Geology: Chapter 5

Cinder Cone

Smallest and most numerous Made of pyroclastic material of all sizes The pyroclastic materials is known as tephra Bombs are blobs of still molten lava that assumes an aerodynamic shape as it flies

Gases from Volcanoes

Sulfur dioxide Forms sulfuric acid in water Corrosive to metals Known as "vog"

Volcanoe Advantages and Disadvantages

Advantages Natural beauty Recreation Lend nutrients to soil Disadvantages Unpredicatable eruptions endanger humans Temporarily disrupt environment Climate issues Unpredictable slopes/mudflows/lahar

Andesite Line

Andesite is an intermediate igneous rock between the mafic basalts and felsic granites and rhyolites Note: Andes mountains are largely Andesite This line divides mostly mafic materials and felsic materials Explosive volcanoes are found on the felsic side, less explosive, gentle volcanoes on the mafic side

Stratovolcano

Builds in layers/multiple flows Thousands of feet high Many miles across at base Upper slopes of pyroclastics Clastics solidified from ejected lava and ash Lower slopes layered lava flows alternating with pyroclastics Mt. Rainier, Mt. Hood, Mt. Fuji, Vesuvius, etc As magmas rise under the future stratovolcano it melts felsic rock which contributes to the explosivity of the magma/lava Stratovolcanoes represent the most direct threat Crater Lake a former volcano that exploded Crater Lake is a caldera that remains after such an event

Where can we find volcanoes?

Caribbean area Twice as many dry land volcanoes north of the equator

Hazards of Volcanic Erruptions

Direct explosion Lava flows Burn everything in its path Ashfalls Pyroclastic flows- Mixtures of hot gases pyroclastic materials that travel with great velocity Lahars- Water mobilizes debris on the side of a volcano, a thick mush of rock, ash, and cinders Tsunami- Wave caused by the displacement of crust in the ocean, or a landslide

Mitigation and Prediction

Diversion of flows Mixed effectiveness Understanding zones of hazards for planning purposes Prediction is more like forecasting, being able to state a probability of an event in a given timeframe based on history and: Tilting of earth Increased seismicity Gravity anomalies

Felsic versus Mafic

Eruptions involving mafic magma tend to be much less violent because: High temperature Low viscosity Low gas content Eruptions involving felsic magma tend to be more violent Low temperature High viscosity Higher gas content

Lava Dome

Expansion from within Lava builds up around the vent because it is too thick to flow far The rising magma mass causes the earth to bulge One formed in the middle of Mt. St. Helens

Phreatic Eruption

Explosive interaction of magma with underground water Produces a lake called a "maar"

Benefits of Volcanic Erruptions

Gave us atmospheric gas in early earth Produces useful materials/nutrients/ore Geothermal energy

Continental flood basalts

Mantle plume makes its way to the crust Flows in continental regions over large areas These extensive flows are possible due to the low viscosity of the magma/lava "Traps" Swedish word for "staircase" Deccan traps in India, Siberian traps

Explosive Erruptions

More viscous magma More felsic, more viscous, sometimes more gas Gets stuck on the way through a vent Pressure builds to explosive levels

Where can we find volcanoes?

Most along tectonic plate boundaries Especially subduction zones Hot spots - Hawaii/Yellowstone Caldera Of the 1350 volcanoes that have erupted in the last 10,000 years 900 have been around the Pacific Rim Also Mediterranean - Vesuvius, Etna, Stromboli

Risk of Volcanoes

No significant threat east of New Mexico There is risk of transported ash and ejecta Lots of activity in the vicinity of the Cascadia Subduction zone in the northwestern portion of the United States Continues south along the Sierras in California

Climate

Particulates blown into upper atmosphere effective in reducing incoming solar radiation Residence time, depending on size of grains, is between 1 - 2 years Dust Veil Index Mt. St. Helens, DVI of 1 had little impact Mt. Tambora, DVI of 1500 caused a year without summer in 1816

Effusive Erruptions

Shield volcanoes Outpouring of low-silica (basaltic) lavas from a central conduit Gently convex upward Hawaii, Galapagos Hawaii "much spreading

Magma and Lava Characteristics

Viscosity - how thick or thin the lava/magma is Low viscosity is runny and flows easily High viscosity is thick and sticky Related to temperature High temperature magmas tend to flow easier Low temperature magmas tend to have more resistance to flow Composition Felsic (light quartz, feldspar containing) magmas tend to be more viscous, or stickier Due to the silica tetrahedra Mafic (darker iron, magnesium containing) More compact molecular structure makes for easier Entrained gases Magma/lava that contain more gases tend to be more explosive Lower viscosity, mafic magmas tend to contain less gas than do felsic, highly viscous magmas

Volcanoes Explosivity

Volcano Explosivity Index or VEI Ranges from 0-8 Based on height and volume of ejecta and the duration of the