V o s l c e a o n

Learning Goal Key Points Volcano Locations Volcano Anatomy Eruptions

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Once magma forms, it rises toward Earth’s surface because it is less dense than the surrounding mantle and crust. Magma that reaches Earth’s surface is called lava. Volcanism- the processes associated with the discharge of magma, hot fluids, and gases

Location of Volcanism Most volcanoes form at convergent boundaries and divergent boundaries plate. Only about 5 % of magma erupts far from plate boundaries.

Convergent Volcanism Tectonic plates collide at convergent boundaries, forming subduction zones As the oceanic plate descends, magma forms the magma moves upward because it is less dense As it rises, the magma mixes with rock, minerals, and sediment from the overlying plate. These volcanoes are characterized by explosive eruption Subduction-(places where slabs of oceanic crust descend into the mantle) Most volcanoes located on land result from oceanic- continental subduction

Major Belts The volcanoes associated with convergent plate boundaries form two major belts Mediterranean Belt The smaller belt Includes Mount Etna and volcanoes in Italy. Its general outlines correspond to the boundaries between the Eurasian, African, and Arabian plates Circum-Pacific Belt The larger belt Called the Pacific Ring of Fire The outline of the belt corresponds to the outline of the Pacific Plate The belt stretches along the western coasts of North and South America, across the Aleutian Islands, and down the eastern coast of Asia.

2/3 of all volcanoes are along the Ring of Fire that surrounds the Pacific Ocean. 8

Divergent Volcanism Tectonic plates move apart and new ocean floor is produced as magma rises to fill the gap. At ocean ridges, this lava takes the form of giant pillows Volcanism at divergent boundaries tends to be nonexplosive, with effusions of large amounts of lava. Subduction-(places where slabs of oceanic crust descend into the mantle) About 2/3 of Earth’s volcanism occurs underwater along divergent boundaries at ocean ridges.

Hotspot Volcanism Volcanoes form far from plate boundaries Hot spots are unusually hot regions of Earth’s mantle where high-temperature magma rise to the surface As the rising magma melted through the crust, it formed volcanoes The hot spot remains stationary while the Pacific Plate moves Over time, the hot spot has left a trail of volcanic islands, the oldest are inactive because the island no longer sits above the hot spot. Subduction-(places where slabs of oceanic crust descend into the mantle) Even older volcanoes to the northwest are no longer above sea level. The world’s most active volcano, Kilauea, on the Big Island of Hawaii, is currently located over the hot spot. Hawaiian islands formation over millions of years

Hot spots can occur beneath oceanic or continental crust 11

Anatomy of a Volcano Volcano- A mound of material (layers of solidified lava) that is extruded to the Earth’s surface from a vent that is connected to a magma chamber via a feeder conduit. Conduit- Lava reaches the surface by traveling through a tubelike to the vent Vent- opening hta allows lava flows out onto the surface, it cools and solidifies around the vent. Crater-At the top of a volcano, around the vent, is a bowl-shaped depression Calderas- Larger depressions, often form after the magma chamber beneath a volcano empties from a major eruption. 12

Types of Volcanoes A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base Shield volcanoes form when layers of lava accumulate during nonexplosive eruptions They are the largest type of volcano

Muana Loa Volcano – the world’s largest volcano. 14

Flood Basalts When hot spots occur beneath continental crust (internal-plate volcanism) Flood basalts form when lava flows out of long cracks in Earth’s crust. These cracks are called fissures. These fissure vent through which lava erupts, usually without any explosive activity

Types of Volcanoes When eruptions eject small pieces of magma into the air, cinder cones form as this material, called tephra, falls back to Earth and piles up around the vent. Cinder cones have steep sides and are generally small; most are less than 500 m high. Cinder cones often form on or very near larger volcanoes.

Cinder cones: Forms an isolated conical mound of tephra Photograph by J.P. Lockwood on 1 December 1975 17

Types of Volcanoes Composite volcanoes are formed of layers of hardened chunks of lava from violent eruptions alternating with layers of lava that oozed downslope before solidifying. Composite volcanoes are generally cone-shaped with concave slopes, and are much larger than cinder cones. Because of their explosive nature, they are potentially dangerous to humans and the environment.

