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Igneous Rocks The Rock Cycle Characteristics of magma Igneous rocks form from molten rock (magma) Characteristics of magma Protolith of igneous rocks Forms from partial melting of rocks Magma at the surface is called lava Characteristics Rocks of magma formed from lava are classified as extrusive, or volcanic rocks Rocks formed from magma are termed intrusive, or plutonic rocks The nature of magma Consists of three components: A liquid portion, called melt, made of mobile ions Solids, if any, are crystallized silicate minerals Volatiles, (dissolved gases), mostly water (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) Crystallization of magma Texture = the size and arrangement of mineral grains Igneous rocks are classified by Texture Mineral composition Igneous textures Texture describes the size, shape, and arrangement of interlocking minerals Factors affecting crystal size Rate of cooling Slow rate promotes fewer and larger crystals Fast rate forms many small crystals Very fast rate forms glass Types of igneous textures Aphanitic Rapid (fine-grained) texture rate of cooling Microscopic crystals May contain vesicles Phaneritic Slow (coarse-grained) texture cooling can be seen Crystals Aphanitic texture Phaneritic texture Types of igneous textures Porphyritic texture Minerals form at different temperatures as well as differing rates Large crystals (phenocrysts) embedded in a matrix (groundmass) Glassy texture Very rapid cooling Results in obsidian Porphyritic texture Andesite porphyry Porphyritic Texture Glassy (vitreous) texture An obsidian flow in Oregon Types of igneous textures Pyroclastic texture Fragments ejected from violent volcanic eruption Textures similar to sedimentary rocks Pegmatitic texture Exceptionally coarse grained Late crystallization stages of granitic magmas Igneous Compositions Igneous rocks are primarily silicate minerals Dark (ferromagnesian or mafic) silicates Olivine Pyroxene Amphibole Biotite Igneous mica rocks are primarily silicate minerals Light (felsic) silicates Feldspars Muscovite mica Quartz Granitic versus basaltic compositions Granitic composition Composed of light-colored silicates Felsic composition High silica (SiO2) Major constituents of continental crust Granitic versus basaltic compositions Basaltic composition Composed of dark silicates and calcium-rich feldspar composition More dense than granitic rocks Comprise the ocean floor as well as many volcanic islands – the most common volcanic rock Mafic Gabbro Basalt Other compositional groups Intermediate At (or andesitic) composition least 25% dark silicate minerals with explosive volcanic activity Associated Ultramafic Rare composition composition, very high in magnesium and iron entirely of ferromagnesian silicates Composed Mineralogy of common igneous rocks Silica content influences a magma’s behavior Granitic magma (high silica) Extremely Higher Basaltic viscous temperatures Molten as low as 700oC magma (low silica) Fluid-like behavior rocks Eruptive fragments Pyroclastic Varieties – ash-sized fragments Welded tuff – ejected hot, “welds” on landing Volcanic breccia – particles larger than ash (similar to sedimentary breccia) Tuff Ash and pumice layers Classification of igneous rocks Origin of Magma Several Factors Involved Generating magma from solid rock Partial melting crust and upper mantle Role of heat Temperature increases with depth in the upper crust (called the geothermal gradient, ~ 20oC to 30oC per km) Estimated temperatures in the crust and mantle Role of heat Rocks in lower crust and upper mantle near melting points Additional heat (from descending rocks or rising heat from the mantle) may induce melting Role of pressure Increase in confining pressure increases rock melting temperature and reducing the pressure lowers the melting temperature When confining pressures drop, de-compression melting occurs Heat and Pressure Affect Melting Decompression melting Role of volatiles Volatiles (primarily water) lower rock melting temperatures important with descending oceanic lithosphere Particularly Evolution of magmas A single volcano may extrude lavas exhibiting very different compositions Bowen’s reaction series and the composition of igneous rocks N.L. Bowen demonstrated that as a magma cools, minerals crystallize in order based on their melting points Bowen’s reaction series During crystallization, the composition of the liquid portion of the magma continually changes Composition changes due to removal of elements by earlier-forming minerals The silica component of the melt becomes enriched as crystallization proceeds Minerals in the melt can chemically react and change Processes responsible for changing a magma’s composition Magmatic differentiation Separation of crystals from