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Igneous Geology. Igneous geology focuses on the process and structure (arrangement of parts) of igneous intrusions and extrusions.

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igneous geology
Igneous Geology
  • Igneous geology focuses on the process and structure (arrangement of parts) of igneous intrusions and extrusions.
  • Magma composition, especially silica content, strongly influences igneous geology. Felsic magmas are cooler, more viscous (even to being more plastic solids than liquids), and more prone to explosive eruptions than mafic magmas, which generally have very gentle eruptions.
  • Partial crystallization and partial melting are important. As crystals form from a melt, the melt becomes depleted in elements that are incorporated in those minerals and enriched in elements that are not incorporated into those minerals. If the crystals are removed from contact with the melt (by settling to the bottom of a magma chamber, for example), the final melt can have a very different composition than the initial melt.
  • As a melt cools and changes in composition, components that were once miscible can become immiscible. (Think of grease separating from chili, or one type of grease separating from another.) This phenomenon is called exsolution and controls formation of some ores and whether volcanoes are explosive.
  • Unlike sediments, which have very predictable distributions due to their mode of deposition, igneous rocks are much more chaotic and difficult to map.

v 0067 of 'Igneous Geology' by Greg Pouch at 2012-09-08 10:45:15

LastSavedBeforeThis 2011-08-31 14:00:25

igneous geology1

Igneous Geology

3 Composition

4 Processes

5 Melting > Generation of Magma

6 Melting > Partial Melting

7 Melting > Partial Melting > Continuous

8 Melting > Partial melting > Eutectic

9 Melting > Partial Melting > Results

10 Melting > How to Melt Rock

11 Melting>Melting Temperature Varies with Pressure

12 Melt > Melting Temperature with Pressure

13 Processes > Ex-solution

14 Processes > Movement

15 Processes > Heating of Country Rock

16 Products


17 Products > Extrusive Features > Volcanoes

18 Products > Extrusive Features > Others

19 Flood Basalts

20 Products > Extrusive Materials


21 Products > Intrusions > Some Vocabulary

22 Products > Intrusions > Tabular

23 Products > Intrusions > Equant, Irregular

24 Products > Intrusions > Features > Other

25 Plate Tectonics and the Origin of Igneous Rocks

26 Plate Tectonics > Divergent Basalt

27 Plate Tectonics > Subduction Andesite

28 Plate Tectonics > Collision Granite

29 Plate Tectonics > Hotspots Basalt, maybe Granite/Rhyolite

39 Plate Tectonics > Transform Boundaries NONE

  • Composition of magma is limited by source rock. The eight major elements plus water and maybe CO2 are common in magmas and control the properties of the melt. Sialic melts are more viscous and less liquid-like (more polymerized) than mafic melts, which are very liquid-like.
  • In addition to the source rock and the degree of melting, magma’s composition can change due to
    • Assimilation of country rock (the surrounding, pre-existing rock),
    • Segregation of early-formed minerals
    • Ingress or egress of volatiles, especially water
    • Mixing with another magma (rare?)
  • Melting
  • Ex-solution
  • Movement and emplacement
  • Heating of surrounding rocks
melting generation of magma
Melting > Generation of Magma
  • Observations: A few compositions of igneous rock are rather common, and most possible compositions are fairly rare (The chart and table in the Igneous Rocks Lecture and in the book shows common igneous rocks, not all possible igneous rocks. A more comprehensive classification scheme is found at http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/IUGS-IgneousClassFlowChart.htm ). The common igneous rocks at the surface are
    • basalt (oceans)
    • granite-granodiorite (continents)
    • and some andesite/diorite (volcanic chains near subduction zones)

Mantle is mostly peridotite (ol+pyr+Ca-plag). So are most meteorites.

  • Conclusion: The narrow range of compositions suggests that magma generation results in certain definite compositions, even from a wide range of materials. Most magmas arise from partial melting of pre-existing rocks.
melting partial melting
Melting > Partial Melting
  • A pure substance like quartz (SiO2) melts at one temperature to a liquid of the same composition.
  • Continuous When solid-solution minerals like olivine (Mg,Fe)SiO4 and plagioclase (CaAl, NaSi)AlSi2O8 melt, continuous partial melting occurs, with melting over a range of temperatures with smoothly-varying composition. The first melt gets more of the low-melting component than the parent, and the restite more of the high-melting component than the source. Continued melting occurs at continuously higher temperatures and later melts include more of the high-melting component, until it's all melted.
  • Eutectic When minerals don't form a solid solution series (separate crystals in the solid phase), like albite+quartz or water+halite, each acts to depress the freezing point of the other, and ANY mixture of the crystals start melting at the same temperature to produce melt of the same composition (eutectic point). The eutectic melting temperature is often MUCH lower than the melting temperature of either component.

