Essentials of Geology, Chapter 4 Igneous Rocks & Intrusive Activity

4.5 What are the three ways magma can be generated?

1. Decompression melting; caused by a decrease in pressure as magma rises 2. the introduction of water, which lowers the melting temp of hot mantle rock 3. heating crustal rocks above their melting temp.

4.1 How does temp impact crystalline rock?

As temp is increased, the tightly packed ions of crystals vibrate more rapidly, colliding with their neighbors. With increased temp, they occupy more space and expand, eventually will overcome their chemical bonds -> melting occurs. Now have unordered ions moving randomly.

4.7 What is a partial melt?

Igneous rocks composed of mixture of minerals -> minerals melt at different temps -> minerals with lowest melt temp will melt first -> composition of melt will eventually be the composition of the rock from which it was derived. However, it never melts completely. This is partial-melt.

4.1 What are the 3 components of magma?

Magma consists of liquid component, solid component, and gaseous component.

4.1 What is the parent material for igneous rock?

Magma is the parent rock.

4.3 A rock with a porphyritic texture is termed a what?

Porphyry.

4.1 What is igneous rock?

Rock formed from the crystallization of magma

4.9 Where are metals deposited during magmatic differentiation?

Since the metals crystalize first as the magma temperature starts to cool, they will settle out to the lower portion of the magma chamber. Some metals: chromite (ore of chromium), magnetite, platinum.

4.9 What is a pegmatite?

Very coarse-grained igneous rock, typically granite, commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a water-rich environs is believed to be responsible for the very large crystals. Rare elements like gold, silver may be in pegmatites.

4.5 Where does magma's originate?

Earth's uppermost mantle; greatest quantities are produced at divergent plate boundaries (sea-floor spreading); lesser amounts at subduction zones where oc plate subducts under continental plate.

4.6 What happens when a magma body forms and exerts pressure to the surrounding area?

In near-surface environs where rocks are brittle, magma pushes upward causing overlying rock to fracture -> force causes host rock to be dislodge -> gets incorporated into magma -> changes overall composition of magma (assimilation). When two distinct chemically different magma bodies are formed, one may rise more rapidly than the other due to viscosity -> they will eventually merge together if close enough to each other -> convective flow stirs the mixture -> now have magma mixing with intermediate composition.

4.1 What are the two forms of igneous rock?

Intrusive (plutonic) Rock - crystallizes under the Earth's crust at depth. They are observed at surface locations as a result of uplifting and erosion that has stripped away the overlying rocks. Eg: Mt Rushmore, Blk hills of S Dakota, Yosemite Extrusive (volcanic) Rock - molten rock solidifies at the surface. Eg. Cascade Range, Andes Mtns

4.5 What is a geothermal gradient?

It is an increase in temperature with depth. In the upper crust it is generally 25 degrees C (75F) for every km. Temp rises with increasing depth.

4.1 How is magma formed?

It is formed by partial melting that occurs at various levels within Earth's crust and upper mantle to depths of ~250 km.

4.5 Where does decompression melting occur?

It occurs at mid-ocean ridges (MOR) where divergent plates are pulling apart. It can also occur at hot-spot volcanoes.

4.3 Rocks that contain vesicles are said to have what?

Vesicular texture.

4.4 What are the 5 mineral compositions? Describe their major and minor minerals.

1. Felsic Maj Min: qtr & K Feld Min Min: Plag Feld, Amph, Musc, Biot Phaneritic: Granite (intrusive) Aphanitic: Rhyolite (extrusive) Porphyritic: Granite porphyry Glass: Obsidian (extrusive) Vesicular: contains voids; Pumice (extrusive) <--> Scoria (extrusive); (FELSIC <--> MAFIC) Pyroclastic: contains fragmentals; Tuff (extrusive) <--> Volcanic Breccia (extrusive); (FELSIC <--> MAFIC) 2. Felsic-Intermediate Maj Min: Min Min: Phaneritic: Granodiorite Aphanitic: Dacite 3. Intermediate Maj Min: Amph, Plag Feld Min Min: Pyrox, Biot Phaneritic: Diorite (intrusive) Aphanitic: Andesite (extrusive) Porphyritic: Andesite porphyry 4. Mafic Maj Min: Pyrox, Plag Feld Min Min: Amph, Oliv Phaneritic: Gabbro (intrusive) Aphanitic: Basalt (extrusive) Porphyritic: Basalt porphyry 5. Ultramafic Maj Min: Oliv, Pyrox Min Min: Plag Feld Phaneritic: Peridotite

