#13 Igneous Rocks

Latin "ignis" = fire - Igneous rocks form by cooling and solidification of hot molten rock either at surface or below ground. Link to another class Web site involving igneous rocks (James Madison Univ.) and to USGS Web site, index of volcanic terms.

I) Importance of understanding igneous rocks (link #2)

Students provide examples






Link for dramatic video introducing rocks; created by Frank Gregorio

Link for dramatic video introducing another relevant topic, created by Frank Gregorio; link for dramatic and beautiful time-lapse video of April 22, 2015 eruption of Calbuco Volcano in Chile, created by Martin Heck.

II) Basic Concepts - Igneous Rocks

A) Magma (link #2) = molten rock within Earth; may be completely molten or (more commonly) fluid mixture of liquid, solid crystals, and dissolved gases. When magma reaches surface = lava (link #2). Magma and lava are very hot (600 - 1,200C).

B) Study of Igneous Rocks - Igneous rocks and igneous processes at surface are studied directly at active or ancient volcanoes. Underground igneous rocks and processes are studied where surface rocks have eroded (roots and feeder systems to ancient volcanoes). Also use geophysical methods (seismic waves, density, magnetism), lab studies (simulate T and P of igneous environments), and computer models (simulate igneous environments).

C) What happens during melting and crystallization at atomic level?

Begin with crystalline solid, > heating causes > atomic vibrations and eventually causes breaking atomic bonds in crystalline solid so atoms vibrate more freely; form liquid magma with no crystal structure. Cool hot, molten magma get < atomic vibrations, eventually allows atomic bonds to form and minerals to grow.

D) Texture and Composition - Igneous Rocks

Texture - What is most important factor controlling texture (grain size) of igneous rock?


Discuss formation of following textures:

1) Coarse-grained (phaneritic, Greek "phaneros" = visible) = large, visible mineral grains

2) Fine-grained (aphanitic, Greek "a phaneros" = not visible) = small mineral grains, barely seen or microscopic

3) Porphyritic = large crystals surrounded by much smaller grains

4) Volcanic Glass = amorphous (random arrangements), e.g., pumice (photo) = bubbling, gas-rich, silicic lava --> highly porous, light-weight glass, obsidian (photo) = silicic lava with little gas --> massive glass with conchoidal fracture (used in arrowheads and cutting tools), and volcanic ash (link #2) = medium-grained glass from explosive volcanic eruptions (photo).

Cooling Rate Grain Size Texture Igneous Rock Type Environment of Formation

Composition - Major elements in igneous rocks = O, Si, Al, Fe, Ca, Mg, Na, K, and S. Composition plus temperature, pressure, and H2O content characterize magma and determine what minerals form. Many classification schemes, all are based on texture (grain size) and composition (mineral, chemical or both). Compositions range from silicic (or felsic) to intermediate, mafic and ultramafic. Major differences in composition for common igneous rocks = silica (SiO2) content (felsic = highest, ultramafic = lowest), proportions of Al, K, and Na (felsic = highest, ultramafic = lowest), and proportions of Ca, Fe, and Mg (felsic = lowest, ultramafic = highest). Alkalic rocks = rich in alkali elements (Na and K). Photo of igneous rocks illustrating common compositional series.



Other important differences = temperature of crystallization (felsic forms at lowest temperatures, ~600 - 800C; ultramafic forms at highest temperatures, >1,200C) and viscosity or resistance to flow (felsic is most viscous, i.e., resists flowing, and ultramafic is least viscous, i.e., flows easily). What are 2 reasons for difference in viscosity?


Ultramafic igneous rocks dominated by mafic silicates (olivine and pyroxene), no quartz, little to no feldspar, and <40% silica. Peridotite = most common ultramafic rock is dark, ~dense, 40 - 100% olivine, and coarse-grained (cooled deep within Earth). Peridotites occur at Earth's surface only when erosion has removed overlying rock, e.g., continental collision zones. Komatiites = rare extrusive ultramafic igneous rocks from early Precambrian.

Mafic igneous rocks = 45 - 55% silica; Ca-rich plagioclase, pyroxene and minor olivine; dark and ~dense; most common rocks of oceanic crust. Basalt (fine-grained) forms upper part of oceanic crust (divergent plate boundaries) and many islands or continental areas associated with hot spots. Gabbro (coarse-grained) underlies basalt in oceanic crust.

Intermediate igneous rocks = 55 - 65% silica; more Na-rich plagioclase, some mafic minerals (amphibole, biotite, and pyroxene) and minor quartz; lighter color (~gray) than mafic. Fine-grained = andesite (Andes mountains in S. America) and coarse-grained = diorite; convergent plate boundaries and explosive volcanoes.

