This lesson is  igneous processes and igneous rocks. The lesson focuses on volcanoes and igneous activity that takes place at Earth's surface. You will learn how volcanic as well as intrusive rocks are classified based on their grain size and mineral content. We recall the rock cycle as presented previeous. This is a conceptual diagram that shows the interrelationship between different types of rocks, i.e., igneous, sedimentary, and metamorphic rocks. After the section on igneous rocks classification, we describe structural relationships between bodies of intrusive rock and other rocks in the earth's crust. This is followed by a discussion of how magmas form and are altered. We conclude by discussing various hypotheses that relate igneous activity to plate tectonic activity. 

Questions

In your own words write a definition for magma and lava.

What are two important criteria used for igneous rock classification?

What is intrusive rock?

What is extrusive rock?

What is the major difference between intrusive and extrusive igneous rocks?

Why the texture of plutonic is different from the texture of volcanic rocks?

What are the most common extrusive and intrusive igneous rocks?

What is the difference between granite and rhyolite and how are they similar?

What is igneous process?

What is volcanic rock?

What is the appearance of a rock?

What is Aphanitic  and Phaneritic ?

What is Porphyritic texture?

What is Glassy texture ? how does it form?

What is Vesicular texture ? How does it form?

What is Pyroclastic texture ? How does it form?

What are different forms of intrusive rocks?

What are different forms of extrusive rocks?

Remember the origin and the characters (texture and mineral composition) of following rocks: granite, basalt, diorite, gabbro, andesite, rhyolite? (hints: students can search in internet for figures and describtion of these rocks)

Extra readings

1. The rock cycle relates the three major rock types, igneous, sedimentary and metamorphic, and the processes by which they are formed. These processes reflect a lack of equilibrium caused by external forces (weathering and erosion) or internal forces (tectonism). The major rock types are also related to one another at convergent plate boundaries.

2. Intrusive rocks crystallize from magmas emplaced into country rock. They possess mineralogies identical to volcanic rocks, but coarse-grained (= slow cooling) textures. Intrusions exhibit both "baked" and chill zone contacts, and they may contain xenoliths.

3. Names of plutonic rocks are the counterparts of extrusive rocks, sharing their mineralogy, but distinguished by their coarse-grained textures. Mineralogically equivalent granite-rhyolite, diorite-andesite, gabbro-basalt. The gabbro-basalt pair is dominated by ferromagnesian minerals and plagioclase feldspar. The granite-rhyolite pair is dominated by feldspars and quartz. The diorite-andesite pair is composed of feldspars and significant ferromagnesian minerals (30%-50%). 

4. Classification systems are arbitrary and there is considerable variation in the composition of granite and rhyolite.

5. Silica content varies significantly among rock types and influences the minerals comprising various rock types. Mafic rocks contain 50% or less silica by weight. They are silica-deficient and have high magnesium, iron, and calcium content. Silicic (Felsic) rocks are silica-rich (greater than 65%), and have significant content of aluminium, sodium, and potassium. Intermediate rocks fall between mafic and silicic (felsic), and Ultramafic rocks, of which peridotite is the most abundant, are composed of pyroxene and olivine and have less than 45% silica. They have no fine-grained counterparts.

6. Intrusive bodies are defined by size, shape and relationship to country rock. Volcanic necks are the solidified throats of volcanoes, dikes are discordant, tabular intrusions, while sills, are concordant, tabular intrusions. Plutons crystallize at great depth, and most are granite. Batholiths are large and discordant, while stocks are small and discordant. Detached bodies of magma that moved to shallow depths are called diapirs.

7. Granite comprises the bulk of continents. Basalt and to a lesser extent gabbro underlay the oceans, while andesite forms most volcanoes along continental margins. Ultramafic rocks are thought to form the mantle.

8. Magmas are melted by a combination of the effects of the geothermal gradient, mantle plumes, water under pressure, pressure release, and mixed mineralogies.

