Chp 3 Igneous Rocks

Igneous Rocks-derived from volcanic processes -volcanoes

-submarine and sub-aerial

Derived from magma chambers deep beneath the surface of the Earth

Chp 3 Igneous Rocks

Magma – a mobile, silicate melt formed in the upper mantle or lower crust, as much as 100 to 300 kilometers below the surface.

- It accumulates at depths in reservoirs called magma chambers. Magma chambers may be only a few km below the surface at spreading centers and below the oceanic crust or only a few tens of km below oceanic subduction zones or continental plates.

-Magma may slowly cool in place forming intrusive igneous rocks.Or, magma may breech the surface in the form of a volcano forming extrusive igneous rocks.

-Lava is the term for magma that has breeched the earth’s surface.

Fig. 3-CO, p.60

Granite rocks in YosemiteNational Park-

Part of the Sierra NevadaBatholith

Mount Rushmore-carved from Harney Peak Granite

Fig. 3-1a, p.62

Crazy Horse Monument, also being carved from Harney Peak Granite

Fig. 3-1b, p.62

Table 3-1, p.63

Magma Types

Fig. 3-2, p.64

Magma Types:Magma Types:

A. Ultramafic-Comprises the upper mantle (asthenosphere)-Very low in silica (45% or less) & low viscosity-Does not breech the earth’s surface usually-Only forms intrusive igneous rocks-High in ultramafic minerals such as olivines -Forms dark greenish-colored rocks

B. Basaltic or Mafic Magma-Low viscosity; fluid-like and flows easily-Low silica content (47 – 50% silica)-Temperature range of 9000 – 12000C-High in mafic minerals: amphibole, pyroxene, olivine-Forms dark colored rocks

Magma Types:Magma Types:

C. Intermediate Magma-A “mixture” of mafic and sialic magmas-Medium viscosity-Silica Content of 50 – 59%-Temperature range of 8000 – 10000C-May contain both mafic and sialic intermediate minerals -Forms medium, grayish-colored rocks

D. Felsic or Sialic Magma-Higher viscosity, thicker, gooier-High silica content (65 – 70% silica)-Temperature range of less than 8000C -High in sialic minerals: quartz, microcline, muscovite-Forms light colored rocks

Fig. 3-3, p.65

Bowen’s Reaction Series - A series of reactions based upon fractional crystallization.

Mafic minerals have a higher point of crystallization and crystallize first, followed by intermediates, followed by sialics,as the magma cools.

Discontinuous Branch – a succession of ferromagnesian silicates crystallize as the temperature of the magma drops: Olivine to pyroxene to amphibole to biotite.

Continuous Branch – Plagioclase feldspars with increasing amounts of sodium crystallize.

This leaves the higher silicate minerals to crystallize last, at the lowest temperatures: potassium feldspars, muscovite, and quartz.

1. Effects of Silica Content on MagmaSilica tetrahedra in magma link together to form polymersThe more silica polymers, the thicker, gooier the magmaThe thicker, gooier the magma, the more explosive a volcano may be..The more silica polymers, the more sialic the magma creating lighter colored rocksThe more sialic the magma, the lower the temperature at which it crystallizes (less than 8000C)

The reverse is true for more mafic magmas…The fewer the silica polymers, the thinner, less viscous the magma and the more free-flowing the magmaThe less viscous the magma, the less explosive the volcanoThe more mafic the magma is creates darker ferromagnesian minerals and darker rocksThe more mafic the magma, the higher the temperature at which it crystallizes (800 – 10000C)

2. Effects of Pressure on MagmaIn the earth, pressure on rocks (or a magma body) from the surrounding rocks (or overburden – the weight of the rocks above the structure) keeps it from expanding and prevents melting.

a. A drop in pressure causes hot rocks to melt. -A decrease in pressure can arise from the erosion or removal of the overburden, causing the rocks to melt.

b. An increase in pressure can arise from tectonic activity or an increase in pressure from the magma chamber. -An increase in pressure causes melting rocks to slow their melting.

As magma is rising upwards through the rocks, pressure is decreasing as it nears the surface, preventing it from solidifying and thus possibly forming a volcano.

3. Effects of Water Content on Magma

a. Water lowers the melting point of magma “Dry Magmas” have a water content of less than 10% “Wet Magmas” have a water content of 10 – 15% water

b. Water at high temperatures is very volatile: At high temperatures water tends to escape as a gas (superheated water vapor)

c. High pressure keeps water from escaping Cracks in the overburden may allow water to escape

d. Sialic (Granitic) magmas usually solidify below the surface intrusively Mafic magmas being less viscous and many times reach the surface as lava The water content in most mafic magmas is very low: 1- 2%

e. Pressure keeps water from expandingNear “mafic” melts usually contain 1 – 2 % H2O, causing it to remain molten and it easily reaches the surface.