Felsic/silicic igneous rocks = >65% silica; K-feldspar, quartz, Na-rich plagioclase, muscovite, and minor mafic silicates (biotite and amphibole); Coarse-grained = granite, (pegmatite = very coarse-grained), fine-grained = rhyolite. Another category between silicic and intermediate = granodiorite (plag > K-spar) is coarse-grained; dacite is fine-grained. All associated with convergent plate boundaries (ocean-continent) and hot spots on continents.

III) Creation of Magma (Melting)

How do rocks melt? Most rocks consist of 1 or more minerals, which have different melting temperatures depending on strength of bonds. Also, single minerals (e.g., plagioclase and olivine) melt over range of temperatures.

Principal controls on magma formation, i.e., Why do rocks melt?





Geothermal gradients in upper part of crust range from ~25 - 100C/km. Different tectonic settings have different geothermal gradients (higher at volcanic areas of divergent and convergent plate boundaries and hot spots where partial melting occurs, lower in cratonic interior and very low in accretionary prism). Eventually depth is reached where temperatures are high enough for rocks to melt, but most of upper mantle and lower crust is solid.

What prevents rock from melting in lower crust and upper mantle?





For solid rock near its melting temperature, sudden < in pressure causes melting. Get < pressure when tectonic plates begin to rift (diverge) due to thinning of lithosphere. Effect of pressure drives partial melting at divergent plate boundaries, i.e., some of rock (with lower melting point) melts, some (with higher melting temperature) remains solid. Sometimes melted portion rises away from solid portion due to its buoyancy.



Water (vapor) - Small amounts of water << melting temperature of rock; water disrupts atomic bonding, allowing lower temperatures for bond breaking to form liquid; especially large effect at high pressures - "wet" rocks are much easier to melt than "dry" rocks. Effect of water drives partial melting at convergent plate boundaries. Sediments associated with the subducting oceanic lithosphere bring water into mantle, which melts partially.

Once magma is generated it rises because magma is less dense than solid rock. Why is basalt a much more common extrusive igneous rock than rhyolite?


Similarly why does felsic magma tend to crystallize underground as granite or granodiorite?

Chemical Composition Different mixtures of minerals will greatly affect melting temperature.

IV) Nature of Solidification (Crystallization)

Sequence of Crystallization (Bowen's Reaction Series) - predictable sequence of minerals that form as magma cools = Bowen's Reaction Series (BRS). 2 pathways along BRS - discontinuous series (olivine forms at highest temperatures, reacts with melt to produce pyroxene, which can react with melt to produce amphibole and then biotite) and continuous series (Ca-feldspar forms at highest temperature and as cooling proceeds, more Na-rich feldspar produced). At lowest temperatures, muscovite, alkali feldspar, and quartz form. Equilibrium crystallization = early formed crystals remain in contact with parent magma and react with it to form new minerals. If magma is mafic, then mafic rock should be produced (olivine, pyroxene, and Ca-feldspar). If magma is felsic, then go through whole sequence (mafic minerals produced early, but they react to produce felsic assemblage of quartz, K-feldspar, and Na-feldspar).

Modifying Magma Composition - Equilibrium crystallization rarely applies in nature. Various processes can affect magma composition such as crystal settling (dense crystals sink to bottom of magma chamber), crystal flotation (light weight crystals float to top of magma chamber), or filter pressing (flow through constriction).



For mafic magma affected by crystal settling, mafic minerals form early and remove those elements (Mg and Fe) from residual magma, which becomes more silicic (i.e., can move farther down BRS) = fractional crystallization (start with single magma and produce range of rock compositions from mafic to felsic by crystal settling). Example = Palisades sill, 300 m thick intrusion which contains layers of different minerals; densest olivines at bottom, least dense plagioclase at top, and mixed plagioclase and pyroxene are in middle.

Bowen thought fractional crystallization of mafic magma could account for all igneous rocks (mafic, intermediate, and felsic), but he was wrong. Beginning with mafic magma, can produce only ~10% felsic rock (and 90% intermediate and mafic rock). Vast areas of felsic igneous rock on continents and no corresponding bodies of intermediate and mafic rock associated with them.

Another process that affects magma composition = partial melting. For solid mafic rock that is heated, first melt produced = felsic. If felsic magma moves away, residual rock becomes more mafic. Another process is assimilation, where magma melts some of surrounding (host) rock and changes magma composition. Works best if mafic magma intrudes into felsic host rock. Why?

