Overview of Ch. 4 Volcanoes and Other Igneous Activity Chapter 4 or 5 I. Nature of Volcanic Eruptions II. Materials Extruded from a Volcano III.Types of Volcanoes IV.Volcanic Landforms V. Plutonic (intrusive) Activity VI.Plate Tectonic Cause of Volcanism I. The nature of volcanic eruptions A. Factors determining the violence or explosiveness of a volcanic eruption Composition of the magma More felsic = more violent eruption B. Viscosity of magma. Viscosity is the resistance to flow of a substance (e.g., cold maple syrup has a high viscosity) More silica content (and less Fe+Mg) then the higher the viscosity of the magma I. The nature of volcanic eruptions C. Implication of silica content and viscosity to explosiveness 1) Less viscous mafic lavas (basaltic) generally produce quiet eruptions, like at Hawaiian-type eruptions. 2) Highly viscous intermediate and felsic lavas (rhyolite or andesite) produce more explosive eruptions (Mount St. Helens) Which image is represents a volcano with a felsic composition? II. Materials extruded from a volcano A. Lava flows 1) Mafic lavas are less viscous and make basalt flows, such as Pahoehoe lava (resembles a twisted or ropey texture) Pillow Basalt basalt forms under ocean water 2) Felsic and intermediate lavas make rhyolite and andesite deposits more viscous, not flow very far 1
II. Materials extruded from a volcano B. Dissolved gases mostly water vapor, carbon dioxide, and sulfur dioxide. (Mammoth Mountain story) II. Materials extruded from a volcano C. Pyroclastic Flows Hot gases Ash dust of glassy fragments Pumice porous rock from frothy lava III. Types of Volcanoes A. General features of volcanoes 1) Opening at the summit of a volcano Crater small, steep-walled depression at the summit, Caldera larger, summit depression, often produced by collapse following a massive eruption 2) Vent opening connected to the magma chamber via a pipe. Magma in pipe can crystallize to a volcanic neck. 3) Magma chamber large pool of magma under volcano, where lava reaches the surface via pipe 4) Parasitic cone smaller cone growing on shoulder of larger cone. III. Types of Volcanoes B. Types of volcanoes Watch Volcano types first click here 1) Shield volcano (Ex. Volcanoes in Hawaii) Broad, slightly dome-shaped Made of basaltic lava Cover large areas Produced by mild eruptions of large volumes of lava Figure 4.12 III. Types of Volcanoes 2) Cinder cone Made of ejected lava (mainly cinder-sized) fragments Very steep slope angle small size 2
Sunset Crater A cinder cone near Flagstaff, Arizona III. Volcanoes 3) Composite cone (stratovolcano) Located along Pacific Ring of Fire due to subduction (ex. Mount St. Helens, Shasta) Large, high elevation (thousands of feet high and several miles wide at base) Steep sides Made of alternating layers of lava flows and pyroclastic debris Explosive, potentially damaging eruptions Copyright 2006 Pearson Prentice Hall, Inc. A composite volcano III.Types of Volcanoes 4) Some other types a) Caldera large wide crater in ground, very explosive, felsic magma, formed by collapse after rapid emptying of magma chamber Ex. Long Valley Caldera, Crater Lake, Yellowstone Caldera b) Fissure eruptions and lava plateaus Low viscosity mafic lava extruded from fractures in crust to make broad plateaus of flood basalts. Ex. Columbia River Plateau Caldera - Formation of Crater Lake, Oregon Click here for animation of Crater Lake Caldera Formation IV. Volcanic Landforms and Plutonic Activity A. Volcanic Landforms 1) Types of intrusive igneous features Dike sheet like, discordant (perpendicular to sedimentary layers typically vertical) Sill sheet like, concordant (parallel to sedimentary layers typically horizontal) Volcanic necks (e.g., Ship Rock, New Mexico) are resistant pipe left standing after erosion has removed the volcanic cone 3
Intrusive igneous structures exposed by erosion IV. Volcanic Landforms and Plutonic Activity 2) Plutonic landforms: Batholith Largest intrusive body Surface exposure of over 100 square kilometers (smaller bodies are termed stocks) Frequently form the cores of mountains, like Sierra Nevada Batholith V. Plate tectonics and igneous activity A. Where are volcanoes located? 1. subduction zones, Especially around the Pacific Ring of Fire. 2. spreading centers (i.e. mid-oceanic ridge), 3. hotspots V. Plate tectonics and igneous activity B. Igneous activity along plate boundaries 1) Spreading centers (like Mid-oceanic ridge) The greatest volume of volcanic rock is produced along the mid-oceanic ridge How?» Lithosphere pulls apart due to mantle convection» As ultramafic mantle rocks rise, pressure is decreased» This causes partial melting of mantle by decompression melting» Which makes a lot of basaltic magma V. Plate tectonics and igneous activity Divergent boundaries with basaltic rocks and magma is found at 1) mid-oceanic ridges on oceanic crust, 2) spreading on the continental crust such as the Great African Rift Valley. 2) Subduction zones Found at deep oceanic trenches Descending plate adds water vapor to overlying mantle which partially melts by wet melting to make mafic magma Mafic magma slowly moves up into lithosphere, which it evolves into more felsic magma via magmatic differentiation (i.e. crystal settling etc). Two types (next page) 4
V. Plate tectonics and igneous activity Two types: a) oceanic oceanic subduction - Rising magma stays mostly mafic and makes an island arc of mafic shield volcanoes - land form is called an volcanic island arc. b) oceanic continental subduction rising mafic magma evolves into more intermediate and felsic magma in the continental crust making composite volcanoes on surface, granite batholiths deep in crust. Landform is called a continental volcanic arc. Volcanic Island Arc oceanic-oceanic subduction Continental Volcanic Arc oceaniccontinental subduction Animation similar to this image. 3) Hot Spots - Intraplate volcanism Activity within a tectonic plate Formed by rising isolated mantle plumes Forms localized volcanic regions in the overriding plate called a hot spot Makes mafic lava when under oceanic crust shield volcanoes in Hawaii Also, makes felsic magma when under continental crust like Yellowstone NPP 5
Volcanism on a tectonic plate moving over a hot spot Figure 4.35 6