Geology 101 Outline of Last Lecture Igneous Textures Common Igneous Rocks Generating Magma from Rocks Evolution of Magma Other Volcanic Landforms Intrusive Igneous Activity Outline of Current Lecture 1980 Mount St Helen s Eruption Nature of Eruptions Materials Extruded from a Volcano General Features of a Volcano Types of Volcanoes Other Volcanic Landforms Volcanic Eruptions and Plate Tectonics and Igneous Activity 1980 Mount St Helen s Eruption o Once had a peak and now is a crater Height reduced a lot 1000 m Peak collapsed Northside o South part of Washington o Process March 20th intrusion of magma generates tremors earthquakes 1 2 on earthquake scale Can predicate eruptions based of the movement of magma April 23 continued intrusion of magma produces bulge Flat side showed the bulge May 18th Earthquake causes collapse of north flank of volcano triggering lateral blast o North side th May 18 Lateral blast devastates 400 sq km Full vertical eruption develops The nature of volcanic Eruptions o Erupt explosively Mount St Helen or relatively quiet Kilauea Magma viscosity More viscous more difficult to flow more explosive Geology 101 o Factors determining magma viscosity Composition higher silica content higher viscosity Temperature hotter magma are less viscous Dissolved volatiles gases the violence is related to how easily gases escape from magma More difficult more explosive Materials Extruded from a Volcano o Lave flow aa Flow rough surface sharp edges Pahoehoe flow smooth surface twisted rope braids Lava tube facilitate lava to flow great distance Pillow lavas on ocean floor tube like o Gases Water vapor main CO2 Sulfur o Pyroclastic materials Hawaii doesn t have this Fire fragments Ash and dust o Fine glassy Cinders o Pea sized Lapilli o Walnut size o coarse Particles larger that lapilli o Blocks Hardened or cooled lava o Bombs Ejected as hot lava Pumice o Porous rock from frothy lava General features of a Volcano o Openings at summit of a volcano Crater a deep walled depression less than 1 km in diameter Caldera depression typically greater than 1 km in diameter produced by collapse following a massive eruption Vent an opening connected to the magma chamber via a pipe Types of Volcanoes o Shield Broad slightly domed shape Geology 101 o o o o Cinder Primarily basaltic lava Covers large areas Mild eruptions of large volumes of lava Mauna Loa in Hawaii Ejected lava Cinder sized fragments Steep slope angle Small size Frequently occur in groups Central vent filled with rock fragment Typically has crater Composite cone stratovolcano Most adjacent to Pacific Ocean Mt Fujiyama and Mt St Helen High Thousands of feet high and several miles wide at base Interbedded lava flows and layers of Pyroclastic debris Mt Shasta CA Comparison of the 3 Shield much larger than other 2 taller Composite cone taller and wider than cinder cone Cinder cone is smallest Animation Cinder Formed by accumulation of cinder lapilli and bombs 30 35 degree angle Don t grow larger 600 m high Only erupt once o Can last a decade Shield Formed by Lava flows o Basaltic Wide Gentle slope 15 degrees 300 10000 m high Repeated eruptions o Makes high peak Composite Many eruptions build it Less fluid shorter distance Slope angle greater than 25 degrees 3000 m high Repeated eruptions o Makes high peak Geology 101 Other Volcanic Landforms o Calderas Mainly caused by collapse into magma chamber If magma released isn t immediately replaced Large circular elliptical Few hour to a few day Pyroclastic materials Loses support Common for a lake to develop Crater lake o Oregon o Fissure eruptions and lava plateaus Fluid basaltic lava extruded from crustal fractures Columbia river plateau o Lava domes Silica rich felsic lavas viscous o Volcanic pipes and necks Pipes Short conduits connecting a magma chamber to surface Neck Resistant pipes left standing after erosion has remove the volcanic cone o Shiprock NM Plate tectonics and Igneous activity o Global distribution of igneous activity is not random Most volcanoes are associated with plate boundaries Basaltic rocks are common in both oceanic and continental settings whereas granitic rocks are rare in oceans Ring of Fire Plate boundaries associated with volcanic eruption Transform Convergent o Subduction The change from convergent to transform o Igneous activity along divergent plate boundaries MOR Greatest volume of volcanic rock Magma formation here Lithosphere pulls apart Decompressional melting Large quantities of basaltic magma are produced o Igneous activity along convergent boundaries subduction zones Rising magma can form either An island arc if in the ocean A volcanic arc if on a continental margin Associated with pacific ocean basing Region around the margin is known as the Ring of Fire Most of the worlds explosive volcanoes are here Geology 101 o o Intraplate volcanism Activity within a tectonic plate Associated with plumes of heat in mantle Form localized volcanic regions n the overriding plate called a hot spot Basaltic magma sources in oceanic crust o Hawaii and Iceland Granitic magma sources in continental crust o Yellowstone National park Animation Continental rifting Extdend and pulled apart until large crust falls apart makes rift valley magma rises up producing volcanoes and lava flows additional spreading continues the down dropping make ocean basins Continental volcanic arcs Subduction Magma is produced by partial melting Magma rises and some of it cools chain of volcanic peaks Resistant to flow Volcanic island arcs Chain of volcanic island Develops along trench Tall volcanoes o Explosive eruptions Mantle plumes Circular column of magma that rises When comes to surface forms hot spots Mantle plume stationary but plate moves making chain