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Lecture Notes 3 Igneous Rocks

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Lecture Notes 3 Igneous Rocks Lecture Notes: Bill Engstrom – Instructor Igneous Rocks GLG 101: Physical Geology In our overview of the Earth, we found out that Earth’s internal heat combined with other mechanisms causes rocks to melt in the asthenosphere and lithosphere. The present is the key to the past = Principle of Uniformitarianism. And…..We have igneous and volcanic activity going on today that we can observe to help us understand the past. NOTE: Before you start reading these notes, skim through them and locate the charts. You can use these as you read to help you remember the information and figure out how it all fits together. We will look at Igneous Rock is…. • A crystalline or glassy aggregate of one or more minerals formed by cooling from a molten liquid; • or by consolidation of pyroclastic material (e. g. ash or other igneous fragments). Distribution of Volcanoes – When we look at the distribution of volcanoes worldwide we see that there is a large concentration associated with plate boundaries (e.g. the “ring of fire” around the Pacific) What is Magma? • Earth’s internal heat combined with other mechanisms causes rocks in asthenosphere and lithosphere to melt. • Magma: a molten or liquid rock or liquid crystal mix underground, often containing dissolved gases such as water, carbon dioxide, sulfur dioxide, and chlorine. • Not all magma is alike‐ there are different kinds‐ this will be useful in interpreting igneous rocks and geologic hazards So….What is lava (and Pyroclastics)? • Lava: a magma that has made it to the surface • Pyroclastics: broken bits of pre‐existing rocks, ash, blasted out lava Igneous Rocks are classified by Texture and Composition – BOTH are used. Figure 4.3 (located below‐ in these notes) shows a general igneous rock classification which includes basic igneous rock types. Texture refers to the size and arrangement of crystals, glass and pyroclasic material in a rock. Texture is NOT how the rock feels. Intrusive/Plutonic textures • Indicate slow cooling beneath surface • Most of the crystals of intrusive rocks are visible to the naked eye Crystalline (Phaneritic (pr. Fan‐eritik)‐coarse grained) ‐ visible interlocking crystals – time for crystals to grow. These are crystals that are similar in size. Porphyritic (pr. Pore‐feritik) ‐ Larger crystals (phenocrysts [pr. Feen‐o‐krysts) in a “matrix” of smaller crystals. This texture is typically formed at first by slow cooling followed by rapid cooling as the magma solidifies. • Phenocrysts = the larger crystals in a porphyritic texture • Groundmass = matrix = the main mass of a porphyritic texture – called phaneritic in intrusives Pegmatitic – Very large ( > 2 cm) crystals – formed in later stages of cooling with lots of vapor. The large crystals are not simply formed by slow cooling, but are formed in a fluid rich (water and other volatiles) environment in the later stages of magma crystallization. Extrusive/volcanic textures • Fast cooling at or near the surface • More interlocking smaller crystals • No time for most crystals to get very large • Most crystals invisible to the naked eye Microcrystalline (Aphanitic [pr. Aff‐anitik) = fine grained) Porphyritic ‐ large crystals in a groundmass (matrix) of invisible (microcrystalline) crystals Vesicular – full of vesicles (holes) where gas has escaped Frothy Texture‐ Extrusive Glassy – No time for any crystals to form Pyroclastic – Glasses weld rock fragments (clasts), ash, etc. – a “clastic” rock Composition – The other ½ of our classification scheme helps us determine where the rocks were formed related to plate tectonics (tectonic environments). We will talk about rock “families”. Rocks that appear different because of texture may have similar compositions. Figure 4.1(at the end of these notes) shows the tectonic environments and compositions. You can follow along with that chart while you look at the rock families. On the left side of the chart you can see how the compositions change based on Bowens Reaction Series. On the right side you can see the types of tectonic settings they occur in and the types of volcanoes and lava flows that occur. In the middle is the rock type. Some students feel that it helps to turn the chart sideways, with the left side on top and the right side on the bottom to better visualize things. • Remember that Classification is based BOTH on texture AND composition AND THAT…. • Composition is actually what will help us figure out tectonic environments Igneous Rock Compositions – Ultramafic Rock Family. Peridotite – Komatiite (Ultramafic) Peridotite‐ an ultramafic intrusive rock Komatiite‐ an ultramafic extrusive rock • Key minerals‐ dominant olivine, minor pyroxene, minor Ca‐rich plagioclase • Color‐ Green to deep green • Density‐ High (Around 3.5 g/cc) • Silica content (amount of Silica relative to iron and magnesium): low ~45% • Temperature of melt: Extremely hot (probably > 1500°C). • Viscosity of Melt: Probably very low but we don't see natural melts of this composition above the mantle anymore. • Gas content of melt: Probably very low in most cases. • Lava Flows & Volcanoes ‐ Youngest komatiite is older than 2 billion years. Suggests that Earth has cooled down. What is important about ultramafic rocks? • Ultramafic intrusive rocks make up the upper mantle portion of the lithosphere and asthenosphere. • These rocks probably formed very early Earth’s history when the much of the planet was probably a giant molten mass. • They are important because partial melting of mantle peridotite ultimately provides the material for all other rocks in the lithosphere. Igneous Rock Compositions – Mafic Rock Family Gabbro – Basalt (Mafic) Gabbro‐ a mafic intrusive rock Basalt‐ a mafic extrusive rock • Key minerals‐ a few olivines, dominant pyroxene, dominant Ca‐rich plagioclase feldspars • Color‐ Dark ‐in plutonic rocks an more dark crystals than light crystals • Density‐ Moderately high (Around 3.2 g/cc) • Silica content (amount of Silica relative to iron and magnesium): low ~50% • Temperature of melt: Very hot (~1200‐1500 °C). • Viscosity of Melt: Low viscosity‐ (fairly fluid relative to other magmas). • Gas content of melt: low due to low viscosity of melt‐ gas escapes easily. • Lava Flows & Pyroclastics ‐ Thin widespread flows‐ basalt (low viscosity promotes thin widespread flows) • Pahoehoe (pr. Pa hoy hoy) = ropey‐ This basalt solidifies when the most gas is still dissolved in the lava. • Aa (pr. Ah ahh) = painful to walk on (when you do you might say “Ah‐ahh”) – This basalt solidifies after the loss of most of the gas from the lava. • Pillow lavas (indicate underwater eruption)‐ This basalt forms like overlapping pillows because it is solidifying under water. • Pyroclastics ‐ cinders (scoria) & bombs common • Volcanoes • Fissure eruptions‐ Columbia Plateau, Snake River Plain, Sunset Point AZ • Shield volcanoes (Hawaii/Galapagos) Cinder cones‐ Sunset Crater Igneous Rock Compositions – Intermediate Family Diorite‐Andesite (Intermediate) Diorite‐ an intermediate intrusive rock Andesite‐ an intermediate extrusive rock • Key minerals‐ hornblende, intermediate plagioclase feldspars • Color‐ Intermediate‐ in plutonic rocks an equal amount of dark & light (e.g. looks like a grey/salt and pepper colored rock) • Density‐ Intermediate (Around 3.0 g/cc) • Silica content (amount of Silica relative to iron and magnesium): moderate ~60% • Temperature of melt: Moderately hot (900‐1200 °C). • Viscosity of Melt: Moderately viscous. • Gas content of melt: moderate percentage due to moderate viscosity of melt‐ gas escapes with difficulty. • Lava Flows & Pyroclastics • Massive or blocky‐ moderate viscosity allows flow but not very far • Pyroclastics: ash and some cinder/scoria deposits common. • Volcanoes • Stratovolcanoes / Composite volcanoes‐ Mt. Rainer, Cascade Volcanoes. Ash and cinders are mixed with lava flows in separate strata/layers. Stratovolcanoes are more explosive types of volcanoes due to the gas content in the magma. Many famous eruptions are the result, including Pompeii, Italy and Mt. St. Helens, Washington. Special Note: Intermediate and Felsic magmas often occur in the same tectonic areas and may even grade into each other. Consequently, they may thus occupy the same volcanoes at different times. Igneous Rock Compositions – Felsic Family Granite‐Rhyolite (Felsic) Granite‐ a felsic instrusive rock Rhyolite‐ a felsic extrusive rock • Key minerals‐ Biotite, muscovite, sodium‐rich plagioclase, K‐spars, Quartz • Color‐ Light or pinkish‐ in plutonic rocks more light (or pink) colors than dark • Density‐ Light (Around 2.7 g/cc) • Silica content (amount of Silica relative to iron and magnesium): low ~75% • Lava Flows & Pyroclastics • Domes‐ Rhyolites (high viscosity prevents thin widespread flows) & Obsidian • Pyroclastics: Ash Flows‐ Rhyolite tuffs and volcanic breccias (because of extremely high gas pressures); Ash Falls (pumice) • Volcanoes • Stratovolcanoes /Composite volcanoes‐ Mt. Rainer, Cascade Volanoes • Collapse structures/large calderas – Superstition Mtns, Crater Lake, Thera Igneous Rock Compositions REVIEW Intrusive (Phaneritic)– Peridotite, Gabbro, Diorite, Granite Extrusive (Aphanitic)– Komatiite, Basalt, Andesite, Rhyolite Rocks Classified by Texture Alone Frothy (may float on water) • Dark colored ‐ mafic and intermediate family = Scoria • Light colored‐ felsic family = Pumice • Frothy rocks are also sometimes classified by their size: Cinders, Ash, and Volcanic Bombs Glassy • Purely glassy rocks are usually of the felsic family in spite of their apparent darkness. • Darkness comes from impurities
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