Igneous Rocks Chapter 4

Igneous Rocks Igneous rocks form as molten rock (magma) cools and solidifies General characteristics of magma Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava

General Characteristics of Magma Rocks formed from lava = extrusive, or volcanic rocks Rocks formed from magma at depth = intrusive, or plutonic rocks

General Characteristics of Magma, cont’d Magma consists of three components: Liquid portion = melt Solids, if any, are silicate minerals Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

General Characteristics of Magma, cont’d Crystallization of magma Cooling of magma results in the systematic arrangement of ions into orderly patterns Silicate minerals result from crystallization in a predictable order Texture - size and arrangement of mineral grains

Igneous Textures Texture - overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals Several factors affect crystal size

Igneous Textures Factors affecting crystal size Rate of cooling Slow rate = fewer but larger crystals Fast rate = many small crystals Very fast rate forms glass % of silica (SiO2) present Presence of dissolved gases

Types of Igneous Textures Aphanitic (fine-grained) texture Rapid rate of cooling Microscopic crystals May contain vesicles (holes from gas bubbles)

Types of Igneous Textures Phaneritic (coarse-grained) texture Slow cooling Large, visible crystals

Types of Igneous Textures Porphyritic texture Minerals form at different temperatures Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

Types of Igneous Textures Glassy texture Very rapid cooling of lava Rock is called obsidian or pumice

Types of Igneous Textures Pyroclastic texture Fragmental appearance produced by violent volcanic eruptions Often appear more similar to sedimentary rocks

Types of Igneous Textures Pegmatitic texture Exceptionally coarse grained Forms in late stages of crystallization of granitic magmas

Igneous Compositions Igneous rocks are composed primarily of silicate minerals Dark (or ferromagnesian) silicates Olivine, pyroxene, amphibole, and biotite mica Light (or nonferromagnesian) silicates Quartz, muscovite mica, and feldspars

Igneous Compositions, cont’d Granitic versus basaltic compositions Granitic composition Light-colored silicates Termed felsic (feldspar and silica) in composition High silica (SiO2) content Major constituent of continental crust

Igneous Compositions, cont’d Granitic versus basaltic compositions Basaltic composition Dark silicates and Ca-rich feldspar Termed mafic (magnesium and ferrum, for iron) in composition Higher density than granitic rocks Comprise the ocean floor and many volcanic islands

Igneous Compositions, cont’d Other compositional groups Intermediate (or andesitic) composition Contain 25% or more dark silicate minerals Associated with explosive volcanic activity Ultramafic composition Rare composition that is high in magnesium and iron Composed entirely of ferromagnesian silicates

Igneous Compositions, cont’d Silica content as an indicator of composition Crustal rocks exhibit a considerable range--45% to 70% Silica content influences magma behavior Granitic magmas = high silica content and viscous Basaltic magmas = much lower silica content and more fluid-like behavior

Naming Igneous Rocks Granitic rocks Granite Phaneritic Over 25% quartz, about 65% or more feldspar Very abundant - often associated with mountain building The term granite includes a wide range of mineral compositions

Naming Igneous Rocks, cont’d Granitic Rocks Rhyolite Extrusive equivalent of granite May contain glass fragments and vesicles Aphanitic texture Less common and less voluminous than granite

Naming Igneous Rocks, cont’d Granitic rocks Obsidian Dark colored Glassy texture Pumice Volcanic Frothy appearance with numerous voids

Naming Igneous Rocks, cont’d Intermediate rocks Andesite Volcanic origin Aphanitic texture Diorite Plutonic equivalent of andesite Coarse grained (phaneritic texture)

Naming Igneous Rocks, cont’d Basaltic rocks Basalt Volcanic origin Aphanitic texture Composed mainly of pyroxene and calcium-rich plagioclase feldspar Most common extrusive igneous rock

Naming Igneous Rocks, cont’d Mafic rocks Gabbro Intrusive equivalent of basalt Phaneritic texture consisting of pyroxene and calcium-rich plagioclase Significant % of the oceanic crust

Naming Igneous Rocks, cont’d Pyroclastic rocks Composed of fragments ejected during a volcanic eruption Varieties Tuff = ash-sized fragments Volcanic breccia = particles larger than ash

Origin of Magma Generating magma from solid rock Role of heat Temperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer Rocks in the lower crust and upper mantle are near their melting points Additional heat may induce melting

Origin of Magma, cont’d Role of pressure Role of volatiles Increases in confining pressure increases a rock’s melting temperature When confining pressures drop, decompression melting occurs Role of volatiles Volatiles (primarily water) cause melting at lower temperatures Important factor where oceanic lithosphere descends into the mantle

Evolution of Magmas A single volcano may extrude lavas exhibiting very different compositions Ice freezes at a single temperature, magma crystallizes through 200 degrees of cooling. If we look at this example from Mt. Mazama, we can see lite colored silica-rich ash near the base grading to dark-colored rocks at the the top. Geologists believe that the magma body had begun to segregate as the less dense, silica rich magma migrated to the top of the magma chamber. The zonation is seen because a sustained eruption tapped deeper into the magma chamber. The rock is an inverted representation of the magma body.