4.3 What are the 6 igneous textures?

1. Phaneritic (coarse-grained) - composed of mineral grains that are large enough t o be identified without a microscope. Lg masses of magma slowly crystallize at great depth (intrusive); inter grown crystals. 2. Aphanitic (fine-grained) - composed of crystals that are too small for the individual minerals to be identified w/o a microscope. Ign. r. forms at surface (extrusive) 3. Porphyritic - composed of 2 distinctly different crystals. Lg mass of magma requires thousand or millions of yrs to solidify (intrusive or extrusive) ; crystals crystallize under different env'l conditions (temp/press) so crystals of one mineral may become large before another one can grow. Rock which has larger crystals (phenocrysts) embedded in a matrix of smaller (groundmass) crystals. 4. Vesicular - Extrusive r. containing voids left by gas bubbles that escape as lava solidifies. Nearly spherical openings called vesicles. Often form in upper zone of lava flow where cooling occurs rapidly to preserve openings produced by gas expanding bubbles. 5. Glassy - composed of unordered atom and resembles dk manufactured glass (obsidian <- hi silica rich magmas solidify). Occurs during explosive volcanic eruption when molten rock is ejected into atmosphere -> quenched (quickly cooled) to become solid. Has conoidal fractures and has sharp edge 6. Pyroclastic - extrusive, produced by consolidation of fragments that may include ash, once molten blobs, or large angular blocks that were ejected during an explosive volcanic eruption. Rocks w/ this texture have pyroclastic texture or fragmental texture.

4.3 What are the three factors that influence the textures of igneous rock? Describe what occurs.

1. Rate at which magma cools - this is the dominate factor in determining rock texture - large magma body many km under earth surface insulated by rock -> takes millions (billions) of yrs for molten rock to cool -> slow cooling promotes the growth of fewer but larger crystals. - rapid cooling (in thin lava flow) ions quickly lose their mobility and readily combine to form crystals -> numerous embryonic crystal nuclei competing for available ions -> result is mass of many tiny intergrown crystals. 2. Amount of silica in the magma Felsic rocks are high in silica content Mafic rocks are low in silica content 3. Amount of dissolved gases in the magma Felsic rocks are low in dissolved magma gases Mafic rocks are high in dissolved magma gases

4.6 Volcanoes may change their lava compositions over time. N.L. Bowen explored the crystallization of magma. Describe the Bowen's reaction series & the composition of igneous rocks formed.

As basaltic magma cools, minerals will crystallize in a systematic fashion based on their melting temp. In descending order (Hi temp ~1200˚C to Low temp ~650˚C) Discontinuous series of crystallization UMafic (Peridotite = Olivine+Pyrox)-> Mafic (gabbro/basalt = Pyrox+Amph) -> Intermediating (Diorite/Andesite = Amph+Biot+Pyrox)-> Felsic (granite/rhyolite = KFeld+Qtz+Musc mica+Biot+Amph+PlagFeld) Continuous series of crystallization UMafic Ca-rich Plag-> Mafic -> Intermediating Na-rich Plag-> Felsic (same as above)

4.5 How does pressure influence the melting of rock (decompression melting)?

As you go deeper into he Earth's the pressure increases. At higher temperatures the volume of material will increase. The confining pressure also increases with depth, exerted by the weight of overlying rocks. Conversely, when confining pressure is reduced, the rocks melting temp is also reduced. When confining pressure drops sufficiently, decompression melting occurs. This occurs wherever hot, solid mantel rock ascends, moving into regions of lower pressure.

4.7 Where are basaltic magmas formed?

Basaltic magmas may occur at MOR or at subduction zones where water is driven from the descending slab of oc crust promoting partial melting of mantle rocks that lie above.

4.8 What are the various intrusive bodies?

Dikes - discordant (cut across existing structure) bodies that form when magma is forcibly injected into fractures and cut across bedding surfaces and other structures in the host rock. Sills - are nearly horizontal, concordant (inject parallel to features such as sedimentary strata) bodies that form when magma exploits weaknesses between sedimentary beds or other rock structures. Columnar jointing - igneous rock cools and develop shrinkage fractures that produce elongated, pillar-like columns that most often have 6 sides. Batholith - massive intrusive igneous body. Eg: Sierra Nevada mountain range Laccolith - intrusive igneous body that can lift sedimentary strata they penetrate. Eg Pine Valley, UT

4.1 What is crystallization and its process?

Cooling reverses the events of melting. As temp drops, ions pack more closely together as rate of movement slows -> chemical bonds re-established between ions to form crystalline arrangement -> SiO2+O2 link first to form silicon-oxygen tetrahedra -> tetrahedra's join other ions to form embryonic crystal nuclei -> ea slowly grows as ions loose their mobility and join the crystal network -> ailment is then transformed into solid mass of interlocking silicate minerals -> Igneous rock

4.6 What is crystal settling and how does it affect magmatic differentiation?

Crystal settling is when earlier formed minerals are denser than the melt and sink toward the bottom of the magma chamber -> then solidifies in place or in another location like fractures in the surrounding rock -> remaining melt will be enriched with silica and should subsequent eruptions occur, the rocks generated will be more silica-rich and closer to the Felsic end of the compositional range than the initial magma. When the mineralogy or chemical composition is different from the parent magma this is called magmatic differentiation.