Sometimes host rock gets broken off by igneous intrusion, but it does not melt. This piece of foreign rock in intrusion = xenolith (photo #1, #2, Greek for "foreign stone"). Mixing of magmas of different composition can also alter their composition.

V) Igneous Intrusions (plutons) - from magma that has cooled underground. Intrusions classified by their shape (sketch). Tabular (slab-like) bodies = dikes (vertical orientation, photo #1, #2) and sills (horizontal orientation, photo #1, #2, #3). Very large (>100 km2 area), irregularly shaped bodies = batholiths, which usually consist of intermediate to felsic rocks. Why intermediate to felsic batholiths (and not mafic)?


Batholiths commonly consist of multiple large intrusions. Sierra Nevada mountains in east-central California (photo, map) contain many plutons (irregular, smaller than batholith) that appear as continuous large batholith, formed over ~130 m.y. span.

VI) Occurrence and Origin of Igneous Rocks

A) Basalt and gabbro - major occurrence = divergent plate boundaries and hotspots. Mafic igneous rocks are ~only igneous rocks in oceanic crust. Source of magma = partial (~30%) melting of ultramafic mantle. Mantle is ~heterogeneous in composition, which causes differences in mafic rock composition. Early in Earth history, upper mantle melted partially and lightest elements rose to form crust. Process depleted upper mantle in light elements, Na, K, and Al. Deeper in mantle, those elements are not depleted. Several types of basalt depending on tectonic setting and source of magma in mantle (shallow vs. deep).

Oceanic basalts include mid-ocean ridge basalt = most abundant volcanic rock. Produced at divergent boundaries (e.g., Iceland and underwater oceanic ridge) from relatively shallow mantle source, therefore low concentrations of Na, K, and Al. Oceanic crust (link #2) consists of ~200 m of mud (clay, carbonate or silica; thinner at ridge and thicker with > distance from ridge), underlain by 2 km of basalt (pillow basalt, link #2 with video, photo #1, #2, due to underwater eruption and rapid cooling; pillows crack open from erupting basalt, ooze out and form another pillow; sheeted dikes (scroll down to Fig. 1) feed erupted pillow basalt), and underlain by 5 - 6 km of gabbro (slow cooling of mafic magma). Below crust is mantle = ultramafic peridotite. Often, these rocks are hydrothermally altered to green minerals, chlorite and serpentine. Evidence for hydrothermal alteration is black smokers (hot underwater geysers, photo) and chimneys (precipitates of metallic sulfide minerals) at oceanic ridges.



Group of rocks comprising oceanic lithosphere = ophiolite suite (link #2), which can get thrust onto land during tectonic plate collisions.

Ocean island basalts form over hot spots (zone of localized upwelling mantle within tectonic plate) over oceanic crust (e.g., Hawaii). Ocean island basalts contain higher amounts of Na, K, and Al, reflecting deeper mantle source.

Continental basalts form at new rift zones (e.g., East African Rift, and Rio Grande Rift) from deep mantle sources (and hot spots on continents, e.g., Columbia River basalts, from deep mantle) and at subduction zones from shallow mantle sources. In both cases, mafic magma rises through 10's of km of continental crust, resulting in incorporation of crustal rocks and more variable composition for continental basalts compared to oceanic basalts.

B) Andesite and Diorite - major occurrence = convergent plate boundaries (subduction zones, e.g., most of margin of Pacific ocean - Mt. Saint Helens, Fuji, Pinatubo, and others). Origin of intermediate igneous rocks = controversial and less understood than mafic rocks. Intermediate igneous rocks form due to partial melting of subducting oceanic plate (wet sediments and altered basalt) and overlying mantle. Dehydration of subducting slab promotes melting of overlying mantle, which sends mafic magma rising. As mafic magma moves through crust, it incorporates some felsic crust, producing intermediate composition rocks.

C) Granite and rhyolite - occur ~exclusively on continents (controversial and poorly understood origin). Vast areas of granite usually associated with subduction zones (e.g., Sierra Nevada Mountains in east-central CA) and smaller amounts at hot spots on continents (e.g., Yellowstone). Similar to intermediate composition rocks, felsic igneous rocks may form by partial melting of rock in lower crust. Heat source is rising mafic plumes that accumulate below lower crust (of intermediate mafic composition), which melts partially and produces viscous felsic magma. Process of accumulating mafic magma at base of crust = underplating. Felsic magma rises slowly and usually cools at depth, producing granite. When it reaches surface (due to high water content), rhyolite eruptions occur.

VII) Extrusive Igneous Rocks - crystallize at Earth's surface (usually under air or water, but rarely ice). Material usually erupted through volcano (mountain produced from eruption of extrusive igneous rock) either non-explosively as lava flow or explosively as pyroclastic debris.

