• a "rock" - an assemblage of minerals bound together - thousands of rock types
• recall: the silicates, by far, are the most common mineral group in the earth's crust
• most igneous rocks contain some silica (SiO2 ) ...... 70% in granitic rocks....... 50% in basaltic rocks ......

• all rocks form from one of the following 3 processes: igneous, sedimentary, and metamorphic processes

• igneous rocks solidify & crystalize from a molton state - from a magma (molton rock beneath the surface)

• magma is either intruded into preexisting strata (country rock); or extruded onto the surface - "lava"

• the rock's texture and degree of crystallization is a function of how quickly the magma cools

• fast cooling = fine grained or glassy
• slow cooling = coarse grained

Fine-Grained Igneous Rock (Rhyolite)

Coarse-Grained Igneous Rock (Granite)

(1) Igneous Structures:

• shape relative to the host rock:

Intrusive Igneous Structures

(a) tabular - igneous body has a flat shape

(b) massive - large and somewhatirregularly shaped

• discordant - cut across existing structures
• concordant - form parallel to existing structures

• a "pluton" - any intrusive igneous structure

• a "batholith" - the largest form of a pluton - an irregular shaped mass with a surface exposure > 100 km2 - they form the mass of many large mountain ranges

• "sills" - plutons that form parallel to layers of sedimentary rocks - tabular and generally concordant - less viscous magma

• "dikes" - plutons that cross layers of country rock - discordant and tabular - are injected into fractures in country rock - more viscous fluid

• "laccoliths" - lens-shaped plutons in country rock - generally concordant - more viscous magma, lifts (arches) overlying strata

• all of these intrusive structures can be exposed by the weathering actions of air water, & ice

Igneous rocks are categorized by texture & composition:

• "Felsic" igneous rocks - derrived from feldspar & silica (minerals) - are high in silica, aluminum, potassium, & sodium - have low melting points, are light in color, & are less dense than mafic rocks

• "Mafic" igneous rocks - derrived from magnesium & iron - are darker in color, & are more dense than felsic rocks

 
• granite is a coarse grained, intrusive, (thus cooled slowly) felsic rock

basalt (comprises the bulk of the ocean floor) is a fine grained, extrusive (cooled quickly) mafic rock

C. Magma Crystallization

• crystallization (defn) - the formation and growth of a crystalline solid from a liquid or gas
• magma - a body of molton rock found at depth; the liquid portion is comprised of freely moving ions; it may also contain suspended crystals and dissolved gases

• crystallization is the opposite of melting
• as the magma cools, the ions in the magma lose kinetic energy, and occupy less space
• with sufficient cooling (loss of KE) the electrostatic forces between ions will begin to "glue" bond the ions together into an orderly crystalline structure
• centers of crystal growth - "embryo crystals" - exist throughout the the magma
• with slow cooling, have just a few centers of growth, yields larger crystals
• with quick cooling, lots of centers of growth competing for ions, thus, have a small (sometimes microscopic) crystals
• if the magma cools very quickly, there is insufficient time for the ions to arrange themselves in a crystalline structure = volcanic glass = randomly distributed ions

• most magma is comprised of 8 elements: Si, O, Al, Na, K, Ca, Fe, & Mg - makeup the silicate minerals
• also find trace amounts of other elements, plus volatiles
• volatiles - materials that are gases at surface temps and pressures - e.g., water & CO2 - visicles - e.g., the rock scoria

• the minerals are not all formed at the same time - some minerals crystallize at higher temperatures than others

Bowens Reaction Series

D. Igneous Rock Texture

defn - describes the overall appearance of the rock - based on size and arrangement of the crystals

• texture is determined predominantly by the cooling rate of the magma

 
• aphanitic, phaneritic, porphyritic (phenocrysts & groundmass), glassy, pyroclasts

E. Igneous Rock Types

• a single magma produces rocks with varying mineral content
• as magma cools, minerals with higher melting points crystallize before minerals with lower melting points
• as crystallization proceeds, the composition of the liquid magma changes, as the minerals rich in Ca, Fe, and Mg crystallize out of the magma
• the remaining liquid magma is rich in Al, Na, and K
• toward the end, the magma is enriched in silica

• additionally, there is interaction between the the solid component (crystals) and the melt - hence: "Bowen's Rxn Series"

Separation of Minerals by "Crystal Settling"

(1) Basaltic Type

• according to Bowen's rxn series, the first minerals to crystallize are Ca feldspar, pyroxene, and olivine - the dark (ferromagnesian silicates)

- peridotite (intrusive) - rarely observed at the Earth's surface - comprised of mostly olivine and pyroxene

- komatiite (extrusive) - rare

"Mafic" Igneous Rocks:

- basalt (extrusive) - pyroxene and Ca Feldspar, smaller amounts of olivine; dark green to black, fine grained; are porphyritic varieties; comprises: volcanic islands (Hawaii, Iceland), ocean floor, large portions of central Oregon & Washington (volcanic arcs)

-gabbro - intrusive equivalent of basalt; dark green to black in color; composed of pyroxene and calcium-rich plagioclase; is not a common constituent of continental crust, but undoubtedly makes up a signifucant percentage of oceanic crust

(2) Andesitic (intermediate) Type

• found near the middle of Bowen's rxn series
• has a mineral composition between that of basalt and granite - amphiboles and plagioclase feldspars

- diorite (intrusive) - coarse-grained, appearance similar to that of granite; distinguished from granite by the absence of quartz crystals - light colored feldspars and dark amphibole crystals yield "salt & pepper" appearance

(3) Granitic Type Igneous Rocks

• the "Felsic" Igneous Rocks:
• the last minerals to crystallize from the magma are K feldspar and quartz (SiO₂)

• granite forms the base of many of the worlds's mountain ranges - Pikes Peak, Mount Rushmore (Black Hills), White Mountains, Stone Mountain (Georgia)
• it is very abundant and resistant to weathering - remains after sedimentary strata have eroded away

Obsidian - volcanic glass ... Pumice

(4) Pyroclastic Rocks

• form from fragments ejected during volcanic eruptions

breccia - composed of particles larger than ash