What controls volcanic eruptions? Composition of Magma – Based on percentage of silica and oxygen present in the magma. – High Silica: Thicker and slower moving. Usually thick and sticky. Viscosity: – Is a physical property that describes the material’s resistance to flow. – Low Silica: Low percentage of silica and more Fe and Mg. Thin and runny.

Dissolved Gases and Temperature – Higher temperature of magma, the more easily it flows. – Gases: H 2 O (v), CO 2, SO 2, H 2 S – The more gases the greater the chance of an explosive eruption.

Types of Magma and Lava Basaltic Magma and Lava – Low percentage of silica. – Low viscosity. – Much thinner, more fluid magma – Tend to pour from the vent and run down the sides of the volcano in a non-explosive eruption. – Pahoehoe lava cools develops ropelike patterns. – Aa lava is stiff, and slow moving. – Underwater eruption forms bubble-like pillow lava.

Granitic Magma and Lava – High percentage of silica. – High viscosity, flows slowly. – Sticky and lumpy. – Trap gases, builds up pressure and produces explosive eruptions.

Types of Volcanoes Shield volcano: – Is a huge, gently sloping volcanic landform that is mainly composed of basaltic lava. – Develop as layer upon layer of gently flowing basaltic lava piles up.

Cinder Cone Volcanoes – Is mainly composed of solid fragments known as Tephra. – Includes fragments of volcanic rock or lava. – Explosive volcanic eruptions. – When lava erupts from the vent, it cools quickly in the air.

Composite Volcanoes – Is mainly composed of alternating layers of lava and tephra. – Quiet and explosive volcanic eruptions due to the composition of magma – Tall mountains.

Volcanoes in California Form at a convergent plate boundary – Part of a volcanic arc. – Form where the Juan de Fuca Plate subducts beneath the N. American Plate

Intrusive Igneous Features Batholiths – Largest intrusive igneous features. – Magma slowly cools and solidifies before reaching the surface. – Many kilometers in width and length and several kilometers thick.

Dikes and Sills – Magma squeezes into cracks in rocks below the surface. – Magma that cuts across rock layers and hardens is called dike. – Magma that is parallel to rock layers and hardens is called sill.

Volcanic Neck – Hardened magma inside the vent is eroded by water and wind. – The cone is much softer than the igneous rock in the vent, so it erodes first.

Lava Domes – Rounded volcanic feature that forms when highly viscous lava erupts from the vent. – Lava piles up. – When gases accumulate, pressure increases within the lava dome, gas, lava and solid materials are ejected in the air.

Lava Tubes – A hollow tube that forms when lava flows through a channel, cools and hardens on the surface. – Often underground. – Magma flows to the sea.

Caldera – Is a large, circular depression. – Forms when the top of the volcano collapses and becomes wedged into the nearly empty magma chamber. – The chamber becomes filled with water, forming lakes and landscapes. – Usually the largest eruptions on Earth.