Geology Lecture 4
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This 69 page Class Notes was uploaded by Shelby Green on Tuesday January 19, 2016. The Class Notes belongs to Geol101 at Clemson University taught by Dr. Coulson in Spring 2016. Since its upload, it has received 64 views. For similar materials see Physical Geology in Geology at Clemson University.
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Date Created: 01/19/16
Announcements • Exam 1 is coming up • Students w/ SDS testing accommodation letters should submit the online form no later than 1 week before the exam • We’ll do some exam review in class next time Geology in the News • 21,000 homes in Flint, Michigan have lead contamination in their drinking water. Lecture 4- Igneous Rocks & Processes PT 1- Magma PT 2- Igneous Rocks PT 3- Volcanoes & eruptions PT 1- Magmas • Why do we care? • Plan ahead for eruptions: good to know when it will erupt, how big the eruption will be, etc • Good to understand the material for constructional/ornamental uses (interior decorations like example below). What is Magma? • Magma: liquid rock below ground • Lava: liquid rock on the surface See file 4b Melting Processes • How To Melt Rock: • 1) Temperature: need high temps (at least 500-600 degrees Celsius) • Partial melting: rocks don’t melt at one set temp • They will melt over a wide range of temp • Why? b/c they’re made up of diff minerals with diff melting points Melting Processes • 2) Wet Melting: the presence of water will lower the melting point of the rock • Needs to be saturated (water inside/out rock) • Water destabilizes the minerals in the rock and cause them to start melting faster Melting Processes • 3)Decompression Melting: • The less pressure you have, the lower the melting point will be • Closer to the surface of the earth, the easier the rock will melt • That’s why high altitudes have lower boiling/melting points Magma Composition • Gases: dissolved gas mixed into magma • SiO2: silicon content is use to classify magma • magma in diff places on earth have very diff properties • Three main types 1) Basaltic Magma Most common type of magma • Source: comes from within the mantle and works its way to the surface • Dry magma: low water content • SiO2content: 50% SiO 2 • Temp will exceed 1100°C • Hottest of the magma types Practice Question • Q: What’s 1100 C in Fahrenheit? • F = (9*C / 5) +32 2)Andesitic Magma • Andesite Line aka Ring of Fire: • Originates from the mantle • Only found in a few places on earth: • Raps the edges of the pacific ocean (not including Hawaii) • Comes from subduction zones: as the crust gets subducted, the crust starts to melt and the magma from the melted crust begins to pass through other plates to reach the surface (how other minerals get mixed in that classify is as andesitic) • Name came fromAndes Mountains • 60% SiO ,2a little higher water content, cooler temp (900-1000 Celsius) 3) Rhyolitic Magma • Cool temp: 700-800 °C • Shallow: forms at a more shallow depth (in the crust) • Only found in continental crust • About 70% SiO 2 • Wet magma: higher water content *Study Hint* • Trying to memorize all the names, temperatures, silica contents, etc is hard • Instead, understand how all those properties are linked/patterns Ex) • Magma with higher water content, means its closer to the earths surface • As temps increase, SiO 2 percentages decrease Freezing of Magma • Crystallization: freezing/making crystals in the process • Partial Freezing: as magma is being crystalized, each mineral will be formed at a specific temperature • Plagioclase: are the crystals that form before the rest of the liquid Plagioclase + liquid Example: • Equilibrium crystallization: the magma you start out with is the same composition as the rock you end up with See file 4c Freezing of Magma • Fractional Crystallization: the rock at the end, has a diff composition than the original magma b/c something was removed during the process Crystallization continued • Minerals form in a specific order as magma cools • Bowens Reaction Series: the sequence minerals will form during crystallization KNOW FOR TEST Ultramafic Mafic Intermediate Felsic Bowens continued • Discontinuous Branch: discontinued that mineral and go onto the next o at 1400 degrees à Olivine, but when 1400 degrees is lowered Olivine à Pyroxene.. Ultramafic Mafic Intermediate Felsic Bowens continued • Continuous Branch: some minerals can form over a large ranch of temps • Plagioclase: calcium replaces sodium as temp cools • higher temp, higher calcium % • Lower temp, lower calcium % PT 2) Igneous Rocks • Two broad types • 1) Plutonic (aka Intrusive): forms below the ground, magma never surfaces before it cools Igneous Rocks • 2) olcanic (aka Extrusive): formed after eruption and cools on surface of the earth Igneous Rocks • Two properties needed for ID: • 1 : Texture measure crystals formed on surface • 2 : Composition what minerals are in my rock? Igneous Rocks nd • 2 , Composition See file 4d 1) Plutonic Rocks • Plutons: large mass of plutonic rock • Many types of pluton: difference between size and shape magma Plutons continued • Dikes and sills: skinny but long • Dikes cut vertically through the rock • Sills cut horizontally through the rock • (tip: shaped like a window sill) Plutons continued • Laccolith: dome shape b/c magma has worked its way to the surface but gets stuck Plutons continued • Batholith: huge size/not distinct shape • Mount Rushmore is an example of a batholith (small by batholith standards) Plutons continued • What kind of pluton is shown? DIKEER: Textures of Intrusive Rocks • Pegmatitic (pegmatite): large crystals (greater than 1cm in diameter) Textures of Intrusive Rocks • Phaneritic: small but noticeable crystals SiO 2 Content Composition drops • Felsic: light colored minerals (white, off white, pink, & red) High SiO 2 • Intermediate: grey (balance between dark/light colored minerals) • Mafic: dark (black and brown) • Ultramafic: green or yellow Low SiO 2 • Color index: how light/dark is the rock Note: this index only works 9 times out of 10 2) Volcanic Rocks ID with same traits 1. Texture 2. Composition (Same compositions apply) T exture- Extrusive Rocks • Porphyritic: mix of little and big grains • Phenocrysts: big grains (formed below surface) • The rest of the rock formed above ground Ex: chocolate chip cookie Phenocrysts Texture- Extrusive Rocks • Aphanitic: crystals are too small to see T exture- Extrusive Rocks • Glassy: the rock looks glassy • Color index will fail: this is actually felsic • Looks smooth(left pic) • Or has fragments of smooth within it (right pic) Felsic! Texture- Extrusive Rocks Vesicular: tiny little openings formed from areas where gas used to be trapped during its magma stage (sponge-like or honeycomb like) Review- Things to Know • Ways to melt rock • Magma types & their properties • Bowens Reaction Series • Igneous rock types • Igneous rock properties Geology in the News • Volcanic glass ‘egg’ found after Kilauea erupted last week PT 3- Volcanoes • Misconception- The US doesn’t have to worry about volcanoes • Japan & Indonesia are the only countries with more volcanoes than the US rd • But us is 3 on the list with the #1 of volcanoes we have • Active United States Volcanoes: • Hawaii = 7 • Alaska = 41 • Contiguous 48 states = 20 Case Study: Krakatoa • Indonesian island volcano • Eruption inAug 26, 1883 • 200 million tons TNT, = 13,000x the yield of the HiroshimaA-bomb • 25 cubic km of ejecta • heard >3,000 mi away • Air pressure waves circled the globe for 5 days, caused waves in the English Channel • Over 30,000 dead, several languages went extinct • 2/3 of island destroyed, new volcano built up since Explosiveness • Volcano Explosivity Index (VEI): understand chart (don’t memorize) Explosive vs Non-Explosive Explosive eruption. This eruption not very explosive. Magma Properties • Viscosity: the resistance to flow • Water à low viscosity • Peanut butter à high viscosity Magma Properties • Viscosity controlled by 2 main things: 1) Temp: The higher the temp, the lower the viscosity 2) Silica content: (How much SiO 2s present) The higher the silica, the higher the viscosity Magma Properties • Viscosity controls gas content • High viscosity: • gas cant work its way up to the surface • Gas pressure builds up • More of an eruption P dropping, less dissolved gas • Low viscosity: • gas can easily finds it