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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

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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 0037 of 'Igneous Geology' by Greg Pouch at 2011-01-19 17:47:30 LastSavedBeforeThis 2011-01-19 17:38:20

C:\Users\GregAdmin\Documents\Geo101\15IgneousGeology.ppt on 'GWPOUCHDELL1720'


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Composition

  • 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?)


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Processes

  • Melting

  • Ex-solution

  • Movement and emplacement

  • Heating of surrounding rocks


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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)

      The mantle is mostly peridotite. So are most meteorites.

  • 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.


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Melting > Partial melting

  • Continuous (Fe-Mg olivine, Ca-Na plagioclase). In continuous partial melting of a mix, the melt has an intermediate composition between the components, but the melt and restite differ from the original. Melting starts/freezing ends at a point (temperature and composition) that depends on initial composition.

  • Eutectic (antifreeze-water, quartz-albite). In eutectic partial melting, the first melt (or last freeze) always happens at the eutectic point. The eutectic melting temperature is often MUCH lower than the melting temperature of either component.

    For iron-carbon (steel): iron melts at 1535C, carbon melts at 4200C, and a eutectic iron-carbon mixture melts at about 1154C and 4.3%wt Carbon.


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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. Melts are usually richer in Si Al Na K and poorer in Fe Mg and Ca than the source rocks. Partial melting results in magma with more Fe than Mg than was in the source.


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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.)


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Melt> Variation of melting temperature 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


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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.


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Processes >Ex-solution

As crystals form in 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. It can also generate porphyries by changing the melting temperature of the remaining material, that becomes the groundmass.

Magmas contain volatiles (gases and liquids) in solution at high temperature. As temperature decreases, the volatiles can ex-solve producing a separate gas or liquid phase. At the surface, this can be explosive. Below the surface, this can result in a fracture network and extensive metasomatic activity.


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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)


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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.


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Products

  • Igneous Rocks (discussed elsewhere)

  • Extrusives

  • Intrusives


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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

    • Cinder cones are usually rhyolitic (granitic), and 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


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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).

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

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)


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Flood Basalts

  • From http://www.geolsoc.org.uk/template.cfm?name=fbasalts


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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.


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Products>Intrusions

  • Intrusives are igneous rocks that were emplaced into solid rock (intruded into rock)

    • Because they cool at depth, intrusions cool more slowly and often have large crystals (phaneritic texture). This mainly applies to large intrusions.

    • Thin intrusions. especially shallow ones, can cool quickly and often have finer texture.

  • 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.


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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.


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Products>Intrusions>Plutons

  • Plutons are massive intrusions, amoebae-like in shape, and are often granitic. They often occur in swarms.

    • Batholiths are over 100 km2 The word batholith can refer to an individual intrusive or to a set of merged plutons.

    • Stocks are under 100 km2


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Products>Intrusions>Features>Other

  • Other shapes

    • Laccoliths 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.


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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.


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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.


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Plate Tectonics>Convergent (oceanic) (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.


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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.

    • One is 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.


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Igneous Geology

  • Partial Melting is a refining process. Certain elements go into the melt, others stay.

  • Origin of Magma

    • As basaltic magma ascends, its melting temperature drops, so basalt usually erupts. Basaltic magmas form by decompressive partial melting of mantle peridotite.

    • As wet granitic magma ascends, its melting temperature increases, so it usually freezes out at depth as a pluton. Granites mostly arise from compressive melting of andesites, granites, and sediments, and underplating (->heating) by basalts.

    • Andesites form in subduction zones, where wet basalt is heated as it is carried into the mantle.

  • Igneous style depends on silica and water content.

    • Silica makes magma more viscous, so felsic and intermediate magmas can explode violently.

    • 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.


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This line is what you would get if you have as much water as needed.


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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.


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