Section 9.2 Volcanic Eruptions

“I was walking along a path with two friends – the sun was setting - suddenly the sky turned blood red - I paused, feeling exhausted, and leaned on the fence - there was blood and tongues of fire above the blue-black fjord and the city - my friends walked on, and I stood there trembling with anxiety - and I sensed an infinite scream passing through nature." Edvard Munch

An agonized figure wails against a blood red Norwegian skyline in Edvard Munch's “The Scream” (1893) in the National Gallery, Oslo, Norway The 1883 eruption of the volcano Krakatoa in Indonesia put so much ash into the atmosphere, it caused unusually intense sunsets throughout Europe in the winter of 1883-4, which Munch captured in his painting.

What FACTORS determine if a volcano erupts violently or quietly?

Quiet eruptions Hawaii & Iceland Pahoa Lava Flow https://www.youtube.com/watch?v=fmCJSS2YAP0

Explosive, violent eruption Destructive landslide associated with 1980 Mt. St. Helen's eruption Krakatoa eruption 2007

Before the 1980 eruption After the VIOLENT eruption The summit and side has been blown away.

Bell-work Pyroclastics consist of …… Cinders Lapilli Glass Ash Cinders Lapilli Glass Pumice/Obsidian Bombs Blocks Tuff (welded ash)

Affiliated with Violent Explosions Lahar: is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. (Volcano peaks often have ice and snow that instantly melt when met with the heat of an erupting volcano.) PYROCLASTIC FLOW: a dense, destructive mass of very hot ash, lava fragments, and gases ejected explosively from a volcano and typically flowing downslope at great speed.

Felsic magma Mafec* magma Factors that affect volcanic eruptions: Pg 121 Felsic magma Mafec* magma Feldspar, silica magnesium and iron Expolsivity depends on the amount of silica and gas in the magma.

So thick it acts like a “plug” to the volcano… FELSIC magma Light colored minerals: Quartz, orthoclase Feldspar More than 60% silica Lower melting point= colder temperatures (“thick”) HIGH viscosity MAFIC magma Dark colored minerals: Magnesium and iron Fe Olivine, pyroxene, Hornblende, augite, biotite Less than 50% silica Higher melting point= hotter temperatures (“thin”) LOW viscosity So thick it acts like a “plug” to the volcano… “Thin” enough that it allows the magma to move more freely…

Volatiles refers to the volatile components of magma [mostly water vapor and carbon dioxide] that affect the appearance and explosivity of volcanoes. Volatiles in a magma with a high viscosity, generally felsic with a higher silica (SiO2) content, tend to produce eruptions that are explosive. Volatiles in a magma with a low viscosity, generally mafic with a lower silica content, tend to vent and can give rise to a lava fountain.

Volatiles Continued: the effect of trapped gases How explosive a volcano will be is dependent on the gases present as well How much gas is present If a lot of gas is present the volcano is more likely to be more explosive How easily the gas can escape If the gases cannot escape easily, the volcano is more likely to be more explosive

What happens to bring magma to the surface? How does the type of magma affect the surrounding pressure? (remember gas is a volatile that affects the reactivity of magma) Volcanic eruptions and what causes them Build up of pressure occurs more frequently with thicker magma (felsic.)….. EXPLOSIVE ERUPTION

Volcanic Explosivity Index How to measure a volcanic eruption There is the Fajita scale for tornadoes, the Safiir-Simpson scale for hurricanes, Richter scale for earthquakes, and the VEI scale for volcanoes

Paroxysmal a sudden outburst of a violent eruption an increase of periodic eruptions An eruption with a (V.E.I.) Volcanic Explosivity Index of 5 (on a scale of 0-8)

Lava Vocabulary Quiz 9.2 pyroclastic Hot, erupted glass, ash, and rock fragments. ________________ All lava flows are this type of rock. _____________________ 3. A basaltic lava that results in smooth, ropelike surfaces: ____________ Magma has a high viscosity if it contains over 60% of ________ If the magma has a high gas content, the eruptions tend to be ________ Because of their high silica content, this rock is light colored. ________ viscosity lava aa pahoehoe pillow lava pyroclastic ash silica cinders igneous metamorphic sedimentary non-explosive mafic explosive felsic igneous pahoehoe silica explosive felsic