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GEOL 105 Test 2 Notes

by: Alaina Notetaker

GEOL 105 Test 2 Notes Geology 105

Alaina Notetaker
University of Louisiana at Lafayette
GPA 3.5

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Covers: Chapter 4 - Igneous Rocks and Intrusive Igneous Activity Chapter 5 - Volcanoes and Volcanism
Geology and man
Elisabeth Boudreaux
igneous, rocks, volcanoes, Volcanism
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This 7 page Bundle was uploaded by Alaina Notetaker on Tuesday September 20, 2016. The Bundle belongs to Geology 105 at University of Louisiana at Lafayette taught by Elisabeth Boudreaux in Spring 2016. Since its upload, it has received 6 views. For similar materials see Geology and man in Geology at University of Louisiana at Lafayette.

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Date Created: 09/20/16
Igneous Rocks and Intrusive Igneous Activity • Black sand/ green sand beaches are results of the material that is broken down • Form from liquid rock materials • The properties and behavior of magma and lava ⿞Magma is molten rock material below the surface ‣ Lower density/ heat causes magma to move upward to the surface ‣ Magma at the surface- lava ‣ Forcefully ejected into the air- pyroclastic material ⿞Extrusive or volcanic rocks ‣ Lava flows ‣ Pyroclastic material ⿞Plutonic or Intrusive rocks ‣ Magma that crystallizes within earth's crust forms plutonic or intrusive igneous rocks • Composition of magma/lava ⿞Defined by silica content ‣ Felsic (Feldspar + Silica) • Silica-rich magma; abundant potassium, aluminum • > 65% silica ‣ 0Intermediate • Compositions between felsic and mafic • 53-65% silica ‣ Mafic (Magnesium, ferrum = iron) • Silica-poor magma • 45-52% silica ‣ Ultramafic • < 45% silica • How hot are magma and lava? ⿞Lava: 700-1,200 degrees Celsius (2192 degrees F!) ⿞(Temp of 1350 degrees C has been recorded) ⿞No direct measurements of magma have been taken ⿞Rock is a poor conductor of heat ‣ Lava flows and plutonic may retain heat for thousands of millions of years ⿞Direct measurements come from low-risk volcanoes such as the mafic lavas of the Hawaiian Island volcanoes • Viscosity ⿞Resistance to flow ⿞Composition ‣ High silica (Felsic) = higher viscosity ‣ Low silica (Mafic) = lower viscosity ⿞Heat ‣ Higher temps = lower viscosity ⿞Dissolved gas content ‣ Higher gas content = lower viscosity ⿞Mafic magma: low viscosity, hot, gases escape easily ‣ Lava flow (Hawaii) ⿞Felsic magma: high viscosity, colder, thick, pasty flow, pressure build up ‣ Explosive (St. Helens) • Bowen's Reaction Series ⿞Shows us the sequence in which minerals crystallize in a cooling magma OR the sequence in which minerals melt • Why do rocks melt? ⿞Heat ‣ Geothermal gradient- temp increases with depth below the surface (avg. = 3 degrees C / 100 meters) ‣ Mantle plumes ("hot spots") ⿞Pressure ‣ Melting point of a mineral increases with increasing pressure ‣ Even when temps are high, extreme pressure can prevent rocks from reaching their melting point ⿞Water ‣ Presence of water at depth helps to break bonds within minerals, thereby lowering the mineral's melting point ‣ Water allows rocks to melt at lower temps • Where do rocks melt? ⿞The origin of magma at divergent plate boundaries ‣ (A) Melting temp rises with increasing pressure ‣ (B) Melting temp decreases when water is present ‣ Melting is initiated by a pressure decrease at spreading ridges ‣ Presence of water further decreases melting temp ⿞The origin of magma at convergent plate boundaries ‣ Partial melting of a mafic crust forms intermediate and felsic magmas ‣ Melting of sediments and contamination with continental crust rocks changes the magma composition to a more felsic magma ⿞The origin