For iron-carbon (steel):

Pure iron melts at 1535C,

Pure carbon melts at 4200C, and a

Eutectic iron-carbon mixture (a cast iron) melts at about 1154C and 4.3%wt Carbon, which is more than 400C colder than iron.

melting partial melting continuous
Melting > Partial Melting > Continuous
  • Some minerals can accommodate different ions at a particular lattice site in their crystal structure. (For example, minerals containing Mg+2 can hold Fe+2 in that same site.) As the composition varies, so do the properties, like melting temperature.
  • A solid-solution mineral melts over a range of temperatures, with melt and restite composition and relative amounts depending on the initial composition and the temperature. (and pressure, water content, …)

At 1 atm pressure: Olivine, the magnesian endmember Mg2SiO4 Forsterite (Fo) melts around 1890°C whereas the Fe endmember Fe2SiO4 Fayallite (Fa) melts at 1205°C.

Plagioclase, the sodic endmember Albite (Ab) NaSiAlSi3O8=NaAlSi3O8 melts at 1118°C, whereas the calic endmember Anorthite CaAlAlSi3O8=CaAl2Si2O8 (An) melts at 1500°C.

  • In the phase diagrams, the white area at high temperature is liquid, the green, banana-shaped area is slush (liquid+crystals), and the light gray area is solid crystals. The blue, upper boundary of the slush region is called the liquidus, and the red, lower boundary is the solidus. At any composition, melting starts when the mixture is heated to the solidus and finishes when it gets to the liquidus (freezing starts at the liquidus and finishes at the solidus). At any temperature, the melt's composition lies on the liquidus and the crystals' composition on the solidus.
  • To find the first melt, go straight up from the parent composition to the solidus to get the temperature; go straight over from there to the liquidus to get the first melt's composition. In the 50%wt Fa mixture shown on the olivine diagram, the first melt happens around 1450°C with an 80%wt Fa melt. The last solid would have a 20% Fa composition.
  • In ferromagnesian minerals the iron-rich endmember has a lower melting temperature than the magnesian endmember but higher density when solid. In Ca-Na minerals, the sodic endmember has a lower melting temperature.
melting partial melting eutectic
Melting > Partial melting > Eutectic
  • Eutectic The melting temperature of a mixture of minerals that don't form a solid-solution but are miscible as liquids is lower than either of their melting temperatures (in CHEM, this is called freezing-point depression, and is what road-salt and antifreeze are for)
  • If you make a mixture of a pair minerals that don’t form a solid solution, like albite+quartz or water_ice+salt, and start heating it, regardless of the composition of the mixture, it will start melting at the same temperature (lower than either separately) and same composition (for that combination), called the eutectic point (which will vary with pressure etc.). Continued heating of the mixture doesn't warm it, it melts more of the eutectic mixture leaving behind one component (unless your mixture happened to have the eutectic composition), until you have only A or B in the solid, at which point the leftovers start melting, at a range of temperatures and altering the composition of the melt, until it matches the starting mixture.
  • For water+NaCl at 1atm pressure, the eutectic point is 23.3%wt and -21.1°C. Below this temperature, water salt combinations are frozen. If you start at -30°C with 90% water and 10% salt and heat it, you won't see any melt until you get it to -21.1°C, at which point you get 23.3% salt+76.7% water mixture, which depletes the mixture in salt (enriches it in water), giving eutectic mixture+water_ice. For a while, continued addition of heat doesn't change the temperature until you run out of salt in the solid, at which point the water_ice starts melting and the temperature starts going up again, until your final solution/melt matches your initial mixture. If you started with 30% salt, you would still get melting starting at -21.1°C, but continued melting would deplete the solution in water, until you started raising temperatures and dissolving/melting the salt.
  • The eutectic melting temperature is often MUCH lower than the melting temperature of either component.
melting partial melting results
Melting > Partial Melting > Results
  • Partial melting is common. It results in a magma (the part that melts) and a restite (the part that rests, or stays behind). The magma includes the more easily melted components.
  • Partial melting is a refining process, in that elements end up getting separated.
  • Compared to the source, partial melting gives melts usually enriched in
    • Fe relative to Mg
    • Na relative to Ca
    • K relative to Na
    • Si relative to Al
  • Eutectic partial melting gives rise to melts of constant composition from a wide variety of sources, and at much lower temperature.
  • Usually, both continuous and eutectic partial melting occur simultaneously.
melting how to melt rock
Melting > How to Melt Rock
  • Heat It is possible to generate a magma by applying heat to a rock. This probably happened early in earth history (first billion or two years) but is rare now, except where material is pushed down into hotter regions in subduction zones.
  • Making it want to melt
    • How many psychologists does it take to change a light bulb?
    • One, but only if the light bulb really wants to change.
  • Similarly, you can melt a rock by making it want to melt by changing its melting temperature: by altering its pressure, or by introducing volatiles (This is mainly how it happens.)
melting melting temperature varies with pressure
Melting>Melting Temperature Varies with Pressure
  • Depending on the composition of the melt and its water content, its melting temperature can increase or decrease with pressure. See diagram. This is why granite (not rhyolite) and basalt (not gabbro) are common.
  • Granite, especially wet granite, has a melting temperature that increases as depth decreases, so granite freezes up as it ascends. Granite mainly comes from compressing wet sediments, andesite, etc.
  • Basalt has a melting temperature that decreases as pressure decreases, so as basalt rises, it gets further above its melting temperature. Basalt mainly comes from decreasing pressure on nearly-melting mantle peridotite
melt melting temperature with pressure
Melt > Melting Temperature with Pressure
  • Magma of basaltic composition (mafic magma) has a PT melting curve that causes more melting to occur as pressure decreases, like most materials. A mafic magma gets further above its melting range as it ascends, so mafic magmas usually erupt, at temperatures well above melting.
  • Granites have a PT melting curve that causes them to freeze as pressure decreases, like water. As a felsic melt ascends, it usually goes below its melting temperature and crystallizes at depth. If a felsic magma erupts, it's usually at or well-below its melting temperature.
  • Rhyolites are usually associated with large granitic intrusions; basaltic volcanoes often occur without plutons or with only small intrusives.
processes ex solution
Processes > Ex-solution

