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I. Effusive eruptions: relatively quiet, non-explosive mostly basaltic lava, flows freely. A. Central Vent Eruptions— lava flows out (sometimes fountaining)

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Presentation on theme: "I. Effusive eruptions: relatively quiet, non-explosive mostly basaltic lava, flows freely. A. Central Vent Eruptions— lava flows out (sometimes fountaining)"— Presentation transcript:

1 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 Low angle slopes of 1-10  Composed primarily of basalt lava flows Largest volcanoes in volume Volcanic Landforms Mauna Kea - 13,792 ft above sea level Mauna Loa - 13,678 ft above sea level Both ~30,000 ft from their base

2 Volcanic Landforms Shield Volcanco Low angle slopes of 1-10  Composed primarily of basalt lava flows

3 Volcanic Landforms Effusive eruptions – Shield volcanoes

4 Volcanic Landforms Effusive eruptions – Shield volcanoes

5 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 km 3 about 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 Volcanic Landforms

6 Fig. 7.18a W. W. Norton

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8 B. Fissure Eruptions on Land: Flood Basalts basalt may flow out of large cracks in the ground (fissures) Volcanic Landforms

9 B. Fissure Eruptions on Land: Flood Basalts Columbia River Basalt: basalt may flow out of large cracks in the ground (fissures) Volcanic Landforms

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11 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! Smallest volcanic features have large craters with steep slopes of  Volcanic Landforms Paricutin, Mexico, cinder cone soon after its birth in 1943 in a Mexican cornfield.

12 B. Composite Cone or Stratovolcano Volcanoes on continents over subduction zones Built up by alternating layers (lava and pyroclastic deposits) Steeper slopes  Cascades, Andes, Aleutian Islands Built over tens to hundreds of thousands of years Volcanic Landforms Izalco, El Salvador, December Small steam eruption and a view of the older lava flow on the side of the cone in the foreground.

13 Composite composite cone Lava flow Blast cloud Summit crater Volcanic neck Layers of lava flows & pyroclastic s Lava flow Pyroclastic flow Eruption on flank of upbuilding composite cone

14 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 Volcanic Landforms

15 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.

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22 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

23 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

24 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

25 Crater Lake today Volcanic Landforms

26 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

27 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

28 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

29 B. Composite Cone or Stratovolcano Mt. St Helens pyroclastic eruption on the volcano flank. Lateral blast

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34 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

35 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

36 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.

37 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

38 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

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40 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

41 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

42 p c original artwork by Gary Hincks 9 km 3 km 0.3 km 1.5 km 15 km 150 km Shield volcano (e.g. Hawaii) Composite volcano (e.g. Vesuvius) Cinder cone (e.g. Sunset crater) Volcanic Landforms

43 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: 1) silica content 2) temperature 3) gas content 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.

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46 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) Felsic magma’s = >65% (Ryolite). Felsic (granitic) magmas = more viscous based on silica content. 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.

47 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. 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. 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.

48 Gas Content Gases in magmas = mostly water vapor, also SO 2, H 2 S, CO 2, HCl, … If the magma has a low viscosity (e.g., basaltic magmas) the gases can escape easily. If they magma has a high viscosity the gases are trapped, they build up until they explode (felsic magmas) Colima Volcano, Mexico. Thick, short andesite flow on the flanks of Colima. Courtesy of J.C. Gavilanes, Universidad de Colima.

49 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. Generally, a liquid is less dense than a solid of similar chemical composition. 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.

50 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) Begins to cool down. 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.

51 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.

52 Eruption Analogy to soda bottle: When soda can is closed and the soda is under pressure the carbon dioxide (CO 2 ) 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.

53 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. 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). Obsidian flow, Long Valley Caldera, California

54 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. Produces distinctive texture in the rock. 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 de- gassed magma have generated viscous rhyolitic domes and short felsic flows.

55 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).

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