Volcanoes Chapter 10 Factors affecting Volcanic Eruptions The

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Volcanoes Chapter 10

Volcanoes Chapter 10

Factors affecting Volcanic Eruptions The three main factors that determine how violent and explosive

Factors affecting Volcanic Eruptions The three main factors that determine how violent and explosive a volcanic eruption can be are magma composition, magma temperature and the amount of dissolved gases in the magma.

Magma Viscosity A magma's viscosity is its resistance to flow. Pancake syrup or honey

Magma Viscosity A magma's viscosity is its resistance to flow. Pancake syrup or honey has a higher viscosity than regular tap water. If you heat syrup or honey it lowers the viscosity. In like manner, the hotter the magma, the lower the viscosity of the magma.

Magma Composition and Viscosity If we use the syrup or honey analogy, the higher

Magma Composition and Viscosity If we use the syrup or honey analogy, the higher the sugar content, the higher the viscosity of the syrup or honey. Similarly, the more silica a magma has, the higher its viscosity. Magma with a felsic or rhyolitic composition has a higher viscosity than magma with a mafic or basaltic composition.

Factors Affecting Magma Viscosity

Factors Affecting Magma Viscosity

Dissolved Gases Magma also contains dissolved gases. Some of those dissolved gases include water

Dissolved Gases Magma also contains dissolved gases. Some of those dissolved gases include water vapor, carbon dioxide, and sulfur dioxide. When the viscosity is high, dissolved gases become trapped in the magma, and the volcano builds up pressure. When viscosity is low, dissolved gases escape from the magma and the pressure is low.

Dissolved Gases When dissolved gases are high, pressure is high, and volcanic eruptions are

Dissolved Gases When dissolved gases are high, pressure is high, and volcanic eruptions are explosive. When dissolved gases are low, pressure is low, and volcanic eruptions are quiet.

Volcanic Materials When people think of volcanoes and the materials produced by them, the

Volcanic Materials When people think of volcanoes and the materials produced by them, the most common thing people think of is lava. Lava is just magma at the earth’s surface. Although lava is one type of material that is produced by volcanoes, there also other types of volcanic materials. Volcanoes also produce hot, broken rock fragments called pyroclastics.

Volcanic Materials

Volcanic Materials

Volcanic Materials The speed of a lava flow depends on its magma composition. Hot

Volcanic Materials The speed of a lava flow depends on its magma composition. Hot mafic/basaltic lavas can move relatively fast. Flow rates for mafic/basaltic lavas vary from 10 to 300 meters per hour. Silica-rich felsic/rhyolitic lavas move so slow that an observer can not detect its movement.

Volcanic Materials When a lava flow hardens and forms a skin with a braided

Volcanic Materials When a lava flow hardens and forms a skin with a braided rope-like texture, it is called pahoehoe (pah HOH ee hoh ee). When a lava flow hardens and forms a rough surface with jagged blocks, sharp edges, and spiny projections it is called aa (AH ah).

Pahoehoe (Ropy, Smooth Lava)

Pahoehoe (Ropy, Smooth Lava)

Aa (Rough, Sharp Lava)

Aa (Rough, Sharp Lava)

Volcanic Materials Particles and fragments of rocks ejected from a volcanic eruption that range

Volcanic Materials Particles and fragments of rocks ejected from a volcanic eruption that range in size from fine dust and ash to boulders that weigh several tons are referred to as pyroclastic materials. The small particles that are pea to walnut sized are called lapilli or cinders. The bigger particles are called blocks when they are hardened lava and volcanic bombs when they are ejected as glowing molten lava.

Lapilli

Lapilli

Cinders

Cinders

Volcanic Blocks and Bombs

Volcanic Blocks and Bombs

Types of Volcanoes There are three main types of volcanoes. Shield Volcanoes – produced

Types of Volcanoes There are three main types of volcanoes. Shield Volcanoes – produced by low viscosity, fluid, basaltic lavas. Shield volcanoes have the shape of a broad, gentle, sloping dome-like shape that resembles a warrior's shield. A good example of shield volcanoes are the volcanoes found on Hawaii.

Shield Volcanoes

Shield Volcanoes

Types of Volcanoes Cinder Cone Volcanoes – These volcanoes are relatively small. Usually they

Types of Volcanoes Cinder Cone Volcanoes – These volcanoes are relatively small. Usually they are no bigger than 30 to 300 meters, but occasionally they can get up to 700 meters tall. Cinder Cones are steep-sided. Cinder Cones get their name from the material they produce. They eject mostly cinders or lapilli which build up steep-sided cones.