As minerals crystallize from a melt, the melt can become depleted in elements that are incorporated in those minerals and enriched in elements that are not incorporated into those minerals (incompatible elements). Magmas contain volatiles (gases and liquids) in solution at high temperature, but the combination might be unstable at low temperature. Amongst others, water concentrates into the melt, as do lots of rare elements like Ag and Pb.

As a melt cools and changes in composition, components that were once miscible can become immiscible, like grease separating from broth, separating into two fluids. This phenomenon is called ex-solution.

The combined volume of the two fluids is often greater than the volume of the single fluid. At the surface, this can be explosive. Below the surface, this can result in a fracture network and extensive metasomatic activity.

Intermediate and felsic magmas often ex-solve into a water-rich phase and a silica-rich phase. Ex-solution can generate porphyries by changing the melting temperature of the siliceous liquid which is suddenly NOT as water-rich, thus suddenly below its freezing temperature, and so freezes, becoming the groundmass.

The water-rich phase contains lots of elements like Au and Cu and Cl that didn't go into early-formed minerals like plagioclase and can lead to cool ore deposits and pegmatites.

processes movement
Processes > Movement

Magmas can move upward in two main ways.

  • Fluid flowing in cracks (basalts and metasomatic fluids).
    • This requires that the country rock be brittle to sustain cracks.
    • Where a fluid magma encounters plastic rocks, the magma can rise only if it is of lower density. If it is of higher density, it gets stopped below the plastic rocks (underplating). This heats and might melt the overlying rocks. Likely source of many granites is basalts underplating continental crust and partially melting crustal rocks.
  • Plastic oozing upward (granites).
    • Requires that the surrounding rocks be able to move out of the way, by flowing, by oozing, or by falling through the magma (stoping)
processes heating of country rock
Processes > Heating of Country Rock
  • Heating As hot magmas passes through colder country rock, the magma cools and the country rock heats. This can result in contact metamorphism and melting of the country rock or making it plastic. The magma might develop chilled margins from the rapid cooling at the edges.
  • Igneous Rocks (discussed elsewhere)
  • Extrusives
  • Intrusives
products extrusive features volcanoes
Products > Extrusive Features > Volcanoes

Extrusives are igneous rocks that erupt onto the earth’s surface (are extruded from the earth)