Fig. 3-5, p.67

Fig. 3-6, p.68

Fig. 3-7, p.69

Fig. 3-8, p.70

Aphanitic=rapid cooling

Phaneritic= slower cooling

Porphyritic= complex cooling history

g= Obsidian (glassy)h= Pumice (vesicular)

i= microscopic view ofFragmental texture

Igneous Rock TexturesThe texture of an igneous rock refers to the size, shape and arrangement of the constituent mineral grains and reflects the rate of cooling.

IGNEOUS ROCK TEXTURES:

I. Glassy Texture – -“resembling man-made glass”-having “concoidal fracture”-possessing a “randomness” of crystal latticesIons have no time to migrate to form crystals-Indicates a rapid rate of extrusive cooling Example = Obsidian, Volcanic Glass

II. Aphanitic Texture –“a” meaning “without”; “phaneros” meaning “visible” – overall meaning is that the rock has crystals, but they can not be identified with the naked eye.Tiny crystals have formed, but require magnification to identifyIndicates relatively “quick” extrusive cooling but at a rate slower than that required for a glassy textureCooled at, or near the earth’s surface.Example = Rhyolite, Andesite, or Basalt

Fig. 3-11a, p.72

Aphanitic= Basalt

III. Phaneritic Texture –“Phaneros” meaning “visible” – whereby there are large crystals present that are easily identified with the naked eye.An individual large crystal is referred to as a Phenocryst.The presence of phenocrysts indicates a slow rate of intrusive cooling occurring deep within the earth’s crust.Pegmatites are igneous rocks that cool extremely slow resulting in “giant” phenocrysts. Examples = Granite, Diorite, Gabbro, or Peridotite

Phaneritic= Gabbro (note light reflecting from crystal faces).

Fig. 3-11b, p.72

IV. Porphyritic Texture –This texture indicates that the igneous rock had two cooling periods – the first one slow and the second one quicker.

The larger phenocrysts form during the first cooling period while the magma is at an intrusive depth in the crust. If it were to continue cooling at this ratet, it would have formed a phaneritic texture. Something occurred to move the still molten magma (containing the first formed phenocrysts) closer to the surface whereby the still molten material cools at a faster rate.This gives the rock two distinct crystal sizes: the larger phenocrysts that formed first are set in a finer “groundmass” (matrix) of either porphyritic, aphanitic, or glassy textures.

Rock texture may be porphyritic/aphanitic, meaning that phenocrysts formed first and the still molten ground mass rapidly cooled. Or, the rock may be porphyritic/phaneritic meaning the the phenocrysts formed and the ground mass cooled having a phaneritic texture. Examples: Basalt Porphyry, Granite Porphry

V. Pyroclastic Texture –Means “fire-broken” – formed from volcanic ejecta: the ash, cinders, and “bombs” expelled from eruptions of volcanoes.All pyroclastic textured rocks are extrusive being produced by explosive volcanic eruptions.Aside from any pre-formed crystals, pyroclastics are generally categorized as to the particle size:

fine ash - X < 0.06 mm

coarse ash - 0.06mm – 2.0mm

cinders - 2.0mm – 64.0mm

“bombs” - X > 64.0mm

Being thrown into the air and later settling out on the ground, they may cool before they settle outforming ”unwelded” pyroclastics, or they may remain glowing hot as they settle out forming “welded” pyroclastics.

“Tuffaceous rocks” is the term for the rocks resulting from the settling of pyroclastic particles, and may result in the formation of “unwelded tuffs” or “welded tuffs”.

Fig. 3-13, p.73

Felsic Igneous Rocks:

a. Rhyolite:

b. Granite

Typically light colored becausethey contain non ferromagnesianSilicate minerals.

Dark spots in granite are biotitemica. White and pink minerals Are feldspars.

Fig. 3-15, p.75

Fig. 3-16, p.75

p.76

Plymouth Rock:

granodiorite

p.76

Igneous Intrusive Bodies – “Plutons”Pluton – an intrusive igneous body that cools and crystallizes deep within the earth’s crust.The geometric shape of plutons may be:massive or irregularTabular, cylindrical, mushroom shaped

Plutons are also described as to whether they are concordant or dis-concordant. Concordant pluton - has boundaries parallel to the to the layering of the country rock (the surrounding rock)Disconcordant pluton – has boundaries that cut across the layering of the country rock.

