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Volcanic Landforms I. Effusive eruptions: relatively quiet, non-explosive mostly basaltic lava, flows freely. A. Central Vent Eruptions— lava flows out (sometimes fountaining) from one central vent, then the lava solidifies in approximately the same volume all around. Shield volcano: a low, broad, cone-shaped structure - looks like a warrior’s shield Mauna Kea - 13,792 ft above sea level Mauna Loa - 13,678 ft above sea level Low angle slopes of 1-10 Both ~30,000 ft from their base Composed primarily of basalt lava flows Largest volcanoes in volume Volcanic Landforms Shield Volcanco Low angle slopes of 1-10 Composed primarily of basalt lava flows Volcanic Landforms Effusive eruptions – Shield volcanoes Volcanic Landforms Effusive eruptions – Shield volcanoes Volcanic Landforms B. Fissure Eruptions on Land— basalt may flow out of large cracks in the ground (fissures) Flood Basalts—large volume of very fluid basaltic lava may gush out at speeds of 25 miles per hour Columbia River Basalts (CRB) 170,000 km3 about 17-14 Mya Over 60 individual flows, covering some areas in over 2 km of basalt! One flow alone could pave I-90 from Seattle to Boston 575 feet deep!) Siberian Flood basalt 900,000 km3 about 245mya Lava Plateaus—thick plateaus of lava spreading over areas thousands of kilometers Fig. 7.18a W. W. Norton Volcanic Landforms B. Fissure Eruptions on Land: Flood Basalts basalt may flow out of large cracks in the ground (fissures) Volcanic Landforms B. Fissure Eruptions on Land: Flood Basalts Columbia River Basalt: basalt may flow out of large cracks in the ground (fissures) Volcanic Landforms II. Pyroclastic Eruptions: Explosive, involve viscous, gas-rich magma. The more gas-rich it is the higher the tephra column; less gas results in pyroclastic flows. A. Cinder Cones: formed by gas-rich lava of any composition (usually basaltic). Built of tephra that is remarkably vesicular (pumice to scoria) Generally short lived eruptions - weeks to a few years until the magma is degassed, then it solidifies in the pipe and flows form from the base After they’re done, they never erupt again! Paricutin, Mexico, cinder cone soon after its birth in 1943 in a Mexican cornfield. Smallest volcanic features have large craters with steep slopes of 30-40 Volcanic Landforms B. Composite Cone or Stratovolcano Volcanoes on continents over subduction zones Built up by alternating layers (lava and pyroclastic deposits) Steeper slopes 10-25 Izalco, El Salvador, December 1949. Cascades, Andes, Aleutian Islands Small steam eruption and a view of the older lava flow on the side of the cone in the foreground. Built over tens to hundreds of thousands of years Composite composite cone 2 1 Lava flow Lava flow 3 Blast cloud Pyroclastic flow Eruption on flank of upbuilding composite cone 4 Summit crater Volcanic neck Layers of lava flows & pyroclastic s Volcanic Landforms 1.Lava Dome Degassed magma may erupt in the crater and harden there without flowing anywhere Produces a plug in the volcanic vent which must be blown away before future eruptions can occur Traps gases inside so they build up pressure. 2. Calderas Energetic eruption, blasts out everything, then collapses Caldera A large amount of magma erupts explosively to form ash fall and ash flow deposits, partially emptying the underlying magma chamber. There is essentially a big "hole" the overlying rock collapses, leaving a depression. 09_10bc.jpg Volcanic Landforms 1.Calderas The following diagrams show the formation of Crater Lake during the climactic eruption of Mount Mazama. Eruption deposits airfall pumice and ash, blown by winds to north and east. Volcanic Landforms 1. Vent enlarges and eruption column collapses. 2. Pyroclastic flows deposit the Wineglass Welded Tuff on north and east flanks of Mt. Mazama Volcanic Landforms Caldera has been partly filled with pumice and ash from the eruption with blocks of rock from the caldera walls Weak, dying explosions within the caldera deposit ash on the caldera rim Pyroclastic-flow deposits develop fumaroles and gradually cool. Volcanic Landforms Crater Lake today Volcanic Landforms C. Ash-flow eruptions Eruption not from a cone, felsic magma 1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock. 2. Creates ring fractures over the bulge. 