Nature of Volcanic Eruptions Volcanoes Nature of Volcanic Eruptions

Factors Affecting Eruptions Volcanoes erupt with different severities Primary factors that determine whether a volcano erupts quietly or violently are: (1) magma composition (2) magma temperature (3) amount of dissolved gases in magma

Magma Composition and Temperature Viscosity refers to a substance’s resistance to flow EX: syrup is more viscous than water and flows more slowly Temperature increase = less viscous liquid

Magma Composition and Temperature Chemical composition of magma greatly affects type of volcanic eruption Viscosity of magma is related to its silica (sand) content High silica content = greater viscosity

Dissolved Gases in Magma During eruptions, gases trapped in magma provide force to eject molten rock from the vent (opening to the surface) Highly viscous magmas slow the upward motion of expanding gases. The gases collect in pockets and bubbles until they increase in size and explosively erupt

Volcanic Material Many different materials are ejected from volcanoes during eruptions (1) Lava flows (2) Gases (3) Pyroclastic Materials

Lava Flows Basaltic lavas have low silica content, and are not very viscous Two types of formations occur as a result of basaltic lava flows: (1) pahoehoe – wrinkled/ropy lava flow (2) aa – rough jagged block surface with spiny projections and sharp edges Pahoehoe Aa

Gases Magma contains varied amounts of dissolved gases Gases are confined to molten rock by pressure; once pressure is reduced, the gases begin to escape

Pyroclastic Materials Pyroclastic material is the name given to particles that are produced during volcanic eruptions Cinders – range from size of small beads to walnuts Blocks – particles larger than 64mm in diameter and composed of hardened lava Bombs – glowing lava larger than 64mm in diameter Bomb Blocks Cinder

Anatomy of a Volcano Volcanic activity begins with fissure/crack that develops as magma is forced towards surface Gas-rich magma moves up through fissure ending at a vent Located at summit of volcano is a steep-walled depression called a crater

Types of Volcanoes Three main types of volcanoes: (1) Shield volcanoes (2) Cinder cones (3) Composite cones Cinder Cone Shield Volcano Composite/Stratovolcano

Shield Volcanoes Produced by the accumulation of fluid basaltic lava Broad, slightly domed structure that resembles a warrior’s shield Most have grown from ocean floor to form islands EX: Hawaiian Islands and Iceland

Cinder Cones Cinder cones mostly composed of gas-rich basaltic magma Usually the product of a single eruption that lasts a few weeks Once eruption ends, magma in pipe connecting the vent to the magma chamber solidifies so the volcano can never erupt again Small in size

Composite Volcano Most dangerous volcanoes Large symmetrical structures composed of lava and pyroclastic deposits Most explosive volcanoes due to high silica content of magma Most are located in Pacific Ocean Rim in an area called the “Ring of Fire” EX: St. Helens

How Volcanoes are Created… Subduction zones Plate Tectonics Hot Spots

Negative Effects of Volcanoes Ash fall, landslides, lahar (mud flows) – all lead to death and destruction Volcanic gases-lower surface temperature by blocking out the sun, released sulfur gases lead to the creation of acid rain Acid rain destroys leaves and can kill trees

Positive Effects of Volcanoes Created up to 80% of Earth’s surface – we need land to live on! Creates rich, fertile soil for agriculture Heat can be harvested for renewable energy source EX: geothermal energy in Japan