  • Volcanoes are mounds of extrusive igneous rock built up by successive eruptions. The style of eruption (runny or viscous) determines the type of volcano that forms.
    • Shield volcanoes (a.k.a. domes) are broad, gently sloped volcanoes produced by runny (non-viscous) lava. Side slopes are usually less than 10º. They are often very big, but don't look very conspicuous because of the gentle slopes. Shield volcanoes are usually not dangerous. Hawaii
    • Domes are usually rhyolitic (granitic, felsic) and more oozed than erupted.
    • Cinder consist of loose material, most of which has been airborne. They usually have steep slopes at the angles of repose for loose pyroclastic material, which has been ejected from the vent. Very difficult to climb. Cinder cones are usually small, but conspicuously volcanoey-looking. Mexico
    • Stratovolcanoes/Composite volcanoes Andesite can be fluid or plastic depending on the volatiles of a particular eruption. Stratovolcanoes consist of strata of both cinders and flows, and have a characteristic shape (very much like a bell-shaped curve). A stratovolcano can be thought of as a cinder cone superimposed on a shield volcano. Japan
products extrusive features others

Products > Extrusive Features > Others

Floods (book calls these plateau basalts)are extensive layers of extrusive igneous rock that moved liquidly and are almost always basaltic.

The lava usually comes out of fissures which are fed by dikes. Areas like the Columbia River Plateau are covered by hundreds of 3-100 meters thick basalt flows, each covering hundreds to tens of thousands of square kilometers. Basalt flows like this frequently show columnar jointing. (see text).

There are not any currently active regions of flood basalts.

Falls are extensive layers of ash and other debris, usually transported by air and are often very violent. Typical of granite/rhyolite. They can include nuée ardente (glowing mix of pyroclasts and hot gases).

Pillow basalts are extruded below water. In a pillow basalt, lava breaks through a hole in the already-frozen-part-of-the-flow and flows out there, resulting in a tube of hardened rock. In cross section, they look like a stack of pillows, with the outer edge showing evidence of quenching (Good video on the CD)

Plugs and Domes are the volcanic equivalent of toothpaste being squeezed out of a tube. Rhyolitic.

flood basalts
Flood Basalts
  • From http://www.geolsoc.org.uk/template.cfm?name=fbasalts
products extrusive materials
Products > Extrusive Materials

Extruded rocks cool quickly, and are fine-grained (aphanitic).