melt forms a different magma composition Assimilation Incorporation Magma of foreign matter (surrounding rock bodies) mixing Involves two magmas intruding one another distinct magmas may produce a composition quite different the originals Two Assimilation and magmatic differentiation Partial melting and magma formation Incomplete melting of rocks is known as partial melting (silica rich minerals melt first) Formation of basaltic magmas Most originate from partial melting of ultramafic rock in the mantle Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones Decompression melting Formation of basaltic magmas As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature Large outpourings of basaltic magma are common at Earth’s surface Formation of andesitic magmas Interactions between mantle-derived basaltic magmas and more silicarich rocks in the crust generate magma of andesitic composition (assimilation) Andesitic magma may also evolve by magmatic differentiation (loss of early olivine and pyroxene) Partial melting and magma formation Formation of granitic magmas Most likely the end product of an andesitic magma magmas are higher in silica and therefore more viscous Because of viscosity, mobility lost before reaching surface (rhyolite rarer than basalt) Tend to produce large plutonic structures Granitic Magmatic Differentiation Mineral resources and igneous processes Many important sources of metals are produced by igneous processes Igneous mineral resources can form from Magmatic chamber segregation – separation of heavy minerals in a magma Hydrothermal solutions - Originate from hot, metal-rich fluids that are remnants of the late-stage magmatic process Origin of hydrothermal deposits The Nature of Volcanic Eruptions Factors determining the “violence” or explosiveness of a volcanic eruption Composition of the magma Temperature of the magma Dissolved gases in the magma The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow with great difficulty) Factors affecting viscosity Temperature - Hotter magmas are less viscous Composition - Silica (SiO2) content Higher silica content = higher viscosity (e.g., felsic lava such as rhyolite) Factors affecting viscosity continued Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt) Dissolved Gases Gas content affects magma mobility Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure The violence of an eruption is related to how easily gases escape from magma Factors affecting viscosity continued In Summary Fluid basaltic lavas generally produce quiet eruptions Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions Materials extruded from a volcano Lava Flows Basaltic lavas are much more fluid Types of basaltic flows Pahoehoe lava (resembles a twisted or ropey texture) Aa lava (rough, jagged blocky texture) Dissolved Gases One to six percent of a magma by weight Mainly water vapor and carbon dioxide A Pahoehoe lava flow A typical aa flow Pyroclastic materials – “Fire fragments” Types of pyroclastic debris Ash and dust - fine, glassy fragments (<2 mm) Pumice - porous rock from “frothy” lava Lapilli - walnut-sized material (2-64 mm) Cinders - pea-sized material Particles larger than lapilli (>64 mm) Blocks Bombs - hardened or cooled lava - ejected as hot lava A volcanic bomb Global Volcanic Effects Volcanoes General Features Opening at the summit of a volcano Crater - steep-walled depression at the summit, generally less than 1 km in diameter Caldera - a summit depression typically greater than 1 km in diameter, produced by collapse following a massive eruption Vent Types – opening connected to the magma chamber via a pipe of Volcanoes Shield volcano Broad, slightly domed-shaped Composed primarily of basaltic lava Generally cover large areas Produced by mild eruptions of large volumes of lava Mauna Loa on Hawaii is a good example Shield Volcano Types of Volcanoes continued Cinder cone Built from ejected lava (mainly cinder-sized) fragments Steep slope angle Rather small size Frequently occur in groups Sunset Crater – a cinder cone near Flagstaff, Arizona Paricutin Types of volcanoes continued Composite cone (Stratovolcano) Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens) Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base) Composed of interbedded lava flows and layers of pyroclastic debris A composite volcano Mt. St. Helens – a typical composite volcano Mt. St. Helens following the 1980 eruption Composite cones continued Most violent type of activity (e.g., Mt. Vesuvius) produce a nueé ardente Fiery pyroclastic flow made of hot gases infused with ash and other debris Move down the slopes of a volcano at speeds up to 200 km per hour May produce a lahar, which is a volcanic mudflow Often A size comparison of the three types of volcanoes A nueé ardente on Mt. St. Helens St. Pierre