VolcanoesChapter 10

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 that are contained in the magma.

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 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

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 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 most common thing people think of is lava.

Lava is just magma at the earth’s surface. Although lava is one type of material there are also

many other types of volcanic materials. Volcanoes also produce broken rock fragments

called pyroclastics.

Volcanic Materials

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 (0.006 – 0.186 miles per hour or 33 – 984 feet 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 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)

Aa (Rough, Sharp Lava)

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

Cinders

Volcanic Blocks and Bombs

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

Types of Volcanoes

Cinder Cone Volcanoes – These volcanoes are relatively small. Usually they are no bigger than 30 to 300 meters ( about 98 – 984 ft.), but occasionally they can get up to 700 meters tall (about 2,296 feet or almost a half a mile).

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 gas-rich 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

Types of Volcanoes

Composite Cones – The world's largest, most explosive, and most powerful volcanoes are the composite cones or 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)

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 (100 MPH).

This is called a pyroclastic flow.

Pyroclastic Flows

Pyroclastic Flows

Aftermath of a Pyroclastic Flow

Types of Volcanoes

Another dangerous aspect of composite cone volcanoes is the mudflows or 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

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 that extrude to the surface cool and harden. Sometimes the sides of the volcano weather and erode to expose the neck or pipe later on.

Caldera (Crater Lake Oregon)

Yellowstone Caldera

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

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

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 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 lens-shaped appearance.

Plutons

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