s eventual identity.

• Mafic minerals form at high temperatures, but are replaced by more and more felsic (“feldspar” + “silica” meaning rich in silicon, oxygen, aluminum, potassium and sodium) minerals at lower temps.

Modification of magma

• At subduction zone, the descending slab loses volatiles and partially melts the mafic or ultramafic rock above, some of the slab material (or continental material) can melt and mix with the mafic magma

Modification of magma

• Fractional crystallization causes higher melting point minerals to crystallize and settle, removing mafic components of magma

Tectonic settings and magma

Magma composition leads to rock name leads to lava behavior

• Mafic intermediate felsic

Lava viscosity leads to volcano types

• Low viscosity lava yields low angle slope volcanoes -- shield volcanoes

Higher viscosity = steeper slopes

• Composite volcano or stratovolcano

Many different volcano types