The Nature of Volcanic Eruptions Chapter 10.1 The Nature of Volcanic Eruptions

Volcanoes give the opportunity to look at what happens deep beneath the Earth’s surface. On May 18, 1980 Mount St. Helens erupted and change the surrounding area in Yakima, Washington. It released a lot of ash and debris into the air and surrounding area.

Factors Affecting Eruptions There are several factors that will determine if a volcano erupts violently or quietly: magma composition, magma temperature, and the amount of dissolved gas in the magma.

Viscosity Viscosity is a substances resistance to flow. Magma that is more viscous usually erupts more violently than magma that is not. Magma that is heated is less viscous so the ability of lava to flow is affected by temperature. As lava cools and hardens it becomes more viscous (less able to flow) and that will eventually stop the lava flow. More silica in magma the more it’s viscosity.

Dissolved Gases The vent is the opening in a volcano to the surface. Explosive eruptions trap gases in the vent. These are gases like water vapor and carbon dioxide. Fluid basaltic magmas allow gases to be released slowly and easily. These are the types of eruptions that happen in Hawaii. Highly viscous lavas trap gases in and they build up until they explosively eject from the volcano. (like Mount St. Helens).

Volcanic Material Lava may seem like it is the main thing ejected from a volcano but large pieces of rock, lava bombs, fine ash, dust, and large amounts of gas are also given off.

Lava Flows Basaltic lavas are very fluid, they flow between 10 to 300 meters per hour. Silica rich lava is too slow to be visible. Pahoehoe flows are lavas that harden and then wrinkle up.

Pyroclastic Materials These are materials that are blown from the vent of a volcano. Fragments that come out a volcano can be very fine dust and volcanic ash to pieces that weigh several tons. Smaller particles are called cinders, larger particles are blocks.

Types of Volcanoes There are 3 main types of volcanoes: shield volcanoes, cinder cone, and composite cone.

Anatomy of a Volcano Volcanic activity starts when a crack starts in the crust and magma is forced toward the surface. Magma will move through the crack, through a circular pipe and end up in a vent. As time moves on and there is more magma that moves into the area it forms a volcano. At the top of many volcanoes is a steep-walled depression called a crater. The type of magma usually determines the type of volcano that is formed.

Shield Volcanoes Shield volcanoes are broad, slightly domed structures - most will form islands, like Hawaii. They are formed by fluid basaltic lava.

Cinder Cones Lava fragments that are the size of cinders (smaller) harden in the air and form a cinder cone. They have steep sides. The volcano is usually formed from one eruption that may last a few weeks to a few years. Usually once that eruption is over the volcanoes magma chamber solidifies and it never erupts again. These are usually smaller volcanoes.

Composite Cones These are the most dangerous type of volcano. Most are found around the Pacific Ocean – an area called the Ring of Fire. Mount St. Helens, Mount Rainier, and Mount Garibaldi in US are part of the Ring of Fire. A composite cone is a large, nearly symmetrical structure made of layers of lava and pyroclastic deposits. They are usually made by very viscous lavas.

Dangers from Composite Cones Pyroclastic flows are the most dangerous – they have hot gases, glowing ash, and larger rock fragments. Some move very quickly. Composite cones may also cause mudflows where volcanic debris becomes saturated with water and move quickly down the side of the volcano.

Other Volcanic Landforms Calderas - a large depression in a volcano. This is cause by the collapse of the top of a composite volcano or from the collapse of the top of a shield volcano. Necks and Pipes – most volcanoes get magma through conduits called pipes that connect a magma chamber to the surface. A neck is when the magma chamber withstand erosion and the rest of the volcano erodes. Lava Plateaus – Sometimes a volcano isn’t formed, instead magma comes out of fissures. These fissures can cover a large area and have many eruptions.