3. May collapse, magma forced into fractures and erupts Forms large calderas Largest and most devastating eruptions in history Volcanic Landforms C. Ash-flow eruptions Eruption not from a cone, felsic magma 1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock. 2. Creates ring fractures over the bulge. 3. May collapse, magma forced into fractures and erupts Forms large calderas Largest and most devastating eruptions in history Volcanic Landforms C. Ash-flow eruptions Eruption not from a cone, felsic magma 1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock. 2. Creates ring fractures over the bulge. 3. May collapse, magma forced into fractures and erupts Forms large calderas Largest and most devastating eruptions in history Volcanic Landforms B. Composite Cone or Stratovolcano Mt. St Helens pyroclastic eruption on the volcano flank. Lateral blast 09_14c.jpg 09_15abc.jpg 09_15d.jpg 09_15e.jpg Volcanic Landforms C. Ash-flow eruptions Eruption not from a cone, felsic magma 1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock. 2. Creates ring fractures over the bulge. 3. May collapse, magma forced into fractures and erupts Forms large calderas Largest and most devastating eruptions in history Volcanic Landforms C. Ash-flow eruptions Examples: Toba, Indonesia 75,000 years ago Caldera is 30 x 60 miles Covered 10,000 square miles!—1000 feet thick! Yellowstone 3 major eruptions in last 2 million years Approximately 1000 times larger than Mt. St. Helens! New felsic magma may be pooling, thermal features are heated by the magma Volcanic Landforms The major eruptions of the volcanic field were exceedingly voluminous, but their products are only surficial expressions of the emplacement of a batholithic volume of rhyolitic magma to high crustal levels in several episodes. The total volume of magma erupted from the Yellowstone Plateau volcanic field since 2.5 million years ago probably approaches 6,000 cubic kilometers. Volcanic Landforms C. Ash-flow eruptions Examples: Long Valley caldera near Mammoth Lakes ski resort in California, north of Bishop, CA Last erupturion 700,000 years ago Over the past 20 years the floor has risen 9 inches Magma recently risen from 5 miles depth to 2 miles Eruption very likely, but timing not certain Volcanic Landforms C. Ash-flow eruptions Examples: Long Valley caldera near Mammoth Lakes ski resort in California, north of Bishop, CA Last erupturion 700,000 years ago Over the past 20 years the floor has risen 9 inches Magma recently risen from 5 miles depth to 2 miles Eruption very likely, but timing not certain Volcanic Landforms Volcanic Landforms Imagine the effects of a large caldera forming eruption Ash covering the US! Clogs all air filters, engines no cars, no electricity, not air travel Abrades all moving parts (ash + water is very heavy building collapses Centimeters of ash on all crops crop failure and famine Volcanic Landforms Imagine the effects of a large caldera forming eruption Ash covering the US! Clogs all air filters, engines no cars, no electricity, not air travel Abrades all moving parts (ash + water is very heavy building collapses Centimeters of ash on all crops crop failure and famine Volcanic Landforms Shield volcano (e.g. Hawaii) 9 km 150 km Composite volcano (e.g. Vesuvius) p.194-195c 3 km 15 km original artwork by Gary Hincks Cinder cone (e.g. Sunset crater) 0.3 km 1.5 km Non-violent vs. explosive eruptions Basalt: flows onto the surface Andesite/Rhyolite: explode, huge eruptive clouds Depends on the viscosity of the lava- resistance of lava to flow (water vs. molasses) Viscosity is controlled by: Santa Maria, Guatemala. Santa Maria had a huge eruption in 1902, from a vent on the other side of the cone as viewed from this direction. The 1902 eruption was not from the summit. Starting in 1929, a lava dome began to grow in the 1902 crater, and it is still active today. It is named Santiaguito. 