of magma and hot spots • Evolution of magmas with different compositions ⿞Crystal settling ‣ Gravity driven ⿞Partial melting ⿞Assimilation- reacting with pre existing rock ‣ Inclusions ⿞Mixing of magmas • Igneous Rock classification ⿞Names are based on: ‣ Texture- size, shape, arrangement of minerals • Extrusive/volcanic/aphanitic/fine-grained • Intrusive/plutonic/phaneritic/coarse-grained ⿞Composition ‣ Amount of silica ‣ Silica tetrahedron (silicon & oxygen) ‣ Ultramafic • Very dark/often green • Less than 45% silica; 45-100% Olivine • Peridotite- intrusive • Komatite- extrusive (rare) ‣ Mafic • Dark • Pyroxene, Feldspar, some Olivine, 45-55% silica • Gabbro- intrusive • Basalt- extrusive ‣ Intermediate • Grey • Pyroxene, Feldspar, Mica, Quartz, 55-65% silica • Diorite- intrusive • Andesite- extrusive ‣ Felsic • Light • Feldspar, Mica, Quartz, >65% silica • Granite- intrusive • Rhyolite- extrusive • Igneous Textures ⿞Glassy- no crystals ‣ Cools very, very fast ⿞Aphanitic ‣ Crystals barely visible ‣ Extrusive, volcanic ⿞Phaneritic ‣ Crystals easily visible ‣ Intrusive, plutonic ⿞Porphyritic ‣ Large crystals (phenocrystals) with very fine groundmass ‣ Slow cooling followed by rapid cooling ⿞Pegmatitic ‣ Pegmatites: large crystals ⿞Vesicular (frothy) ‣ Bubbles trapped in rock o ‣ "Volcanic foam"- pumice ⿞Pyroclastic- explosive volcanic activity ‣ Ejecta and accumulations of ash- called a tuff ‣ Quickly cooled • Plutons or Intrusive Igneous Rock Bodies ⿞Intrusive Structures ‣ Classification of plutonic based on • Size • Shape • Orientation ‣ Intrusive Igneous Bodies • Dikes- form where cracks formed; cuts across • Sills- parallel to the layers • Laccolith- has a flat bottom; causes the top layer to dome over • Volcanic Neck • Batholiths- very large pluton ⿞Made up of numerous diapirs • Stock- smaller pluton Review- Intrusive Igneous Rocks • Classified by texture and composition • Plutonic- intrusive igneous rocks • Now- extrusive What's in it for us? • Can be catastrophic or beneficial ⿞Fertile soil ⿞Geothermal energy ⿞Climate can be affected I eruption spews ash into atmosphere, CO2 ⿞Volcanic catastrophes • Volcanoes and Volcanism ⿞Volcano ‣ Landform created by eruption of lava, etc. • Quiet (Hawaii) • Explosive (Krakatau, Mt. St. Helens) ⿞Volcanism ‣ Geologic processes involved in eruptions of lava ⿞Form extrusive rocks from cooling lava ⿞*95% of all volcanoes are at or near plate boundaries. Pacific surrounded by 60%-75% of all volcanoes.* ⿞What is a volcano? ‣ Hill or end around a vent where lava, pyroclastic material, and gases erupt. ‣ A very large crater • Caldera ⿞Collapsed structure around the crater ⿞Forms when the magma chamber empties and the roof collapses ⿞Activity of Volcanoes ‣ Active • Currently erupting or has erupted recently ⿞During the past few thousand years ‣ Dormant • No activity or the past few thousand years, but may in the future ‣ Extinct • No activity or the past tens of thousands of years and not expected to erupt again ⿞What causes a volcano to erupt? ‣ Hot spots ‣ Convergent plate boundaries- change in the angle of subduction can cause magma to rise to the surface in a different location ‣ Failed rift valleys ‣ Active to passive margins ‣ ETC. ⿞What is erupted from a volcano? ‣ Solids • Pyroclastic materials ⿞Ash (<2mm) ⿞Lapilli (2-64mm) ⿞Blocks and Bombs (>64mm) ⿞Particles that are produced by explosive magma, collectively called Tephra • Pyroclastic Flows ⿞High gas content ⿞Fast moving, approx. 100mph ⿞Hot, approx. 