9. Bowen's Reaction Series explains the variation in rock composition that can be produced from a single magma. Crystallization proceeds simultaneously along two branches: a discontinuous branch for ferromagnesian minerals that remain reactive with the magma, and a continuous branch for plagioclase feldspars that exhibit zoning from changes in calcium and sodium content. These minerals are formed by silicon-oxygen tetrahedra that control their silica content. Any magma left after the discontinuous and continuous branches are complete is enriched in silica, and the last minerals to form are potassium feldspar, muscovite and quartz. Differentiation, crystal-settling, partial melting, assimilation, and magma mixing also account for compositional differences in magmas.

10. Basaltic magmas are produced at diverging plate boundaries from partial melting of the asthenosphere and build oceanic crust. Mantle plumes produce intraplate volcanism that is basaltic under oceanic crust, and rhyolitic under continental crust. Converging plate boundaries produce andesite by partial melting, and magmatic underplating that promotes melting of the lower continental crust for granite production. 

Technical terms

andesite  

Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar with the rest being ferromagnesian minerals.

basalt  

A fine-grained, mafic, igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.

batholith  

A large discordant pluton with an outcropping area greater than 100 square kilometers.

Bowen’s reaction series  

The sequence in which minerals crystallize from a cooling basaltic magma.

chill zone  

In an intrusion, the finer-grained rock adjacent to a contact with country rock.

coarse-grained rock  

Rock in which most of the grains are larger than 1 millimeter (igneous) or 2 millimeters (sedimentary).

contact  

Boundary surface between two different rock types or ages of rocks.

country rock  

Any rock that was older than and intruded by an igneous body.

crystal settling  

The process whereby the minerals that crystallize at a high temperature in a cooling magma move downward in the magma chamber because they are denser than the magma.

diapir  

Bodies of rock (e.g., rock salt) or magma that ascend within Earth’s interior because they are less dense than the surrounding rock.

differentiation  

Separation of different ingredients from an originally homogeneous mixture.

dike  

A tabular, discordant intrusive structure.

diorite  

Coarse-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar and the rest is ferromagnesian minerals.

extrusive rock  

Any igneous rock that forms at Earth’s surface, whether it solidifies directly from a lava flow or is pyroclastic.

fine-grained rock  

A rock in which most of the mineral grains are less than one millimeter across (igneous) or less than 1/16 mm (sedimentary).

gabbro  

A mafic, coarse-grained igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.

geothermal gradient  

Rate of temperature increase associated with increasing depth beneath the surface of Earth (normally about 25°C/km).

granite  

A felsic, coarse-grained, intrusive igneous rock containing quartz and composed mostly of potassium- and sodium-rich feldspars.

igneous rock  

A rock formed or apparently formed from solidification of magma.

intermediate  

Rock with a chemical content between felsic and mafic compositions.

intrusion (intrusive structure)  

A body of intrusive rock classified on the basis of size, shape, and relationship to surrounding rocks.

intrusive rock  

Rock that appears to have crystallized from magma emplaced in surrounding rock.

lava  

Magma on Earth’s surface.

mafic  

Silica-deficient igneous rock with a relatively high content of magnesium, iron, and calcium.

magma  

Molten rock, usually mostly silica. The liquid may contain dissolved gases as well as some solid minerals.

mantle plume  

Narrow column of hot mantle rock that rises and spreads radially outward.

peridotite  

An ultramafic rock composed primarily of the mineral olivine.

pluton  

An igneous body that crystallized deep underground.

plutonic rock  

Igneous rock formed at great depth.

rhyolite  

A fine-grained, felsic, igneous rock made up mostly of feldspar and quartz.

rock  

Naturally formed, consolidated material composed of grains of one or more minerals. (There are a few exceptions to this definition.

rock cycle  

A theoretical concept relating tectonism, erosion, and various rock-forming processes to the common rock types.

silicic (felsic)  

Silica-rich igneous rock or magma with a relatively high content of potassium and sodium.

sill  

A tabular intrusive structure concordant with the country rock.

stock  

A small discordant pluton with an outcropping area of less than 100 square kilometers.

texture  

For igneous rocks, refers to the size of mineral crystals in the rock.

ultramafic rock  

Rock composed entirely or almost entirely of ferromagnesian minerals.

volcanic neck  

An intrusive structure that apparently represents magma that solidified within the throat of a volcano.

xenolith  

Fragment of rock distinct from the igneous rock in which it is enclosed.