p.76

Pluton Types:I. Dikes – discordant intrusive bodies usually emplaced in pre-existing fractures cutting across the country rock as the magma rises. Characteristics:They are discordant cutting across the layering of the country rock in zones of weakness, such as cracks. Most are 1 to 2 meters wide, can range from a few centimeters to more than 100 meters thick.They form whenever magma is forced into pre-existing fractures of the country rock, or when the fluid pressure in the dike itself creates its own fractures.Many can form “wall-like” structures radiating outward from volcanoes like the spokes on a wheel.II. Sills – concordant intrusive bodies that are sheet-like emplaced between layers of the country rock.Characteristics:Sills are concordant emplaced whenever fluid pressure is so great that it lifts the overlying rocks, filling in with magma in a horizontal manner. They are tabular or disk-like in shape with many usually a meter or less thick. Some are much thicker, up to 300 meters or more (i.e. the Palisades of NY and New Jersey)Most have intruded into sedimentary rock, but many are also commonly found injected into piles of volcanic rock.Sill inflation prior to a volcano erupting may account for volcanoes swelling just before exploding.

p.81b

Shiprock, in New Mexico:

a. Volcanic neck

b. Dikes intruded laterallyfrom the volcanic neck, nowexposed at surface due toerosion.

Fig. 3-14, p.74

Pegmatite Dike inBlack Hills of SoDakota (a)

C: Tourmalinecrystals from Pegmatite in Maine

III. Laccoliths – sill-like in that they are concordant, but with a “mushroom shape”. Characteristics:Laccoliths are concordant mushroom-shaped intrusive bodies.They tend to have a flat floor with a domed up center.Like sills, they lift up the overlying strata of the country rock, but usually being larger than sills, the overlying strata bends to conform to the curved shape.They also are relatively shallow intrusions.

IV. Volcanic Pipes and Necks – discordant cylindrical conduits of volcanoes.Characteristics:A volcanic pipe is the term for the actual conduit of magma upward from the magma chamber deep below.Through this structure magma rises to the surface.When a volcano ceases to erupt, surface processes begin to erode the cone while the once molten volcanic pipe solidifies.Whenever the solidified volcanic pipe is exposed by erosion, it is termed a volcanic neck. (i.e. ”Shiprock” in northwestern New Mexico)

p.81a

Volcanic neck in Le Puy, France.

Volcanic neck= the pathwaythrough which magma escapesfrom Earth’s interior upwards,eventually leading to volcanoesor sills/dikes.

p.81c

Devil’s Tower: in Wyoming

a. Maybe a volcanic neckb. or eroded laccolith

Vertical ‘lines’ arefractures, describedas columnarjoints.

V. Batholiths and Stocks – are the largest of all plutons.Characteristics:These are very large intrusions created by repeated, forceful injections and voluminous intrusions of magma in the same area. Many times these intrusions continue for millions upon millions of years (i.e. thecoastal batholiths of Peru took about 60 million years; and the Llano Uplift of Central Texas).

To be called a batholith, the body must be greater than 100 km2 of total surface area.A stock is similar in formation but has a surface area less than 100 km2.Some stocks are simply parts of large plutons that once exposed by erosion are batholiths (the tip of the iceberg”) Most are granite in composition, but some may be diorite. (mostly sialic magmas, with someintermediate magmas)Most are formed near continental margins during episodes of mountain building or great uplift (during an orogeny or tectonic activity).

Fig. 3-18, p.82

V. Batholiths and Stocks – are the largest of all plutons.Characteristics:

As the solutions at the tops of the intrusions penetrate cracks in the overlying strata, concentrations of minerals dissolved in the solutions may become concentrated. (i.e. gold, copper, silver)Granitization – the process whereby the surrounding country rock is transformed into granite in a severe form of metamorphism. This mayaccount for the great amounts of granite formed in some batholiths that shows a gradation from granite into some other rock at the borders.

Some batholiths show a direct igneous origin of its granite since its borders are “sharp” at the transition from granite to country rock.

The presence of inclusions of country rock especially at the top of batholiths indicates that it was igneous in origin.

Fig. 3-19, p.83

Batholith emplacement through time: parts of host rock, termed‘country rock’ are detached and engulfed by magma as they Intrude into the host rock.

Fig. 3-17, p.82

Berkeley Pit Mine in Butte, Montana: nearly 1.5 billion tons of copper have been removed from rocks adjacent to the Boulder batholith.

Fig. 3-9, p.71

Classification of Igneous Rocks

Chapter 3 Summary: Igneous RocksChapter 3 Summary: Igneous Rocks

a. Magma – source of igneous rocks-3 things effect magma: pressure, temperature, water

b. Textures of igneous rocks: function of cooling and composition; 5 types 1. aphanitic

2. phaneritic3. porphyritic4. glassy5. pyroclastic

c. Intrusive igneous bodies are known as ‘Plutons’1. Dikes2. Sills3. Laccoliths4. Volcanic necks5. Batholiths and stocks

Fig. 3-4, p.66

Fig. 3-10, p.71

Ultramafic= Peridotite

Fig. 3-12a, p.72

Intermediate: Andesite

Fig. 3-12b, p.72

Intermediate: Diorite (note salt and pepper appearance)