Introduction to Geology GEO-101-004 Class 6: Igneous Rocks

1. Introduction to GeologyGEO-101-004Class 6: Igneous Rocks From http://volcano.und.nodak.edu

2. Last Question of the Day Name one mineral that you have seen and/or used in the last two hours Graphite, muscovite, barium, gypsum, etc

3. Question of the Day What is the major reason for some igneous rocks being coarse grained, and some being fine grained?

4. Titan Earth-like processes on Titan This image from Huygens shows detail of a high ridge area including the flow down into a major river channel from different sources – evidence of fluvial activity From http://www.nasa.gov/mission_pages/cassini/multimedia/pia07236.html

5. And on Mars….. The Mars Exploration Rover Opportunity has found a pitted, basketball-size meteorite made of iron and nickel. This is the first ever meteorite seen on the surface of the planet Metal-rich meteorites are rare (making up only 2% of meteorites on Earth). Such meteorites must come from a destroyed planet that was big enough to differentiate into a metallic core and a rocky mantle

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

7. General characteristics of magma • General characteristic of magma • Rocks formed from lava = extrusive, or volcanic rocks • Rocks formed from magma at depth = intrusive, or plutonic rocks

8. 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.

9. General characteristics of magma • The nature of magma • Consists of three components: • Liquid portion = melt • Mobile ions, Si, O (readily combine to SiO2), Al, K, Ca, Na, Fe, Mg • Solids, if any, are silicate minerals • Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) • Remain in magma until it moves nearer the surface, or it crystallizes

10. General characteristics of magma • Crystallization of magma • As a rock melts the ions begin to vibrate more rapidly, colliding with their neighbors • The rock expands • When the melt is hot enough the bonds break, and the rock becomes a liquid • A liquid is composed of unordered ions moving randomly

11. General characteristics of magma • Crystallization of magma • Cooling of magma results in the systematic arrangement of ions into orderly patterns – the silicates (tetrahedra) • The silicate minerals resulting from crystallization form in a predictable order – ions lose their mobility and join a crystal network • Earliest formed minerals have better developed crystal faces – room to grow • Later formed minerals fill the remaining space • When all the melt has cooled = igneous rock

12. Igneous textures • Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals • Factors affecting crystal size • Rate of cooling • Slow rate = fewer but larger crystals • Fast rate = many small crystals • Very fast rate forms glass

13. 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.

14. Igneous textures • Types of igneous textures • Aphanitic (fine-grained) texture • Rapid rate of cooling • Crystal can only be seen under magnification • May contain vesicles (holes formed by gas bubbles) – vesicular texture

16. Igneous textures • Types of igneous textures • Phaneritic (coarse-grained) texture • Slow cooling • Large, visible intergrown crystals • Form deep in the Earth so are only exposed at the Earth’s surface after uplift and/or erosion

17. Igneous textures • Types of igneous textures • Porphyritic texture • Minerals form at different temperatures – some crystals can become quite large before others start to form • If environment changes (e.g. eruption) the remaining melt may cool rapidly • Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

18. Igneous textures • Types of igneous textures • Glassy texture • Very rapid cooling of lava • Resulting rock is called obsidian – no crystalline structure • Chain structure of high silicate content minerals can impede ionic transport and thus restrict crystal growth • Granitic magmas (higher silicon content) more commonly form obsidian than basaltic • Basaltic magmas form glassy texture when cooled rapidly

19. Igneous textures • Types of igneous textures • Pyroclastic texture • Fragmental appearance produced by violent volcanic eruptions • Welded tuff • Fragments solidified during flight, forming a rock on landing – will not have interlocking crystals • Often appear more similar to sedimentary rock http://volcanoes.usgs.gov/

20. 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.

21. Igneous textures • Types of igneous textures • Pegmatitic texture • Exceptionally coarse grained • Form in late stages of crystallization of granitic magmas • Volatiles such as chlorine, water, fluorine, and sulfur enhance ionic transport and aid in crystal growth • Great environment for rare minerals to form – beryl, topaz

22. Igneous compositions • Igneous rocks are composed primarily of silicate minerals • Dark (or ferromagnesian) silicates • Olivine, pyroxene, amphibole, and biotite mica • Rich in iron, magnesium and comparatively low in silicon • Light (or nonferromagnesian) silicates • Quartz, muscovite mica, and feldspars • Greater amounts of potassium, sodium, and calcium rather than iron and magnesium

23. Igneous compositions • Graniticversus basaltic compositions • Granitic composition • Light-colored silicates • Termed felsic (feldspar and silica) in composition • High amounts of silica (SiO2) – about 70% • May contain 10% dark silicate minerals (mica, amphibole) • Major constituent of continental crust

24. Igneous compositions • Graniticversus basaltic compositions • Basaltic composition • Dark silicates and calcium-rich feldspar • Termed mafic (magnesium and ferrum, for iron) in composition • Higher density and darker than granitic rocks • Comprise the ocean floor and many volcanic islands

25. Igneous compositions • Other compositional groups • Intermediate (or andesitic) composition • Composition between granitic and basaltic rocks • Contain 25% or more dark silicate minerals (amphibole, pyroxene, biotite mica, plagioclase feldspar • Associated with explosive volcanic activity on continent margins • Ultramafic composition • Rare composition that is high in magnesium and iron • Composed entirely of ferromagnesian silicate • Peridotite is believed to be the main constituent of the upper mantle

