Chapter 13 Volcanoes Section 2 Volcanic Eruptions Section

Chapter 13 - Volcanoes Section 2: Volcanic Eruptions

Section 2 Objectives � Explain how the composition of magma affects volcanic eruptions and lava flow. � Describe the five major types of pyroclastic material. � Identify the three main types of volcanic cones. � Describe how a caldera forms. � List three events that may signal a volcanic eruptions.

Windows to Earth’s Interior � 2 › general types of lava Mafic: magma or rock rich in Mg and Fe; commonly dark in color �Commonly makes up oceanic crust �May also make up continental crust › Felsic: magma or rock that is rich in lightcolored silicate materials �Makes up continental crust

Types of Eruptions � Viscosity – the resistance of a liquid to flow; determined by magma’s composition �This can affect the force with which a volcano may erupt �Mafic magmas are runny with low viscosity and typically cause QUIET eruptions (typically oceanic volcanoes – Hawaii) �Felsic magmas are very sticky and therefore highly viscous. �They can trap large amounts of dissolved gases, which leads to more EXPLOSIVE eruptions.

Lava Flows � Pahoehoe – mafic flows that cool & form a crust, continues to flow causing wrinkles in crust; “ropy” lava flows � Aa – forms if crust deforms rapidly or grows too thick to wrinkle, and surface breaks into jagged chunks. › Same composition but different textures due to gas content, rate and slope of flow � Blocky – Higher silica and is more viscous, forming large blocks of volcanic rock when cooled

Explosive Eruptions � Felsic lavas of continental volcanoes such as Mount St. Helens tend to be heavier and stickier (more viscous) � Contain large amounts of trapped gases, such as carbon dioxide and water vapor � During an eruption, gases escape and molten and solid particles are thrown into the air › Pyroclastic material: fragments of rock that form during a volcanic eruption

Types of Pyroclastic Material � Particles are classified according to size �Volcanic ash: particles < 2 mm in diameter �Volcanic dust : particles < 0. 25 mm in diameter �Lapilli: “little stones” < 64 mm in diameter �Generally fall near the vent �Volcanic bombs: clumps of red-hot lava that spin and cool in the air; round or spindle �Volcanic blocks: solid rock blasted from the vent; may be as large as a small house

Types of Volcanoes � Table 1, page 328 � Lava &/or pyroclastic material ejected during both quiet &/or explosive eruptions build up around the vent � Forms 3 Types of Volcanic Cones › Shield volcanoes: Broad at base with gentle sloping sides; quiet eruptions; mafic lava › Cinder cones: Very steep slope; explosive eruptions with cones of pyroclastic material › Composite volcanoes: alt. layers of hardened lava flows and pyroclastic material; a. k. a stratovolcanoes; lg. volcanic mtns.

Calderas � Large circular or basin-shaped depressions in the ground › Form when the magma chamber below a volcanic cone collapses › Figure 4, page 329 › Eruptions discharge large amounts of magma can cause a magma chamber to collapse › Krakatau: volcanic island in Indonesia exploded in 1883 forming a caldera with diameter of 6 km › May fill with water to form lakes

Predicting Volcanic Eruptions � Earthquake activity may signal a volcanic eruption before it occurs › Due to increased pressure, increased temps, or fracturing of surrounding rock › Also increased strength and frequency of quakes � Patterns › › in Activity Sides of volcano may bulge as magma moves Instruments measure tilt of ground on slopes Knowledge of previous eruptions Daily measurements vs. past measurements