A) Lava flows - non-explosive eruptions of moving stream of red-hot lava, most commonly basalt, which has low viscosity due to its high temperature and low silica content. Mafic lava flows can move ~fast (up to 15 km/hr) or very slow (cm/day).

Example of mafic lava flows = Hawaii volcanoes, erupt two kinds of basalt textures: pahoehoe (smooth, ropy surface from fast moving flows) and aa (rough, jagged surface from slow moving flows). Typically, get both types from single lava flow; one occurs near crater (opening of volcano) and other forms far from crater. Which occurs where and Why?


Other features of mafic lava flows = vesicular basalt (rich in pea-size holes due to escaping gas), known as scoria. Columnar jointing = hexagonal-shaped columns produced by contraction as lava cools (photo). Examples = Devil's Postpile in eastern CA (photo) and Devil's Tower in NE WY (photo). Subaqueous eruptions of mafic lava produce pillow basalt.

Intermediate composition lavas do not flow as quickly or as far as mafic lava flows (due to higher viscosity). Usually don't get ropy (pahoehoe) surface because intermediate lava is too viscous to stretch into ropy structure; get vesicles, rough (aa) surface, columnar jointing, and pillow structure. They commonly stop around crater and cause pressure to build up, > chance of explosive eruption.

Silicic lava flows are relatively rare. Why?


When silicic lava does reach surface, it usually erupts explosively as solid volcanic glass instead of lava flow. Silicic lava flows are usually short and thick (due to high viscosity). Sometimes gas-rich silicic magma bubbles out of vent cooling quickly and producing pumice (extremely porous silicic volcanic glass), which can be so lightweight that it can float.

B) Pyroclastic materials - explosive eruptions send lava (which cools very quickly to solid pieces of volcanic glass) and rock fragments (pieces of volcano ripped away during eruption) forcefully into atmosphere. This material = pyroclastic materials (Greek, pyro = fire, klastos = fragments) can range in size from very small pieces to huge blocks. Erupted materials are sent upward as plume of dispersed particles (ashfall eruptions (link #2), e.g., Mt. Saint Helens in 1980) or along ground as dense flow (ashflow or pyroclastic flow eruptions).

(1) Tephra (link #2, #3) = quickly cooled lava from pyroclastic eruption. Tephra particles are classified based on size including volcanic dust (~1m) and volcanic ash (fine - coarse sand, <2 mm). Volcanic dust can travel large distances downwind of erupting volcano and remain in upper atmosphere for up to 1 - 2 years. Volcanic ash usually stays in air for few hours to days. Both particles can cause major problems with machinery located downwind (e.g., auto carburetors and jet engines). Near-disasters of aircraft engine stalling (and then restarting) after entering cloud of gritty, volcanic ash, e.g., British Airways Flight 9. Coarse grains of tephra that fall relatively close to erupting volcano = cinders or lapilli (2 - 64 mm) and volcanic bombs (>64 mm).

Tephra typically accumulates in layers (airfall tuff) that < in thickness and grain size with > distance from volcano source. Commonly there are crystals (e.g., K-feldspar and biotite) that can be dated to provide absolute age in sequence of sedimentary rocks.

(2) Volcanic mudflow - forms when hot pyroclastic debris falls on ice-covered slopes of volcano (photo-Fuji) producing hazardous and destructive mudflows called lahars (link #2, photo). Alternatively, erupted fine dust can produce rain clouds and heavy rains that generate lahars. They can move very quickly (up to 10's of km/hr) if water content is high. Grain size can range from finest ash to 100-ton blocks. Examples of Mt. Saint Helens in 1980, Mt. Pinatubo in Philippines in 1991, and Nevado del Ruiz volcano in Columbia which produced lahar that killed 23,000 in town of Armero.



The Eruption of Mt. St. Helens video (lateral blast, ashfall, lahars, Otto Sieber, 13:11 - 17:20)

Lahars in Japan video (YouTube, 1:24)

(3) Pyroclastic Flow (ashflow) (link #2) - forms when amount of explosively erupted material is very large and particles are too large to be carried up in atmosphere. Gravity pulls erupted material back down volcano and sends it rushing downslope as pyroclastic flow (photo) or nuee ardente ("glowing cloud"). Trapped air and magmatic gases keep ashflow buoyant and < friction with ground allowing speeds of up to 150 km/hr. Particles can be red-hot when flow slows down and grains settle. Red-hot particles can fuse together producing welded tuff (ignimbrite, photomicrograph of welded texture). Example = Mt. Pelee (link #2) eruption in 1902; pyroclastic flow killed 28,000 in ~30 seconds in city of St. Pierre in Caribbean island of Martinique.