Types of Volcanoes Cinder cones are usually produced from gasrich, mafic/basaltic lavas. Cinder cones

Types of Volcanoes Cinder cones are usually produced from gasrich, mafic/basaltic lavas. Cinder cones are usually short lived and usually are made by a single eruption. Cinder cones are often found in volcanic fields surrounded by other cinder cones, or occasionally on the sides of larger volcanoes like in the case of Mount Etna.

Cinder Cone Volcanoes

Cinder Cone Volcanoes

Types of Volcanoes Composite Cones – These are some of the world's largest, most

Types of Volcanoes Composite Cones – These are some of the world's largest, most explosive, and most powerful volcanoes. They are also called stratovolcanoes. Most of these volcanoes are located along the Circum Pacific Belt or Pacific Ring of Fire. Composite cones are produced by gas-rich, intermediate/andesitic lavas.

Composite Cone (Stratovolcanoes)

Composite Cone (Stratovolcanoes)

Types of Volcanoes Composite cone volcanoes have silica-rich, viscous lavas that generate explosive eruptions

Types of Volcanoes Composite cone volcanoes have silica-rich, viscous lavas that generate explosive eruptions of pyroclastic materials. One of the most destructive features of a composite cone volcano is when hot gas, ash, and rock fragments flow down the side of the volcano at speeds up to 200 km/hr. This is called a pyroclastic flow.

Pyroclastic Flows

Pyroclastic Flows

Pyroclastic Flows

Pyroclastic Flows

Aftermath of a Pyroclastic Flow

Aftermath of a Pyroclastic Flow

Types of Volcanoes Another dangerous aspect of composite cone volcanoes are the mudflows (lahars).

Types of Volcanoes Another dangerous aspect of composite cone volcanoes are the mudflows (lahars). Lahars are mudflows created by volcanoes when the heat from a volcano melts snow and ice on a volcano and sends torrential flows of mud, water, and debris down the volcano.

Lahars

Lahars

Lahars

Lahars

Lahars

Lahars

Other Volcanic Landforms Calderas – When a volcano collapses after an eruption, a deep

Other Volcanic Landforms Calderas – When a volcano collapses after an eruption, a deep depression called a caldera is formed. Sometimes a large lake fills in the caldera. Yellowstone National Park is a caldera where a large eruption occurred. Volcanic Necks and Pipes – These form when the magma in the conduits come to the surface and cool and harden. Sometimes the sides of the volcano weather and erode to expose the neck or pipe later on.

Caldera (Crater Lake Oregon)

Caldera (Crater Lake Oregon)

Yellowstone Caldera

Yellowstone Caldera

Shiprock, NM and Devil's Tower, Wy, (Volcanic Necks and Pipes)

Shiprock, NM and Devil's Tower, Wy, (Volcanic Necks and Pipes)

Other Volcanic Landforms Lava Plateaus – When fluid, basaltic magma comes to the surface

Other Volcanic Landforms Lava Plateaus – When fluid, basaltic magma comes to the surface through fissures or cracks in the ground, they can form large plateaus called lava plateaus. The Columbia Plateau in the northwestern United States is an example of a lava plateau.

Lava Plateau

Lava Plateau

Plutons When magma cools and hardens under the ground, it forms intrusive igneous bodies

Plutons When magma cools and hardens under the ground, it forms intrusive igneous bodies of rock called plutons. Plutons are classified by their shape, size, and relationship to the surrounding layers of rocks. A pluton called a sill forms when magma intrudes between two horizontal sedimentary rock layers close to the surface of the earth.

Plutons Laccoliths are similar to sills because they intrude in between horizontal sedimentary layers

Plutons Laccoliths are similar to sills because they intrude in between horizontal sedimentary layers near the surface of the earth. The shape of laccoliths is different because the magma that forms a laccolith is more viscous and this causes the sill to take on a bulging lens-shaped appearance.

Plutons

Plutons

Plutons Dikes – Dikes are plutons that form when magma cuts across preexisting sedimentary

Plutons Dikes – Dikes are plutons that form when magma cuts across preexisting sedimentary rock layers. Sometimes these plutons travel diagonally, vertically, or sometimes they can radiate in all directions. Batholiths – These are the largest plutons. In order to be considered a batholith, it must have a surface area exposure greater than 100 kilometers squared. (10 km x 10 km)

Plutons

Plutons