Volcanoes Windows Into the Earth

Volcano – a mountain formed by lava & pyroclastics Crater – opening at the top of a volcano Caldera – summit depression >1 km-usually caused by collapsed magma chamber Important Vocabulary

Active – erupted in recent history (within the last few 100 years) Dormant – no recent eruptions (not within the past several 100 years), not eroded or worn down Extinct – no recent eruptions (not within the past 1000 years) and eroded

Where are all the Volcanoes? 1,900 volcanoes are active today or are known to have been active during historic times. 90% of these volcanoes are on the Ring of Fire– a band of volcanoes circling the Pacific Plate

Practice Questions What is the difference between a crater and a caldera? –size Is the picture below a crater or a caldera? –caldera

Practice Questions What is the difference between a dormant and an extinct volcano? –Time since last eruption & amount of erosion Where are most of the volcanoes in the world located? –Ring of Fire –Convergent and Divergent Plate Boundaries (not transform)

Magma vs. Lava Both are molten (liquid) rock with suspended mineral grains and dissolved gases Magma –interior of the earth Lava –magma that reaches the earth’s surface

Magma and Viscosity Viscosity can be affected by many variables  Mineral composition  Temperature  Dissolved gases  Viscosity is a fluids resistance to flow

Practice Question Rank the following substances from high to low viscosity. –Ketchup –Vitamin Water –Magma –Maple syrup 1.Magma 2.Ketchup 3.Maple syrup 4.Vitamin Water

Mineral Composition Remember silica? –Silica is a very common mineral composed of silicon and oxygen (SiO 2 ). Silicates make up about 95% of the earth’s crust A magma’s viscosity is directly related to its silica content  High silica magmas (granitic) are highly viscous with short thick lava flows  Low silica magmas (basaltic) are more fluid and may travel up to 90 miles and have low viscosity

Temperature High temperatures decrease viscosity and make magmas and lavas more fluid= low viscosity As a lava flow cools it begins to thicken and movement decreases= high viscosity

Dissolved Gases As magmas reach the surface, the pressure is reduced and gases expand with explosive results Gas composition varies, but it usually consists of: –Mostly H 2 O (water vapor) & some CO 2 (carbon dioxide) –Minor amounts of Sulfur, Chlorine, and Fluorine gases The more gas, the more explosive the eruption! However, gas content affects viscosity as well: –High Gas = Low Viscosity –Low Gas = High Viscosity

Practice Questions What are 3 things that affect the viscosity of magma? –Silica content –Heat –Gas content How does each of these affect viscosity?

Viscosity Review 1)Temperature -  temp.  Viscosity 2)Silica Content -  silica  Viscosity 3)Dissolved Gas -  gases  Viscosity

Deep below the surface of the earth, the pressure is great. Volcanic gases are dissolved in the liquid….

If the magma has low viscosity, the gas bubbles can easily escape... If the magma has high viscosity, the gas bubbles are trapped in the thick, gooey liquid. And gas pressure builds up until….

Kaboom! An explosive eruption! So low viscosity magmas that allow gas to escape produce non- explosive eruptions. High viscosity magmas that trap gas produce explosive eruptions.

Types of Eruptions Non-Explosive  Lava Flows – Low viscosity  Ex. Hawaii & Mid-Ocean Ridge Explosive  High viscosity lava flows & high gas content  Pyroclastics – “fire fragments”  Ex. Mount St. Helens, Krakatoa

Soda Bottle Analogy Dissolved gases under pressure inside bottle. Soda (lava) escapes violently as pressure drops!

Practice Questions What type of eruption would happen if the magma has a high silica content and a high gas content? –Explosive What type of eruption would happen if the magma has a low silica content and a low gas content? –Non explosive

Lava Flows Dark colored = mafic lava (rich in magnesium & iron, low in silica) –low viscosity –Also called basaltic lavas  Pahoehoe – smooth, rope-like lava  aa – clinkery, rough lava Light colored = felsic lava (rich in silica) –High viscosity

Pyroclastics “Fire Fragments” Ash- very fine glassy fragments Welded tuff – fused glassy shards Cinders – pea sized pyroclasts Lapilli – walnut sized pyroclasts Lava blocks – large hardened pyroclasts Lava bombs – incandescent lava that cools as it flies through the air, football shaped Pumice – frothy material w/ air spaces

Volcanic Locations 1)Divergent Plate Boundaries  Where two plates are pulling apart--Mid Ocean Ridge & Continental Rift 2)Subduction Zones  Where an oceanic plate converges with another oceanic plate or a continental plate--Pacific Ring Of Fire 3)Hot Spots  Fixed source of magma in the center of a plate-- Hawaii, Yellowstone

Types of Volcanoes Shield Volcano Cinder cone Composite (stratovolcano) cone Fissure eruptions

Shield Volcano Mafic, basaltic lava Rapid streams of low viscosity lava flow easily form gentle slopes. Very flat and low compared to diameter Each flow is only a few meters thick Mauna Loa  2.5 miles above see level ( 13,679 ft)  6 miles above sea floor  60 miles across at base

Cinder Cone Small, steep cone shaped volcanoes Usually very forceful release of gas Little to no lava flow Magma and rock are flung from volcano Pea size pyroclastics (cinders) pile up and form distinctive steep sided cone

Composite Cone Alternate layers of lava and pyroclastics build up Viscous, gas charged andesitic lava Violent eruptions rth/animations/volcanoes/index.ht mlhttp:// rth/animations/volcanoes/index.ht ml

Fissure Eruptions A fissure is a fracture or crack in rock along which there is an obvious separation Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.

Practice Questions Arrange the types of volcanoes from least explosive to most explosive. (in general)  Shield, cinder cone, composite/strato Which volcanoes have lava as part of the eruption?  shield, fissure eruptions & composite/strato Which volcanoes have pyroclastics as part of the eruption?  cinder cone & composite/strato

Volcanic Hazards Lava and PyroclasticsLahars – volcanic mudflows created by ash and water Nuée Ardente – incandescent debris w/ hot gases that moves like an avalanche  Glowing cloud   F   120 mph Toxic Gases – CO, CO 2, Sulfur, HCl  Lake Nyos - Cameroon Steam Explosion  Phreatic eruption  Seawater is heated and explodes  Krakatau  100 million tons of TNT

Future Eruptions Decade Volcanoes –16 volcanoes that have been designated as Decade Volcanoes by the International Association of Volcanology and Chemistry of the Earth's Interior b/c of their eruptive history and proximity to major population centers.

Volcano Pictures Mount Etna, Italy Pacific Northwest – Rainier and Mt. St. Helens Hekla, Iceland Olympus Mons, Mars

Eyjafjallajökull, Iceland 2010

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