4.2 What are the 4 basic igneous compositions and characteristics?

Felsic - rich in K Feldspar, Qtz, Na Plag; less in Amphibole, Biotite mica, Muscovite mica. - most commonly form granite (granitic composition) - major constituents of continental crust - Very viscous (thick) - Can erupt at low temps (650˚C) Felsic-Interm - Fall between Felsic and Intermediate Intermediate - rich in Na Plag-Ca Plag, Amphibole; less in Pyroxene, Biotite mica, K Feld, Qtz - andesitic composition - contain 25% dark silicate material (amphibole, pyroxene, biotite mica) Mafic - rich in Ca Plag, Pyroxene; less in Amphibole and Olivine - most common basalt (basaltic composition) - at least 45% dark silicates (ferromagnesian minerals) - make up ocean floor and many volcanic islands within ocean basins - form extensive lava flows on the continents - Low in silica, more fluid lava flows (less viscous) - erupt at higher temps(1050 - 1250˚C) UltraMafic - rich in Olivine, Pyroxene; less in Ca-Plag - almost entirely of ferromagnesian minerals - main constituent of upper mantle (rare at Earth's crust)

4.7 Describe how partial melt of the mantle rock peridotite can generate a basaltic (mafic) magma.

Liquid magma rises to the surface only where the magma is less dense than the adjacent rock -> if the magma is more dense than the rock then the magma will remain trapped below the surface -> when mafic magma forms in the mantle it is less dense than the surrounding peridotite r. and moves upward -> mafic magma is denser than the continental crust so the magma stalls at the base of the crust -> if deep fractures penetrate the crust the press of the rock pushing down on the mafic magma will force it up to the surface to make volcanoes -> heat conducts from the mafic magma that is stalled at the base of the crust, this heating partially melts the crust to form felsic magma -> felsic magma is less dense than the continental crust and rises toward the surface, some of this magma erupts as volcanoes

4.1 Describe the three components of magma

Liquid portion: called melt is composed mainly of mobile ions of basic eight elements found in Earth's crust (Silicon, O2, and lesser amounts of Al, K, Ca, Na, Fe, and Mg) Solid portion: (if any) are crystals of silicate minerals. As the body cools, the size and number of crystals increase. At last stage of cooling looks like crystalline mush with little melt left. Gaseous portion: called volatiles, are materials that vaporize (form a gas) at surface pressures. Most common is H20, CO2, SO2.

4.3 Which magma forms more viscous lava?

Magmas with hi silica (granitic) content tend to form long chainlike structures before crystallization is complete -> slows migration of ions -> impedes crystal formation -> increases viscosity (thickness) of granitic magma. By contrast, basaltic magma (low in SO2) forms very fluid lava. however, rapid quenching of basaltic lava will also form thin, glass skin.

4.7 What is the source of the magma that crystallized to form andesitic (intermediate) and granitic (felsic) rocks?

Silica-rich magmas erupt mainly along continental margins. The continental crust which has a lower density than oceanic crust and provides the higher silica content for the recipe needed for Felsic rocks. Mantle-derived basaltic magma -> magmatic differentiation -> slowly making way thru continental crust -> basalt magma has ferromagnesian minerals crystallize first -> leaves remaining melt has an andesitic composition...OR Basaltic magma's assimilate crustal rocks that tend to be rich in silica -> partial melt of basaltic rocks -> ponds below continental crust due to greater density to continental crust -> basaltic magma partially melts overlying crustal rock (rich in silica w/ lower melting temp) -> producing large quantities of granitic magmas.

4.5 How would you recognize on a geothermal gradient graph when a rock will melt?

This chart shows you how to tell when rock melts. If the geothermal temperature is shifted to the right, over the solidus (temp at which a material changes from solid to liquid or vice a versa) line, there will be partial melt. This is true with the exception of Island Arcs where the solidus line shifts to the left of the geothermal gradient.

4.5 How does water affect melting rock and at which plate boundary does it occur?

Water lowers the melting temp of rock. The introduction of H2O to generate magma occurs mainly at convergent plate boundaries.where cool slabs of ocean lithosphere descend into the mantle. As oc plate sinks, heat and press drive H2O from the subducting oc crust and overlying sediment. At about 100 km (60 mi) the hot mantel wedge will react with H2O where mantel rock peridotite generates hot basaltic magma with temps over 1250 C (2300 F)

4.8 What is host or country rock and why are they important?

When magma rises through the crust it forcefully displaces preexisting crustal rocks called host or country rock. When magma is emplaced in preexisting rocks, intrusions or plutons are formed. They are formed below Earth's surface and only become exposed after uplifting and erosion has exposed them.

4.3 When does pumice form?

When silica-rich lava is ejected during an explosive eruption.