800 C ⿞Also known as nuees Ardennes (noo-ay ar-dent), or glowing cloud ‣ Liquids • Lava flows ⿞Low viscosity ‣ Basalt ⿞Cooling and contraction of a lava flow creates Columnar Joints • Lava tube • Basaltic Lava ⿞Paths are predictable ⿞Rarely a danger to human life ⿞Two types are recognized from Hawaiian flows ‣ Pahoehoe • Fast moving; fluid; ropy • Gases can escape more easily ‣ A'a • Slow moving; blocky; chunky • Low gas content ⿞How does Pele's Hair form? ‣ When basaltic flows (pahoehoe) in Hawaii are blown by the wind (often lava fountains or lava cascades), the lava can cool in thin strands ⿞Eruption of Basaltic Lava underwater creates pillow basalts • Andesitic and Rhyolitic (FELSIC) Lava ⿞Explosive ‣ High viscosity ⿞Pumice ‣ Low viscosity ‣ Gases • 1%-9% of most magmas are gases • 70% water, 15% CO2, 5%N2, 5% S and others • Gases contained in rising magma expand and can contribute to violent eruptions ⿞Types of volcanoes ‣ Sheild volcanoes • Low, rounded profiles, composed of numerous flows of mafic composition and little explosive activity • Volume - Largest of all volcanoes ‣ Cinder Cone Volcanoes • Composed of pyroclastic materials that accumulate around the vent; steep slopes • Usually short-lived and may represent final Eruptive stages • Smallest ‣ Composite Volcanoes (Stratovolcano) • Alternating layers of pyroclastics and lava flows • High viscosity magma: Composition is intermediate, with andesite common • Eruptions are infrequent, violent, and may involve lahars (volcanic mud flow) ‣ Lava domes • High-viscosity, felsic magmas move slowly upward to form steep-sided lava domes • Violent eruption!! • Nuée ardent ‣ Maars • When suddenly heated, one cubic meter of water converts into 1,600 cubic meters of steam ⿞Not all eruptions build a volcano ‣ Fissure Eruptions and Basalt Plateaus • Columbia River basalt flowed from fissures to cover large areas in WA or OR. • Low viscosity, mafic lavas spread out and built up a basalt plateau ‣ Pyroclastic Sheet Deposits • Erupted from long fissures rather than a central vent • Fissures formed during the origin of calderas ⿞Yellowstone ⿞Crater Lake ⿞Long Valley Caldera (Bishop Tuff) ⿞Where are the volcanoes in the world? ‣ The circum-Pacific belt- about 60% ⿞Plate tectonics and the distribution of volcanoes and plutons ‣ Divergent Plate Boundaries • Mid-Oceean Ridge System ⿞Mafic lava - basalt ⿞Under water ‣ Pillow basalts ⿞Above sea level ‣ Iceland ⿞Rift Valley ‣ East African Rift Valley ‣ Convergent Plate Boundaries • Subduction under oceanic plate / continental plate • Intermediate to Felsic in composition • Aleutian Islands in Alaska; Japan / Andes Mountains in SA; Cascades in NA ‣ Intraplate Volcanism • Occurs as a plate moves over a stationary "hot spot" in the upper mantle ⿞Eruptive styles and landforms ‣ Volcanic craters • Bowl shred depression around the vent ⿞How BIG are volcanic eruptions? ‣ Eruptions are ranked by the volcanic explosivity index or VEI • Ranges from 0 (unexplosive) to 8 (explosive) • Based on volume of material explosively elected, height of eruption plums • Volume of lava, human and property damage are not considered ⿞Is it possible to predict eruptions? ‣ Volcanic monitoring • Physical and chemical changes ⿞Tiltmeters, seismic activity, past history ⿞Changes in magnetic and electrical fields ⿞Gas emissions, groundwater level, and temperature • While timely warnings have been issues in the past, volcanoes remain unpredictable and only a few are regularly monitored ‣ Predicting Eruptions • Satellite temperature • Observe escape of steam • Seismographs • Tiltmeter • Mount St. Helens ⿞Eruption May 18, 1980 after 123 years of silence ⿞Pyroclastic flow destroyed everything within 30km ⿞Everything within 12km covered with thick ash ⿞Trees up to 20km broke like matchsticks ⿞Tephra up to 25km altitude ⿞Death toll: 60 humans, 500 deer, 200 bears, 1500 elk, countless birds and small animals • North America's Volcanoes ⿞Alaska ⿞Cascade Range


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