  • Basaltic
    • Pahoehoe is smooth and ropy.
    • Aa is jagged and sharp.
    • Pillows form under water.
  • Pyroclastic (granitic and andesitic) materials are hot airborne fragments, and include dust, ash , cinders, lapilli, and bombs/blocks
  • Bubbles can be frozen into a rock, resulting in vesicular (scattered bubbles) to scoriaceous to pumaceous (mainly bubbles) textures.
  • If a lava doesn't crystallize, but instead just gets cold and very viscous, you get volcanic glass obsidian
  • Porphyries are common, due to ex-solution at shallow depths or magma loitering in a magma chamber before eruption.
products intrusions some vocabulary
Products > Intrusions > Some Vocabulary
  • Intrusives are igneous rocks that were emplaced into solid rock (intruded into rock), also called plutonic rocks. The body of rock is called a pluton or an intrusion.
    • Large deep intrusions cool at depth, so cool more slowly and often have large crystals (phaneritic texture).
    • Thin intrusions. especially shallow ones, can cool quickly and often have small crystals (aphanitic).
  • Country rock is the rock that was there before the intrusion.
  • Xenoliths are fragments of some foreign rock (often country rock, sometimes from the source region) in an igneous body.
products intrusions tabular
Products > Intrusions > Tabular
  • Sheet intrusions are common at shallow depths and with fluid magmas, and tend to be basaltic. Sheet intrusions imply runny, flowing magma.
    • Sills are parallel to layering in country rock (concordant). They often occur below beds that flow plastically, like shale.
    • Dikes cut across country rock (discordant). If “cut across country rock” is undefined, it’s a dike.
products intrusions equant irregular
Products > Intrusions > Equant, Irregular
  • Rather than being sheet-like, plutons can be equant (similar sizes in all directions), amoebae-like in shape, and are often granitic, granodioritic or dioritic. They often occur in swarms.
    • Batholiths are over 100 km2 The word batholith can refer to an individual intrusion or to a set of merged plutons.
    • Stocks are under 100 km2
products intrusions features other
Products > Intrusions > Features > Other
  • Other shapes
    • Laccoliths (sometimes domes) are “hemi-spherical” with the convex side up. They are usually granitic.
    • Lopoliths are “hemi-spherical” with the convex side down. They are usually basaltic.
    • Pipes a.k.a. necks are “circular” and “vertical” and often feed volcanoes
    • Veins are irregular and are filled with material you might count as igneous or metamorphic.
plate tectonics and the origin of igneous rocks
Plate Tectonics and the Origin of Igneous Rocks
  • Plate tectonics explains current igneous activity fairly well. Most igneous intrusions are associated with plate boundaries. There is also igneous activity associated with hot-spots.
  • Older igneous activity, especially more than 2.5 Ga old (Archean), has a different style, suggesting that modern plate tectonics was not dominant.
plate tectonics divergent basalt
Plate Tectonics > Divergent Basalt
  • Oceanic basalts are derived by decompressive partial melting of mantle material, and occur extensively at mid-ocean ridges and some oceanic hotspots. Most volcanic activity occurs as pillow basalts at divergent plate boundaries. Early in the rifting apart of a continent, bi-modal volcanism (rhyolites and basalts) occurs.
plate tectonics subduction andesite
Plate Tectonics > Subduction Andesite
  • At ocean-subducting convergent boundaries, wet basalt is heated as it subducts, resulting in partial melting of basalt to produce andesite.
  • Subduction-zone andesites are derived by water- and pressure-induced partial melting of basalt and sediments that are being subducted. There may also be partial melting of mantle peridotite leading to basaltic magma. Sub-equal volcanic and intrusive activity occur in a continent-ocean collision.
  • The constancy of composition of andesite in collision zones suggests it’s a partial melt, rather than a mixing phenomenon.
plate tectonics collision granite
Plate Tectonics > Collision Granite
  • At continent-continent convergent boundaries (collision zones), wet andesitic, granodioritic, and sediments and metamorphic rocks are compressed and yield granitic magma which freezes as it ascends.
  • Granitic magmas appear to have several sources.
    • Most are due to compressive melting of water-rich sediments, as occurs in deep burial or continent-continent collisions.
    • Another is secondary melting, due to underplating by basaltic or andesitic magmas and heat transfer.
  • Andesitic magmas might incorporate sediments and move towards a granitic composition. Continent-continent collisions mainly result in intrusive granites.
plate tectonics hot spots
Plate Tectonics > Hot Spots
  • Hot spots are tracks of volcanism (age increases away from current activity) that are not associated with plate boundaries (and earthquakes and structural deformation). The Hawaiian islands are one in an ocean (an aseismic ridge), Yellowstone lies on another (look at the geologic map of North America in your text). A lot of Archean volcanism looks like hotspot tracks on continental crust.
  • We think hot spots are where a plume of hot material has welled up from the mantle (sort of like a thunderhead). Under oceans, they almost always erupt as flood basalts or shield volcanoes. Under continents, they might erupt as flood basalts or shield volcanoes, or they might heat the continental material enough to make it plastic and block further mafic eruptions, but trigger secondary underplating granites and rhyolites.
plate tectonics transform none
Plate Tectonics > Transform NONE
  • Transform boundaries really don't seem to have igneous activity at all.
igneous geology2
Igneous Geology
  • Partial Melting is a refining process. Certain elements go into the melt, others stay in the restite. Crystallization can be a refining process
  • Origin of Magmas
    • As mafic magma ascends, its melting temperature drops, so basalt usually erupts. Basaltic magmas form by decompressive partial melting of mantle peridotite.
    • As wet felsic magma ascends, its melting temperature increases, so it usually freezes out at depth as a granitic pluton. Felsic magmas mostly arise from compressive melting of andesites, granites, and sediments, and underplating (=>heating) by basalts.
    • Intermediate melts form in subduction zones, where wet basalt is heated as it is carried into the mantle, resulting in andesitic volcanoes and dioritic plutons.
  • Igneous style depends on silica and water content.
    • Silica makes magma more viscous, so felsic and intermediate magmas can explode violently, while mafic magmas erupt gently.
    • Concentration of incompatible elements into the melt results in a melt that differs markedly from the original melt. Felsic and intermediate magmas often ex-solve into a water-rich phase and a silica-rich phase, forming porphyries, pegmatites, and hydrothermal ores.
  • Plate tectonics provides a good framework for understanding modern igneous activity, but there are some problems, especially with Archean rocks and anorthosites.
melt melting temperature with pressure1
Melt > Melting Temperature with Pressure
  • Magma of basaltic composition has a PT melting curve that causes more melting to occur as pressure decreases. Granites have a PT melting curve that causes them to freeze as pressure decreases.
  • Basaltic magmas are well above their melting point if they erupt. Granites/rhyolites are often at or well-below their melting point when they erupt. Rhyolites are usually associated with large granitic intrusions; basaltic volcanoes often occur without plutons or with only small intrusives.

Figures are from Petrology by Ehlers & Blatt p97. Y-axis is pressure in kilobars. Multiply by 3 to get depth in kilometers of rock.