Mount Mageik volcano, Alaska Photograph by R. McGimsey on 15 July 1990 19

Mt. St. Helens Before Mt. St. Helens After 20

Eruptions: Making Magma What makes the eruption of one volcano quiet, and the eruption of another explosively violent? The activity of a volcano depends on the composition of the magma! What are the factors that determine the composition of magma? It’s temperature and pressure Amounts of dissolved gas Very significantly — the amount of silica a magma contains. The composition of magma determines the type of rock that forms when it cools and its behavior during an eruption.

Composition of Magma Viscosity- The physical property that describes a material’s resistance to flow Temperature Depending on their composition, most rocks begin to melt between 800°C and 1200°C These high temperatures are found in the crust and upper mantle Temperature increases with depth beneath Earth’s surface cooler magma has a higher viscosity, tends to resist flowing Pressure Pressure increases with depth because of the weight of overlying rocks. As pressure increases, melting temperature of a substance also increases The effect of pressure explains why most of the rocks in Earth’s lower crust and upper mantle do not melt Melting temp (solid to liquid)

Composition of Magma Amount of Silica Present Amounts of Dissolved Gas minerals in the mantle, such as albite melt at high temperatures The presence of dissolved water vapor lowers the melting temperature of minerals, causing mantle material to melt into magma. This eventually forms volcanoes and fuels their eruptions. SiO2 content controls the viscosity Amounts of Dissolved Gas as the amount of gases in magma increases, the magma’s explosivity also increases gases in magma are water vapor (most common), carbon dioxide, sulfur dioxide, and hydrogen sulfide. The greater the viscosity the more gas trapped in the magma.

Types of Magma The silica content of magma determines not only its explosivity and viscosity, but also which type of volcanic rock it forms as lava cools. Basaltic magma Formed when rock in the upper mantle melts Less than 50% silica Reacts very little with overlying continental crust or sediments. Low silica content = low-viscosity magma. Dissolved gases escape easily volcano is characterized by quiet eruptions.

Types of Magma Andesitic magma 50 to 60 % silica Found along oceanic-continental subduction zones The source material for this magma can be either oceanic crust or oceanic sediments The higher silica content = magma that has intermediate viscosity intermediate explosivity

Types of Magma Rhyolitic magma Formed when molten material rises and mixes with the overlying continental crust rich in silica and water More than 60 % silica The high viscosity = slow movement High viscosity = large volume of gas trapped within = very explosive

Overall Magma Type Chemical Composition Temperature (degrees C) The behaviour of the magma determines the type of volcano that develops: Low SiO2 magmas, with little gas and low viscosity, flows readily through their vents and across the land surface when the lava escapes the vents. High SiO2 magmas, gaseous and with high viscosity, tend to plug their vents until the force of escaping magma blows the vent clear; such magmas cause explosive volcanoes. Magma Type Chemical Composition Temperature (degrees C) Viscosity Gas Content Basaltic 45-55% SiO2; 1000 - 1200 Low Andesitic 55-65% SiO2; 800-1000 Intermediate Rhyolitic 65-75% SiO2; 650-800 High 27

Eruptions

Explosive Eruptions Occur when lava is too viscous to flow freely from the vent, pressure builds up in the lava until the volcano explodes, throwing lava and rock into the air. The erupted materials are called tephra pieces of lava that solidified during the eruption pieces of the crust carried by the magma before the eruption Tephra is classified by size: diameters less than 2 mm, are called ash, Largest tephra thrown from a volcano are called blocks (can be the size of a car)

The Impact of Volcanic Eruptions Lava flows- Damage limited to the vicinity. Fatalities rare due to slow speed of advancing lava flow. Ash fall- Significant ash in the upper atmosphere can circle the globe in a matter of weeks. Pyroclastic flows- Rapidly moving clouds of tephra mixed with hot, suffocating gases. They can have internal temperatures of more than 700°C, and move nearly 200 km/h. Lahars- Water and debris rushed down the slopes, picking up more debris along the way. Volcanic Gases- CO2, SO2 and CO are the most abundant of harmful gases.