26. Figure 4.7

27. Igneous compositions • Naming igneous rocks – granitic rocks • Granite • Phaneritic • Over 25% quartz (clear to light gray), about 65% or more feldspar (not as glassy, white to gray or salmon pink) • Minor muscovite mica and amphibole • Very abundant - often associated with mountain building • The term granite includes a wide range of mineral compositions • May have porphyritic texture (large feldpar crystals)

28. Igneous compositions • Naming igneous rocks – granitic rocks • Rhyolite • Extrusive equivalent of granite • May contain glass fragments and vesicles • Aphanitic texture • May contain phenocrysts of quartz or potassium feldspar • Less common and less voluminous than granite

29. Igneous compositions • Naming igneous rocks – granitic rocks • Obsidian • Dark colored • Glassy texture • Unordered ions – no crystal structure • Native Americans used it to make arrowheads and cutting tools • Pumice • Volcanic • Glassy texture • Large amounts of gas escaping create a frothy appearance with numerous voids • Sometimes will float on water (dangerous for shipping)

30. Igneous compositions • Naming igneous rocks – intermediate rocks • Andesite • Volcanic origin (Andes, Pacific rim) • Porphyritic texture (phenocrysts of plagioclase feldspar, amphibole). • Minor quartz (compared to ~25% for rhyolite) • May look similar to rhyolite (distinguish using a microscope) • Diorite • Plutonic equivalent of andesite • Coarse grained • Absence of visible quartz (distinguishes it from granite)

31. Igneous compositions • Naming igneous rocks – basaltic rocks • Basalt • Volcanic origin • Aphanitic texture or porphyritic (with phenocrysts of feldspar or olivine) • Composed mainly of pyroxene and calcium-rich plagioclase feldspar, lesser amounts of olivine and amphibole • Most common extrusive igneous rock • Upper layers of oceanic crust, many volcanic islands

32. Igneous compositions • Naming igneous rocks – mafic rocks • Gabbro • Intrusive equivalent of basalt • Phaneritic texture consisting of pyroxene and calcium-rich plagioclase • Significant % of the oceanic crust • Formed underground reservoirs that fed basaltic lava flows

33. Igneous compositions • Naming igneous rocks – pyroclastic rocks • Composed of fragments ejected during a volcanic eruption • Varieties • Tuff = ash-sized fragments • Volcanic breccia = particles larger than ash • Flows speeds approaching 100 km/h

34. 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.

35. Origin of magma • Generating magma from solid rock • Earth’s crust and mantle are composed primarily of solid rock – why does rock melt? • Role of heat • Temperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer (1200-1400oC at 100 km) • Rocks in the lower crust and upper mantle are near their melting points • Any additional heat may induce melting (friction) • Hot mantle rocks can rise and intrude crustal rocks

36. Origin of magma • Role of pressure • Increases in confining pressure cause an increase in a rock’s melting temperature • When confining pressure drops, decompression melting occurs

37. Origin of magma • Role of volatiles • Volatiles (primarily water) cause rocks to melt at lower temperatures • Important factor where oceanic lithosphere descends into the mantle • Heat and pressure drive water from subducting rocks • Lowers melting temperature of mantle rocks

38. Evolution of magmas • A single volcano may extrude lavas exhibiting very different compositions

39. Evolution of magmas • Bowen’s reaction series • Minerals crystallize in a systematic fashion based on their melting points

40. Evolution of magmas • Bowen’s reaction series • During crystallization, the composition of the liquid portion of the magma continually changes • When about one third of the magma has crystallized the melt will be nearly depleted in Fe, Mg, and Ca (constituents of first formed minerals) • Remaining portion of magma is richer in silica • If the solid portion of the magma is removed, the melt will crystallize to for a more felsic rock

41. Evolution of magmas • Bowen’s reaction series • If the solid portion of the magma remains in contact with the melt, the mineral will chemically react and evolve into the next mineral in the sequence • This arrangement of minerals is known as Bowen’s reaction series

42. Evolution of magmas • Processes responsible for changing a magma’s composition • Magmatic differentiation • Separation of a melt from earlier formed crystals • Crystal settling – early formed dense minerals sink to the bottom of a magma chamber • Remaining melt will form a rock with a different composition to the parent magma

43. Evolution of magmas • Processes responsible for changing a magma’s composition • Assimilation • Changing a magma’s composition by the incorporation of surrounding rock bodies into a magma • Magma mixing • One magma body intrudes another

44. Evolution of magmas • Partial melting and magma formation • Incomplete melting of rocks is known as partial melting • Low temperature minerals melt first (quartz) • Formation of basaltic magmas • Most originate from partial melting of ultramafic rock in the mantle at oceanic ridges • Large outpourings of basaltic magma are common at Earth’s surface

45. Evolution of magmas • Partial melting and magma formation • Formation of andesitic magmas • Produced by interaction of basaltic magmas and more silica-rich rocks in the crust • May also evolve by magmatic differentiation • Granite may be the end product of the crystallization of an andesitic magma or partial melting of silica rich continental rocks • Granitic magmas usually lose their mobility before reaching the surface and tend to form large plutonic structures • If they reach the surface they form large explosive eruptions (Mount St. Helens)