after Pelee’s Eruption Other volcanic landforms Pyroclastic flows Associated with felsic & intermediate magma Consists of ash, pumice, and other fragmental debris Material is ejected a high velocity e.g., Yellowstone Plateau Calderas Steep walled, roughly circular, depressions at the summit of a volcano Size generally exceeds 1 km in diameter Formed by the collapse of the structure Caldera Formation Type 1 Empty magma chamber under volcano leads to collapse (like Crater Lake) Type 2 Magma chamber empties laterally through lava tubes (like Mauna Loa) Type3 Magma chamber produces ring fractures and empties with explosive eruption producing silica-rich deposits and very large caldera (like Yellowstone, and Long Valley) Crater Lake, Oregon is a good example of a caldera Long Valley Caldera Eruptions from 3.8 to 0.8 Ma (from basalt to rhyolite) ended with Glass Mountain eruption (0.76 Ma) that formed ring-structure caldera and produced the Bishop Tuff More recent Mono-Inyo Crater system (0.4 to present, also from basalt to rhyolite) includes Mammoth Mountain (made of a series of lava domes and flows) Currently active, probably reflecting smaller magma bodies Long Valley Caldera Bishop Tuff Mono-Inyo Craters Activity Explanation of Mono-Inyo Craters Trend Fissure eruptions and lava plateaus Fluid basaltic lava extruded from crustal fractures called fissures May travel as much as 90 km from source e.g., Columbia River Plateau individual flows accumulate to more than a km thick Fissure Eruptions The Columbia River basalts Lava Domes Bulbous mass of congealed lava formed post eruption Most are associated with explosive eruptions of gas-rich magma Volcanic pipes and necks Pipes are conduits that connect a magma chamber to the surface A lava dome on Mt. St. Helens Kimberlite Pipe Volcanic pipes and necks continued Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone Formation of a volcanic neck Shiprock, New Mexico – a volcanic neck Volcanic Neck Plutonic igneous activity Most magma is emplaced at depth in the Earth An underground igneous body, once cooled and solidified, is called a pluton Classification of plutons Shape Tabular Massive Classification (sheetlike) (massive refers to shape, not size) of plutons continued Orientation with respect to the host (surrounding) rock – cuts across sedimentary rock units Concordant – parallel to sedimentary rock units Discordant Types of intrusive igneous features – a tabular, discordant pluton Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York), uplifts overlying rock, so must be a shallow feature Laccolith Dike Similar to a sill or mushroom-shaped mass Arches overlying strata upward Lens A sill in the Salt River Canyon, Arizona Intrusive igneous features continued Batholith Largest intrusive body and massive Surface exposure of 100+ square kilometers (smaller bodies are termed stocks) Frequently form the cores of mountains like our Sierra Nevada Batholith Discordant Intrusive Igneous Structures Batholith Distribution Plate tectonics and igneous activity Global distribution of igneous activity is not random Most volcanoes are located within or near ocean basins (e.g. circum-Pacific “Ring of Fire”) Basaltic rocks are common in both oceanic and continental settings, whereas granitic rocks are rarely found in the oceans Distribution of some of the world’s major volcanoes Igneous activity along plate margins Spreading centers The greatest volume of volcanic rock is produced along the oceanic ridge system Mechanism of spreading Lithosphere pulls apart Less pressure on underlying rocks Results in partial melting of mantle Large quantities of basaltic magma are produced Igneous activity along plate margins Subduction zones Occur in conjunction with deep oceanic trenches plate partially melts (fluxed with water) Magma slowly moves upward Rising magma can form either An island arc if in the ocean A volcanic arc if on a continental margin Descending Subduction zones Associated with the Pacific Ocean Basin around the margin is known as the “Ring of Fire” Most of the world’s explosive volcanoes are found here Region Intraplate volcanism Activity within a tectonic plate Intraplate volcanism continued Associated with plumes of heat in the mantle (perhaps from the core-mantle boundary, de-compression melting after rise) Form localized volcanic regions in the overriding plate called a hot spot Produces basaltic magma sources in oceanic crust (e.g., Hawaii) Volcanic Activity Volcanic Activity A hot spot affecting a tectonic plate Key Terms Chapter 6 Volcano (shield, stratovolcano, composite cone, cinder cone) Lava Magma Pyroclastic Pyroclastic flows Viscosity Fractional melt, fractionation, fractional crystallalization Crystallization Volcanic and plutonic rocks Pluton (batholith, stock, laccolith, sill, dike)