Bowen’s Reaction Series Minerals crystallize in a systematic fashion based on their melting points During crystallization, the composition of the liquid portion of the magma continually changes So we know that magmas evolve. How does this happen? A geologist named N.L Bowen performed lab studies that demonstrated…

Evolution of Magmas, cont’d Processes responsible for changing a magma’s composition Magmatic differentiation Separation of a melt from earlier formed crystals Assimilation Changing a magma’s composition by incorporating surrounding rock bodies into a magma Magma mixing Two chemically distinct magmas may produce a composition quite different from either original magma

Partial Melting and Magma Formation Incomplete melting of rocks is known as partial melting Formation of basaltic magmas Most originate from partial melting of mantle rocks at oceanic ridges (decompression melting) or at subduction zones (presence of water) Large outpourings of basaltic magma are common at Earth’s surface Remember that crytallization occurs over 200 degrees. That means that melting also occurs over 200 degree. As rock begins to melt—which will melt first? The last to crystallize—the ones with the lowest melting temperatures. If melting continues, minerals with higher melting points begin to melt, then the composition of the magma will steadily approach the composition of the rock from which it was derived. Most times the melting is not complete—Partial Melting. A process that produces most if not all magma. The minerals with the lowest melting point are the granitic minerals—quartz and K-spar. So how do basalts get tot the surface? Partial melting of peridotite, a major constituent of the upper mantle. Peridotite is an ultamafic rock composed mainly of olivine

Partial Melting and Magma Formation, cont’d Formation of andesitic magmas Produced by interaction of basaltic magmas and more silica-rich rocks in the crust May also evolve by magmatic differentiation

Partial Melting and Magma Formation, cont’d Formation of granitic magmas Most likely form as the end product of crystallization of andesitic magma Granitic magmas are more viscous than other magmas – tend to lose their mobility before reaching the surface Produce large plutonic structures

End of Chapter 4

Lava flows are typically finer grained than intrusive igneous rocks Lava flows are typically finer grained than intrusive igneous rocks. Why? 04.01 Intrusive magma is cooler because it is well insulated by the surrounding rock. Intrusive magma flows onto the Earth's surface and cools very slowly, allowing many small mineral grains to grow. The extrusive magma cools quickly so the mineral grains do not have time to grow. The extrusive magma, because it is deep below the surface, cools very slowly, producing very small mineral grains.

What does Bowen’s Reaction Series describe? 04.02 What does Bowen’s Reaction Series describe? The pressures that different minerals are formed in metamorphic rocks Which minerals are recrystallized in a sedimentary rock The temperatures at which different minerals crystallize out of a melt All of the above None of the above

Which of the following describes best the difference between magma and lava? 04.03 Upon eruption, magma becomes “lava,” the form of the magma that flows out of the volcano When magma gets to the top of the volcano, it releases gas found within the magma, and the lava that escapes the volcano has therefore lost those gases. Lava is the name given to the ejected magma from a passive (fluid) volcano, but it is still called “magma” if it is erupted explosively, like at Mt. St. Helens. If the magma has no crystals or gases within it, it is called “lava.” Both 1 and 3 Both 3 and 4 All of these.

Which of the following rocks is likely to have the most quartz within it and why? 04.04 Granite; intrusive rock that formed from cooling of relatively high silica magma. Rhyolite; extrusive rock that formed from cooling of relatively low silica magma. Diorite; intrusive rock that formed from the cooling of relatively intermediate silica magma. Granite; intrusive rock that formed from cooling of relatively intermediate silica magma. Basalt; extrusive rock that formed from cooling of relatively low silica lava. Basalt; extrusive rock that formed from cooling of relatively high silica lava.

What is the most important factor for whether magma cools slowly or quickly? 04.05 Pressure of the environment – higher P = slower magma cooling. The presence or absence of volatiles (gases) – less gases = slower magma cooling. Temperature of the environment – lower T = slower magma cooling. The presence or absence of volatiles (gases) – more gases = slower magma cooling. Temperature of the environment – higher T = slower magma cooling. Pressure of the environment – lower P = slower magma cooling.

Igneous rocks are produced largely by ________. 04.06 Igneous rocks are produced largely by ________. the changing of a rock from one set of minerals to another the compaction of metamorphic rocks the melting of sedimentary rocks the cooling of magma Both c and d are correct.

Glassy igneous rocks form when the magma _______. 04.07 Glassy igneous rocks form when the magma _______. cools so fast that mineral grains cannot crystallize and grow cools so slowly that only one mineral is formed is composed of basalt is a rhyolitic type cools at an extremely high temperature

04.08 Quartz is ________. resistant to weathering and is an important component of sand in river beds and beaches a main constituent of many igneous rocks a main constituent of many sedimentary rocks the most stable of all minerals at Earth’s surface temperatures and pressures all of the above