Dramatic video of pyroclastic flow from Mt. Unzen, Japan, 1991 (Volcano by National Geographic, 1:12)

Personal video of pyroclastic flow from Montserrat (Montserrat, 12 - 15)

VIII) Causes of Explosive (photo) vs. Non-explosive (photo) Volcanic Eruptions

To get explosive eruption, you need highly viscous (thick) magma with lots of gas (mainly water vapor). Gas expands (boils away) as magma reaches surface, causing explosion. Explosive eruptions also require viscous magma, making it difficult for gas to escape from lava and > build up of pressure.

Non-explosive basalt lava flows are most common at divergent plate boundaries and hot spots over ocean crust, both involve partial melting of mantle (low in silica and water). Explosive, andesite - rhyolite pyroclastic eruptions are most common at convergent plate boundaries (subduction zones), where crust (continental and oceanic, richer in silica and water) and mantle undergo partial melting.

IX) Types of Volcanoes

A) Non-explosive (effusive) volcanoes usually involve mafic lava flows (and lava fountains, if gas pressure builds up) at shield volcanoes (link #2) - very large, gently sloping, dome-like volcanoes composed of multiple lava flows (e.g., Hawaii volcanoes, Mauna Loa, Kilauea).




Occasionally, magma chamber below completely or partially empties, causing overlying rock to collapse and form large, ~circular depression (caldera). Craters = ~several 100 meters wide and deep; calderas = up to 15 km wide and 1 km deep. Weight of collapsed summit can close off central crater, causing flank eruption along side. Kilauea (one of world's most active volcanoes) on Hawaii has built up from flank eruptions from Mauna Loa (link #2). Both consist of 1000's of lava flows. Amazing photos of the world's largest lava lake in the crater of Mount Nyiragongo, an active volcano in the Great Rift Valley of Congo, Africa; video of scientist who probably gets too close to that lava lake.

Video of crazy volcanologists descending into a crater with lava frothing about (Ambrym Island, Vanuatu in southwest Pacific).

Fissure eruptions occur along series of linear cracks produced by diverging plates. These eruptions can involve huge quantities of fast-moving basalt = flood basalt (link #2) that can spread over enormous areas. Successive flows build up very thick lava plateaus (link #2). Examples = Iceland (divergent boundary), Columbia River Plateau Basalt in eastern WA and northern OR (area of 50,000 km2) (hot spot) and India (75,000 km2).

B) Explosive (pyroclastic) volcanoes involve andesite (and rhyolite) ash fall - large vertical plume of fine-grained volcanic glass, which settles from air. Produces composite (strato) volcano (link #2) - steep-sided, cone shape - Orsono volcano, Chile containing ash-fall and lava flows (e.g., Vesuvius, Saint Helens, Fuji - dramatic photo, and many others around Pacific rim).




Other explosively erupting volcanoes

Cinder cone (link #2) - consists entirely of pyroclastic debris (no lava flows); symmetrical, small cones, photo of Sunset Crater, AZ; usually have ~ short lifespans, e.g., Paricutin, Mexico; can be any composition (gas-rich).

Lava dome (link #2) - bulbous mass of extremely viscous lava (silicic) that piles up around vent, photo of lava dome at Chaiten, Chile; which can get plugged; potential for extremely explosive eruption.

Caldera - rare, but extremely violent eruptions that produce huge crater (10's of km across, photo of Mt. Pinatubo caldera, Philippines); eruptions commonly produce ash-flows (ignimbrites). Calderas begin as composite cone, then enormous eruption blows away top part of volcano; magma chamber empties, leaving large underground cavity and ground over it collapses, e.g., Crater Lake (link #2) in Oregon (formed ~6,800 years ago by eruption of Mount Mazama).







Other caldera-forming eruptions include:

Krakatau (link #2) in SW Pacific, in 1883 it created crater that extended 300 m below sea level and produced giant tsunami that killed >30,000 people.

Santorini (link #2) in Greece (1600 BC) destroyed Late Minoan civilization (lost city of Atlantis?).

Yellowstone (link #2, loads of photos) (3 large caldera-forming eruptions over past 2 m.y. (1.9, 1.3, and 0.6 m.y.a.), eruptions were ~1000 times larger than Mount St. Helens (excellent summary with maps of destruction area), continues to show signs of activity with hot springs and geysers).

Photos of non-explosively erupting volcanoes

Photos of explosively erupting volcanoes (except for bentonite, nuee ardente, and Crater Lake)