Chapter 5 Contrast 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 viscous magma 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 can see 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 or rhyolite 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 far from 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 to re 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 flows Crater circular depression at top of volcanic edifice caused as material accumulates around sum mit vent Caldera depression filled with ash caused by sudden draining of magma chamber Cinder cone cone shaped piles of basaltic lapilli and blocks Shield 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 ash Island arc convergent plate boundary as least one oceanic plate at a subduction zone Continental arc subduction zone with one continental plate Rift either ocean ridge or continental drift continental hot spot Yellowstone oceanic hot spot forms on oceanic lithosphere basaltic magma erupts at sea floor volcano grows and becomes island basalt flows pile up lava flows basaltic lava pyroclastic flows move especially fast ash falls during a pyroclastic eruption buries crops spreads toxic chemicals directed blast some volcanos like Mt St Helens explode sideways like a bomb explosion landslides trigger landslides Debris consists of ash and solidified lava lahars ash with water resembles wet concrete Denser and more viscous than water volcanic 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 causing surface to bulge or increase in gas and steam emission gas bubbles out of magma and moves up through 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 Earth