Ch 13-Volcanoes

• What causes most volcanoes?

– Movement of tectonic plates • Magma-liquid rock produced under Earth’s surface – Temp of rock rises above melting pt of the minerals the rock is composed of, rock melts – Enough pressure is removed from the rock, melting pt will decrease and rock will melt – Addition of fluids, such as water, may decrease the melting pt of some minerals in the rock and cause rock to melt

Volcanism

• Any activity that includes the movement of magma toward or onto Earth’s surface • Magma rises upward through the crust cuz the magma is less dense than surrounding rock • Lava-magma that flows onto Earth’s surface • Vent-opening that lava flows through • Volcano-vent or fissure in Earth’s surface through which magma and gases are expelled

Major Volcanic Zones

• Where do they occur?

– Convergent and divergent boundaries of tectonic plates – Pacific Ring of Fire • Subduction Zones-one tectonic plate moves under another – Oceanic to oceanic, continental to oceanic • Mid Ocean Ridges-largest amount of magma comes to surface – Plates pull apart magma flows upward along rift zone – Fissures-cracks through which lava flows to Earth’s surface • Hot spots-volcanically active area of Earth’s surface – Areas of volcanism w/in interiors of lithosphere plates

Intrusive Activity

• Magma intrudes overlying rock, magma that doesn’t reach Earth’s surface cools and solidifies inside crust • Plutons-igneous rock • Dikes-small tabular plutons • Batholithes-large plutons

Sec 2-Volcanic Eruptions

• Types of magma – Mafic-magma or rock that is rich in magnesium and iron, dark – Felsic-magma or rock rich in light colored silicate materials • Mafic-oceanic crust • Felsic and mafic-continental crust

Types of Eruptions

• Viscosity-resistance to flow, of magma affects force w/ which particular volcano will erupt • Mafic-runny lava, low viscosity, quiet eruptions • Felsic-sticky lava, high viscosity, explosive eruptions • Quiet eruptions – Oceanic eruptions, mafic magma. Low viscosity=gases escape so volcanoes are quiet • Lava flows – Lava cools rapidly, crust forms and lava continues to flow, it forms rock=pahoehoe – Crust deforms rapidly, forms wrinkles, surface breaks into jagged chunks=aa – Blocky lava-high viscosity, produces large chunks, and lava continues to flow

• Explosive Eruptions – Felsic lavas of continental volcanoes-cooler and stickier – Large amounts of trapped gases-CO2 and water vapor – Explodes and throws pyroclastic material into air – Pyroclastic material-fragments of rock that form during volcanic eruption

Types of Pyroclastic Material

• Volcanic ash-less than 2 mm in diameter • Volcanic dust-volcanic ash less than .25 mm in diameter • Lapilli-large pyroclastic particles, less than 64 mm • Volcanic bombs-large clots of lava, red hot when thrown out • Volcanic blocks-largest pyroclastic material, form from solid rock blasted from vent

Types of Volcanoes

• Crater-funnel shaped pit at top of volcanic vent, forms when materials are blown out • Types of cones – Shield Volcanoes-broad at base, sloping sides, wide area, quiet eruptions – Cinder Cones-steep slopes, explosion eruptions, made up of pyroclastic material – Composite volcanoes-alternating layers of hardened lava flows and pyroclastic material

Volcanic Eruptions

• Calderas-large circular depression, forms when magma chamber below volcano partially empties, causes ground to sink – Krakatau in Indonesia • Predicting Volcanic Eruptions – Earthquake activity-most important sign is changes in earthquake activity around volcano – Patterns in activity-upward movement of magma beneath the surface, causes surface of volcano to bulge outward before eruption