Transcript Chapter 3
Igneous Rocks The Rock Cycle The continuous and reversible processes that illustrates how one rock changes to another. “One rock is the raw material for another”. The Rock Cycle Rock Cycle Processes – Crystallization Rock Cycle Processes - Weathering Rock Cycle Processes - Lithification Rock Cycle Processes Metamorphism Magma and Lava Differences • magma is in the interior of the earth; lava is at the surface. • magma contains volatiles, dissolved gases, that escape at the surface and so are not present in lava. • magma cools very slowly; lava cool relatively rapidly, leading to differences in crystal size. Similarities • Parent material of igneous rocks • Forms from partial melting of rocks at depth •both consist of melt and possibly solid crystals Rate of cooling and crystallization A slow cooling rate promotes an interlocking mass of mineral crystals, all visible to the naked eye. This granite crystallized from slow-cooling magma. Rate of cooling and crystallization A rapid rate of cooling doesn’t allow the mineral crystals to grow large enough to see with the naked eye. This basalt crystallized from lava. Rate of cooling and crystallization If lava cools too rapidly, the orderly repeating crystalline structure, associated with minerals, does not have time to form. This obsidian is not crystalline, instead, it is a considered a glass. Igneous Rock Classification Criteria #1: Texture Overall appearance of rock based on size and arrangement of mineral crystals – Texture indicates the environment in which the rock crystallized since crystal size is determined by the cooling environment – Extrusive (volcanic) rocks cooled quickly at the surface from lava – Intrusive (plutonic) rocks cooled slowlyh at depth from magma – Includes secondary factors such as vesicles (cavities left by escaping gas) Aphanitic (fine-grained) • Rapid rate of surface cooling results in microscopic crystals • The top sample is rhyolite, which has the same compostion as granite • Aphanitic rocks may exhibit a secondary vesicular texture, like this basalt, as gas escaped from the lava. Phaneritic (coarse-grained) • Slow cooling in Earth’s interior results in visible crystals, like this granite • Crystals approximately the same size. Pegmatitic (very coarse-grained) • Composed of crystals >1 cm in diameter • They form late in the crystallization stage of a magma. Porphyritic - large crystals embedded in a fine-grained groundmass • Crystallized in (at least) 2 different environments • Large crystals, phenocrysts, crystallize first • Matrix of smaller crystals, the groundmass, crystallize last • See sample #10. Glassy • When molten rock cools too quickly, the resulting rock lacks crystalline structure and is called a glass, like the obsidian (top). • A glassy-textured rock can also have a vesicular texture, like the pumice shown in the picture. Pyroclastic – Various-sized fragments ejected during a violent volcanic eruption – the most common fragment is ash-sized and the resulting rock is called welded tuff. – Often appear similar to sedimentary rocks – Larger fragments form Pyroclastic Larger-sized fragments form other types of pyroclastic rocks, such as this vesicular scoria (see sample #7), made from cindersized fragments. Decide if each igneous texture below indicates an extrusive (volcanic), or intrusive (plutonic) origin, based on its texture: A and C are extrusive and B is intrusive D probably started out in an intrusive environment and ended up in an extrusive environment Igneous Rock Classification Criteria #2: Mineral Composition Igneous rocks are composed primarily of silicate minerals • Dark (ferromagnesian) silicates – – – – Olivine Group Pyroxene Group (Augite) Amphibole Group (Hornblende) Biotite Mica • Light (nonferromagnesian) silicates – Quartz – Muscovite mica – Feldspars Mafic (Basaltic) composition – Composed of ferromagnesian silicate minerals and calcium-rich feldspar (e.g. labradorite). – Approximately 50% silica (SiO2) content. – More dense (heavy) than granitic rocks – Comprise the ocean floor and many volcanic islands, although also found on continental crust as lava flows, and intrusive bodies. Ultramafic - Peridotite • The lowest silica (SiO2) content of the igneous rocks • Composed entirely of ferromagnesian silicates, with a relatively high iron and magnesium content. • Rarely found in crust; main constituent of the upper mantle. Intermediate ( andesitic) composition • Contains at least 25 percent dark silicate minerals. • Andesite, named after the Andes Mountains in South America, is associated with explosive volcanic activity. • None in our collection Felsic (Granitic) composition •Composed of primarily of light silicates •Granite not shown here, is a common example. •Contains up to 70% silica (SiO2). •Major constituents of continental crust. •Sample #10 is a felsic porphyry called trachyte. Rhyolite Trachyte porphyry Mineralogy of igneous rocks Origin of Magma • Earth’s crust and upper mantle primarily composed of solid rock. • Earth’s outer core is considered molten, but magma has the same composition as mantle and crust, not iron core. • Geologists conclude that magma originates when solid rock of crust and mantle melts. • What causes this to happen? Role of Heat: Geothermal gradient • Rocks in lower crust and upper mantle are already near melting points. • Any additional heat (e.g. basaltic magma beneath silicarich rocks) may induce melting. What causes rock to melt • Role of pressure (see animation on mantle melting) – Melting point increases with depth due to increased pressure, so rocks that would melt on the surface remain solid at depth. – Reducing the pressure lowers the melting temperature; decompression melting occurs. – Occurs at divergent boundaries, where rock is buoyant and ascending so pressure is low. Decompression melting Figure 3.14 What causes rock to melt • Role of volatiles (i.e. water and dissolved gases) – Volatiles (primarily water) cause rocks to melt at lower temperatures. – This is particularly important where wet oceanic lithosphere descends into the mantle. – See animation on mantle melting addition of volatiles is called wet melting Addition of volatiles (water) lowers the melting point of subducting plate Bowen’s Reaction Series: Systematic crystallization of silicate minerals based on their melting points • High temperature silicates have high melting points (up to 1200 degrees C): – first minerals to crystallize from molten rock, last to melt from solid rock. – Includes ferromagnesian silicates and Ca-rich plagioclase feldspar. Bowen’s Reaction Series: Systematic crystallization of silicate minerals based on their melting points • Low temperature silicates have “low”melting points (as low as 750 degrees C – Last to crystallize from molten rock, and first to melt from solid rock. – non-ferromagnesian silicates, Na-rich plagioclase feldspar and K feldspar. Partial Melting and Magma Formation Partial melting – incomplete melting of rocks due to differences in mineral melting temperature • Low temperature minerals melt first and form a magma which migrates upward. Low temperature minerals are the ones with a high silica content (quartz, feldspars etc.) • Therefore, the product magma or rock of partial melting always is more silica-rich (i.e. granitic) composition than the parent rock. Partial melting of the solid rock on the left results in a more mafic solid portion and a more felsic magma that is free to rise. Formation of Mafic Magmas • Mafic (basaltic) magmas originate from partial melting of ultramafic rock in the mantle. • These magmas form at mid-ocean ridges by decompression melting and at hot spots. • Much of the oceanic crust is formed by partial melting of ultramafic material. Formation of Andesitic and Felsic Magmas These magmas originate from: • partial melting of subducting mafic ocean crust which has had water added to it. • assimilation of pieces of continental crust, which is usually felsic. • What is the composition of the Andes of South America?