1) silica content 2) temperature 3) gas content Basaltic lava Basaltic lava Silica content As silica tetrahedra bond together they make the magma thicker, or more viscous. Similar to slushy as ice bonds start to form. Slushy is thicker and more viscous than water. Thus more silica = more viscous Mafic magmas = <50% silica Intermediate magmas >60-65% (Andesite) Sarigan volcano, Northern Mariana volcanoes. It has not had any recorded eruptions but it is very young. That is just a regular cloud over its summit. Felsic magma’s = >65% (Ryolite). Felsic (granitic) magmas = more viscous based on silica content. Temperature The hotter a liquid is the less viscous it is. Example: heating honey or molasses to make them flow more readily. Breaks bonds. Temperatures required to melt the minerals in mafic vs. felsic magmas. Lascar Volcano, Chile, The most active stratovolcano in the central Andes. Note the two massive andesite flows exhibiting thick flow margins tens of meters high and well-developed lava levees. Courtesy of Peter Francis. Minerals in mafic magmas have higher melting temperatures- therefore the magmas must exist at higher temperatures. Minerals in felsic (granitic) magmas have lower melting temperatures- cooler, thus more viscous. Gas Content Gases in magmas = mostly water vapor, also SO2, H2S, CO2, HCl, … If the magma has a low viscosity (e.g., basaltic magmas) the gases can escape easily. Colima Volcano, Mexico. Thick, short andesite flow on the flanks of Colima. Courtesy of J.C. Gavilanes, Universidad de Colima. If they magma has a high viscosity the gases are trapped, they build up until they explode (felsic magmas) Buoyancy There are several factors that are important in allowing magma to move to the surface and erupt as a volcano. The first is buoyancy. Buoyancy is the tendency for a less dense substance to move up or float. Pacaya, Guatemala is a volcanic complex of two small strato-volcano cones and older lava domes. It has erupted over twenty-two times since its birth in 1565 and nearly annually since 1965. Generally, a liquid is less dense than a solid of similar chemical composition. Buoyancy Since magma is liquid rock, surrounded by solid rock, the magma will tend to move up through the crust toward the surface. As a magma is moving toward the surface, it is moving into cooler areas of the crust (geothermal gradient) Pacaya, Guatemala. Eruptions are generally characterized by explosions, but recent eruptions have also produced lava flows. Here, an ash eruption shortly after the February 4, 1976, magnitude 7.5 earthquake. Begins to cool down. Buoyancy What happens to magma when it cools down? It begins to crystallize. If the magma gets to about 50% crystallized, it will stop moving up. ( “crystal mush”). All those crystals make the liquid too sluggish to flow very easily, and it simply stops moving. Therefore, whether or not a magma makes it to the surface is really a race between how fast it moves up and how fast it crystallize The hotter a magma starts out, the more likely it is to get to the surface before it has reached that 50% crystallization. Eruption Analogy to soda bottle: When soda can is closed and the soda is under pressure the carbon dioxide (CO2) is dissolved in the soda. Open the bottle, release the pressure, the carbon dioxide comes out of solution and bubbles out. Gas expands and escapes. Can happen slowly, controlled, or violently. Magmas generally have a high gas content. Like soda, the gas is dissolved within the magma when the magma is under pressure. Pressure builds up until there is an eruption, this releases the pressure. Again, gases expand and escape. Obsidian flow, Long Valley Caldera, California If gases escape easily and gradually (non-viscous) it is a non-violent eruption (basalt), If gases can't escape easily and gradually it results in a violent eruption (felsic magma). Magmas generally have a high gas content. Like soda, the gas is dissolved within the magma when the magma is under pressure. Gas bubbles and froth on surface of the lava, similar to bubbles on top of soda. Obsidian flow, Long Valley Caldera, California, was created by crustal collapse associated with an explosive eruption about 650,000 years ago. Since that time, felsic eruptions of degassed magma have generated viscous rhyolitic domes and short felsic flows. Produces distinctive texture in the rock. Types of explosive volcanoes: Composite or stratovolcanoes these types of eruptions, which often alternate with more effusive eruptions, produce composite or stratovolcanoes. Very steep sided volcanoes that are characterized by interbedded or alternating deposits that result from explosive (pyroclastic rocks) and effusive eruptions (lavas). 09_29.jpg 09_30a.jpg 09_30b.jpg 09_View.jpg