way to the surface • not an explosive eruption High P, more dissolved gas Class Exercise • What is the viscosity of this magma? (high/low) • What is its temp? (700/900/1100 C) • What is its silica content? (50/60/70%) • Amount of dissolved gas retained? (high/low) Non-Explosive Features • Pahoehoe • Early stage of cooling • The surface of the lava starts to crust over, but the lava continues to flow underneath that crust • Spaghetti like appearance Non-Explosive Features • Aa • More advanced cooling stage, weeks later • Lost the soft look • More brittle • Vesicles • where gas escaped, leaving holes in the rock Run? • How fast does lava flow? • ‘fast’= 16 km/hr • Q: = 10 mi/hr Explosive Eruptions • Several hazards beyond lava Explosive Eruptions • Lahar • A mud flow associated with volcanic eruptions • Can happen several days into an eruption • Ground temp rises as lava comes to surface à melts snow/ice à fast moving mudslides **can occur before actual eruption • Ex: mnt saint hellen Explosive Eruptions • Pyroclasts • Larger solid particles ejected during eruptions • Divided by size: Bombs > 64 mm Explosive Eruptions • Pyroclasts • Smaller solid particles that get ejected during eruptions • Divided by size: Lapilli 64-2 mm Explosive Eruptions • Pyroclasts: smallest debree (not the same material as ash from a fire) • Volcanic ash • Massive amounts, can burry cars • If gets sucked into an engine, tears up engine • Why there are no fly zones • Health hazard: acts like cement if ingested • Divided by size: Ash < 2 mm Pyroclastic Flow • http://www.youtube.com/watch?v=vPRoTQYXwuY • Less dense then earth atmosphere so when it looses its momentum, falls back down to the earth • b/c its gas, moves very quickly • Cannot out run and hot enough to kill • Can occur before eruption begins Volcanoes Diff types, based on type of eruption & erupted material Think of volcano as a vent on surface that the magma comes up through. Each time the magma comes up, it adds a layer to the volcano. Volcano T ypes Shield volcano: • Their profile looks like a lot like a shield • Broad face/low slope • Lots of basaltic magma • Low viscosity à gradual build up, hence the low slope V olcano T ypes • Tephra (cinder) cone: • Steeper, cone shaped, but decently small • Don’t get particularly big b/c they’re not the primary volcano, usually at the base of another larger volcano • Mostly erupts with pyroclastic material V olcano T ypes • Stratocones • aka composite volcanoes, stratovolcanoes • Class “Hollywood” volcano • Greater/powerful explosions • Viscous magma • Magma flow not very far à tend to be taller See file 4f Stratocone example Supervolcanic Eruptions • Not a type of volcano, but a type of eruption • One that’s big enough to effect global climate • Only 1 in history everyone agrees on: Tambora (1815) 100 km ejecta Changed temp in Irelandà cold summer, crops didn’t grow Frost in NY on the 4 of July Supervolcanic Eruptions • Yellowstone Huckleberry Ridge eruption • 2 Ma (million yrs ago), 2500 km ejecta See file 4h Hot Spots • The Hawaiian Problem More volcanoes=more subduction zones, right?? like Hawaii…e volcanoes that were formed where there aren’t any subduction zones, How? Hot spots. Hot Spots Volcano on plate on top of hot spot will form an island, but the plate will move away from the hot spot and move the island formed from magma with it Thus, there is 1 active volcano in hawaiià the one on the hot spotsland gets moved too.. Review- Things to Know • Viscosity • Types of eruptive features • Types of volcanoes • Hot spots 1 exam this coming tuesday In this room during class time 50 MC questions Focus on lecture notes, if there’s a topic in the book that we never covered in class, it wont be on the exam 50% of the class got a D or on Exam 1 last semester 50% of the class got an A or B on Exam 4 last semester wont be able to use a calc/wont need one bring pencils and erasers scantrons will be provided Mistakes to avoid Erase mistakes completely (no re-grades) Don’t put in wrong version pile =-20 points to re-grade Cheat = F in course Cannot leave during exam =0 on exam Cannot take exam with another section =0 on exam
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