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Chapter 5Contrast what you would see at a volcano that is erupting lava flows, vs. one that is releasing py-roclastic debris. What sorts of hazards do these different eruption types pose to people near or far from the volcano? How do the effects vary with mafic-intermediate-felsic nature of a magma?· Lava flows – magma viscosity influences flow velocity, so the area covered by a lava flow depends both on the volume of magma that erupts, and its viscosity. A very viscousmagma won’t expand very far from the vent, so that flow would be short and very thick, compared to the same volume of low-viscosity magma. Contrast basalt, andesite and rhyolite viscosities (which is most viscous? Least? In-between?) – so, would basalt be more likely to form a shield volcano, for example? How does columnar jointing form – what does it look like? (See the photos in Figure 5.3.) If you see an outcrop of pillow basalt – what does that tell you about the environment where the basalt erupted? You cansee a very viscous, rhyolite “dome” that didn’t travel very far from the vent, in Figure 5.4.· Volcaniclastic Deposits – “Pyroclastic” means fragmental (clastic) rock that formed during a volcanic (pyro) eruption. Volcaniclastic is another word that is sometimes used – clastic rock (fragmental) where the clasts (particles) are all or mainly volcanic. Ex-panding gas bubbles drive a rising column of bubble-rich magma (most often, andesite orrhyolite but sometimes basalt has enough dissolved gas to do this also) up to the surface, and the energy of the expanding gas bubbles can lift small-to-large fragments of magma into the air, where they cool, and land back on the earth. Tiny particles of frozen glass are “volcanic ash”; thumbnail-size clasts are “lapilli”; and larger clasts than lapilli are “blocks” (if angular) or “bombs” (if rounded or flattened). Large clasts won’t fly very farfrom the vent before falling to earth, whereas tiny clasts can either land near the vent, or very far away, depending on how high they are driven in the atmosphere by the eruption updraft, and which way the wind might be blowing on the day of the eruption. Figure 5.6 shows the difference between an ash fall vs. an ash flow (nuee ardente) – a glowing cloud of ash that travels downslope. If the glass particles in an ashflow are hot enough tore-weld together, they will form a rock (rather than a layer of loose sandy ash), called a welded tuff, or ignimbrite. If rain falls on loose ash high on steep slopes of a volcano, a muddy mass of water and ash can race down slope, called a “lahar” (mudflow).· Volcanic gas is important in understanding why we have pyroclastic eruptions.Fissure eruption- crack that makes lava flowsCrater- circular depression at top of volcanic edifice caused as material accumulates around sum-mit ventCaldera- depression filled with ash caused by sudden draining of magma chamberCinder cone- cone-shaped piles of basaltic lapilli and blocksShield volcano- broad gentle domes caused when eruption has low viscosity (weak, so spreads out)Stratovolcano- large, steep, cone-shaped (Mt. Fuji)Effusive eruptions-relatively low gas content, can be any magma type, as long as there isn’t a lot of dissolved gas. Sometimes a volcano will start with explosive, pyroclastic activity for a few days or weeks, and later, if there is still magma to erupt but the gas pressure was reduced by the initial pyroclastic eruptions, “quiet” effusion of lava of the same chemistry (minus the gas) can flow sedately down from the crater, covering some of the pyroclastic layers that were deposited earlier in the eruption.Explosive eruptions-pyroclastic activity generally goes with intermediate to felsic magma chem-istry, though basalt volcanoes can generate large volumes of volcanic ashIsland arc- convergent plate boundary, as least one oceanic plate at a subduction zoneContinental arc- subduction zone with one continental plateRift- either ocean ridge or continental driftcontinental hot spot- Yellowstoneoceanic hot spot- forms on oceanic lithosphere, basaltic magma erupts at sea floor, volcano grows and becomes island, basalt flows pile uplava flows- basaltic lavapyroclastic flows-move especially fastash falls- during a pyroclastic eruption, buries crops, spreads toxic chemicalsdirected blast- some volcanos, like Mt. St. Helens, explode sideways (like a bomb explosion)landslides- trigger landslides. Debris consists of ash and solidified lavalahars- ash with water resembles wet concrete. Denser and more viscous than watervolcanic gases- carbon dioxide and sulfur dioxide*Active vs. dormant vs. extinct volcanoes- examine historical record, age of erupted rocks, tec-tonically active area, landscape character*Predicting eruptions- change in shape (magma fills magma chamber, pushing outwards, causingsurface to bulge) or increase in gas and steam emission (gas bubbles out of magma and moves upthrough cracks, indicating that magma has entered the ground below)*Dust or gas (Sulfur oxides, etc.) in the atmosphere can block sunlight from getting through the atmosphere to warm the air or the surface. Following a large pyroclastic eruption, the earth’s cli-mate would be colder because it keeps energy from reaching


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NU HONR 1206 - Chapter 5

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