Types, Structures, and Eruptions Volcanoes! Types, Structures, and Eruptions

http://qrius.si.edu/watch/par%C3%ADcutin-birth-volcano

What is a Volcano? A volcano is… A rupture in the surface of a planet that… Allows molten rock, ash, and/or gases to escape from lower levels onto the surface It can be large or small It can be violent or quiet

The Anatomy of a Volcano All volcanoes have the following: A magma chamber A main vent A crater Volcanoes may also have: Secondary cones, vents, and craters Lava domes Pyroclastics (ash, rocks, bombs)

The Anatomy of a Volcano

Types of Magma Magma is classified based on its composition, temperature, and viscosity Composition includes the amount of mafic minerals and silica and the amount of dissolved gasses Viscosity refers to its resistance to flow Three basic types (although intermediate magmas are common)

Types of Magma Basaltic Magmas High amounts of mafic minerals Low silica content (45-55%) Low viscosity (flows like water) High Temperature (1000-1200 C) Produces pahoephoe and aa lavas (based on temperature) Gases escape easily

Types of Magma (Pahoehoe)

Types of Magma (Aa Flow)

Types of Magma (Aa Flow)

Types of Magma Andesitic Intermediate Lava Moderate amount of mafic minerals Moderate silica content (55-65%) Moderate gas content Moderate viscosity Moderate temperature (800-1000C)

Types of Magma Rhyolitic Poor in mafic minerals Rich in silica (more than 65%) High gas content High amount of ash High viscosity Low temperature (650-800 C)

Types of Volcanoes The major types of volcanoes include: Shield (Hawaiian-type) Cinder Cones Stratovolcanoes (Composite volcanoes) Other (less common) volcanoes include: Fissures Lava Plateau Caldera

Shield Volcanoes Shield volcanoes are dominated by their bases They have low slopes and are built entirely from basaltic lava flows Most associated with divergence or hot spots Ex: Hawaiian islands

Shield Volcanoes Eruption style Very gentle Typically produces little ash or pyroclastics Frequently results in lava fountains or lava lakes Lava tubes are also produced

Shield Eruptions

Cinder Cones Cinder Cones are the smallest type of volcano and the shortest lived Rarely taller than 1000 ft Have very steep sides Lava erupts from the base of the volcano, not the crater! Have large, deep craters and steep sides Formed by eruptions of brittle, rocky lava and pyroclastic material

Cinder Cone Eruptions Typically moderately explosive eruption Basaltic lava with a higher gas content Leads to the production of cinders – cooled small pieces of lava blown into the air Obsidian (volcanic glass) and scoria are common

Stratovolcanoes (Composite) “Typical” volcanoes Tall, with steep sides Built by alternating layers of andesitic and rhyolitic lava and pyroclastics Common around convergent boundaries Most major volcanoes are of this type http://www.cotf.edu/ete/movies/strato.mov

Stratovolcanic Eruptions Most violent volcanoes! Eruptions produce high quantities of ash, pyroclastics, gas, and lava (relatively little lava) Can also result in nuees ardentes (glowing clouds) – huge pyroclastic clouds that rush down mountain at sonic speeds

Fissures and Lava Plateaus A fissure is a crack in the surface of the Earth that opens along a divergent boundary A lava plateau is related to a large scale field of fissures that open due to an intense magma plume (rare!) Can be related to mass extinctions

Fissures and Lava Plateaus Fissure eruptions and lava plateaus Fluid basaltic lava extruded from crustal fractures called fissures e.g., Columbia Plateau

Almost everything about this volcanic province is impressive Almost everything about this volcanic province is impressive. The Columbia River Flood Basalt Province forms a plateau of 164,000 square kilometers between the Cascade Range and the Rocky Mountains. In all, more than 300 individual large (average volume 580 cubic km!) lava flows cover parts of the states of Idaho, Washington, and Oregon. At some locations, the lava is more than 3,500 m thick. The total volume of the volcanic province is 175,000 cubic km. Eruptions filled the Pasco Basin in the east and then sent flows westward into the Columbia River Gorge. About 85% of the province is made of the Grande Ronde Basalt with a volume of 149,000 cubic km (enough lava to bury all of the continental United States under 12 m of lava!) that erupted over a period of less than one million years. Flows eventually reached the Pacific Ocean, about 300 to 600 km from their fissure vents. The Pomona flow traveled from west-central Idaho to the Pacific (600 km), making it the longest known lava flow on Earth (the major- and trace-element compositions of the flow do not change over its entire length).

Fissures and Lava Plateaus

Calderas Calderas are the largest volcanic craters on Earth Can be tens of miles in diameter (just the crater!) Form when the roof of a large magma chamber collapses during an eruption of any other type of volcano http://www.cotf.edu/ete/movies/caldera.mov Results in a massive eruption, destroying the volcano and leaving a circular depression Examples – Yellowstone, Krakatoa, Santorini, Crater Lake

Calderas

Caldera

Calderas

Eruptions Eruptions produce the following: Lava (molten rock) Pyroclastics Ash (smaller than 2 mm) Lapilli (between 2 mm and 64 mm) Blocks and Bombs (larger than 64 mm) Gases

Eruptions

Types of Eruptions The severity of an eruption depends on: Viscosity of magma Higher viscosity – more violent eruption Gas content Greater gas content – more violent Silica content Higher silica content – more violent In other words – basaltic magmas will be less violent than rhyolitic magmas

Volcanoes and Other Igneous Activity Tutorial http://media.pearsoncmg.com/ph/esm/esm_tarbuck_escience_13/geode/3DVolcanoes/3DVolcanoes.html

Volcanoes and Plate Tectonics

Intrusive Igneous Activity Plutons: structures that result from cooling and hardening of magma beneath the Earth’s surface. Pluton derived from Pluto, Roman god of the underworld

Igneous Rocks Type of rock that forms when: magma cools and hardens beneath the surface of the Earth (intrusive) or magma cools and hardens on the Earth’s surface from volcanic eruptions (extrusive or volcanic)

Plutonic Structures Two types of plutons: Concordant – parallel to rock beds Discordant – cutting across rock units

Concordant Plutons Laccolith - mushroom shaped Sills – small flat intrusions

Sill and Laccolith Formation Sills and Laccoliths are plutons that form when magma intrudes between rock layers close to the surface.

Sill and Laccolith Formation Form between layers of sedimentary rock. Form at shallow depths. Why??

Sill and Laccolith Formation: Why are Shapes Different? Magma that forms laccoliths has higher viscosity than magma that forms sills.

Discordant Plutons Dike – small feature that cuts across rock units

Relationship between concordant and discordant plutons Concordant pluton Discordant pluton

Batholiths and Stocks Batholith Stock The largest bodies of intrusive igneous rock. A batholith has a surface exposure of over 100 square kilometers!! Stock Similar to batholiths, but under 100 kilometers in size.

Batholith Formation Begin as magma deep beneath the surface. Plutons slowly rise through the crust. Batholiths are formed when individual plutons join together.

Batholiths Batholiths form the core of many mountain ranges! The Idaho batholith forms part of the northern Rocky Mountains and is more than 40,000 square kilometers!